CHAPTER VII

DIPTERA—OR FLIES; APHANIPTERA—OR FLEAS; THYSANOPTERA—OR THRIPS

ORDER VII. DIPTERA

_Wings two, membranous, usually transparent and never very large; behind the wings a pair of small erect capitate bodies—halteres—frequently concealed under membranous hoods. No distinct prothorax, all the divisions of the thorax being united to form a large mass. Mouth-parts very variable, formed for suction not for biting, frequently assuming the form of a proboscis that can be retracted and concealed in a cleft of the under side of the head. The metamorphosis is very great, the larvae bearing no resemblance whatever to the perfect Insects, but being usually footless grubs or maggots; frequently the head is indistinct, small, and retracted. Pupa variable, either exposed and rather hard, with the appendages of the body more or less adherent; or enclosed in a scaly capsule looking like a seed, and when extracted, soft and delicate, with the appendages not fastened to the body incapable of movement._

This definition of the Diptera, or two-winged flies, is framed without reference to the fleas, which are wingless, or to a few other parasitic wingless Diptera, such as the sheep-tick. Although the Order is of enormous extent, these exceptional cases are remarkably few. About 40,000 species of Diptera have been discovered, but these are only a tithe of what are still unknown to science. The Order is not a favourite one with entomologists, and by the rest of the world it may be said to be detested. Flies do not display the sort of intelligence we appreciate, {439}or the kind of beauty we admire, and as a few of the creatures somewhat annoy us, the whole Order is only too frequently included in the category of nuisances that we must submit to. Moreover, the scavenger-habits that are revealed, when we begin to study their lives, are very repugnant to many persons. It is therefore no wonder that flies are not popular, and that few are willing to study them, or to collect them for observation. Nevertheless, Diptera have considerable claims to be classed as actually the highest of Insects physiologically, for it is certainly in them that the processes of a complete life-history are carried on with the greatest rapidity and that the phenomena of metamorphosis have been most perfected. A maggot, hatching from an egg, is able to grow with such rapidity that the work of its life in this respect is completed in a few days; then forming an impenetrable skin it dissolves itself almost completely; solidifying subsequently to a sort of jelly, it in a few days reconstructs itself as a being of totally different appearance and habits, in all its structures so profoundly changed from what it was that the resources of science are severely taxed to demonstrate any identity of the organs of the two instars.

[Illustration: Fig. 212—A Dipteron (Fam. Syrphidae), _Cheilosia chrysocoma_. Britain. A, Adult larva; B, the pupa; C, nymph, extracted from pupa; D, imago. (From Weyenbergh.)]

A good study of the comparative anatomy of Diptera has never been made; Baron Osten Sacken, one of our most accomplished Dipterologists, has recently stated that "the external characters of the Diptera have as yet been very insufficiently studied." We shall therefore only trouble the student with a few observations on points of structure that are of special importance, or that he will find frequently alluded to. The head is remarkable {440}for its mobility, and is connected with the thorax by a slender concealed neck that permits the head to undergo semi-rotation. A large part—sometimes nearly the whole—of the exposed surface of the head is occupied by the faceted eyes. It is usually the case that the eyes are larger in the male than in the female, and the sexual discrepancy in this respect may be very great. When the eyes of the two sides meet in a coadapted line of union the Insect is said to be "holoptic," and when the eyes are well separated "dichoptic."[344] The holoptic condition is specially characteristic of the male, but in some forms occurs in both sexes. There is no definite distinction between holoptic and dichoptic eyes. The eyes may be enormous, Fig. 238, without actually uniting, and in the cases where actual contiguity occurs, it takes place in different manners.[345] The eyes are frequently during life of brilliant colours and variegate with stripes or spots; this condition disappears speedily after death, and it is uncertain what the use of this coloration may be.[346] The eyes are frequently densely set with hairs between the almost innumerable facets. These facets frequently differ in size according to their position in the organ. The curious double eye of the male _Bibio_ (cf. Fig. 224) is well worth notice. There are usually three small ocelli placed very near together on the middle of the summit of the head.

The antennae are of considerable importance, as they offer one of the readiest means of classification. The families placed by systematists at the commencement of the Order have antennae similar to those of the majority of Insects, inasmuch as they consist of a series of segments approximately similar to one another, and arranged in a linear manner (Fig. 213, A). The number of these joints is never very great, but reaches sixteen in certain Tipulidae, and falls as low as eight in some Bibionidae. In certain cases where the antennae of the male are densely feathered (_Chironomus_, e.g.), the number of joints is in that sex greatly augmented, but they are imperfectly separated. This form of antenna gives the name Nemocera to the first series of Diptera.

{441}[Illustration: Fig. 213—Antennae of flies. A, The two antennae of _Glaphyroptera picta_ (Mycetophilidae); B, antenna of _Hexatoma pellucens_ (Tabanidae); C, of _Asilus crabroniformis_ (Asilidae); D, of _Leptis scolopacea_ (Leptidae); E, of _Dolichopus undulatus_ (Dolichopidae); F, of _Volucella bombylans_ (Syrphidae). (After Wandolleck.)]

The majority of flies have antennae of another form, peculiar to the Order, viz. three segments, the outer one of which is of diverse form, according to the genus or species, and bears on its front a fine projecting bristle, frequently feathered, as in Fig. 213, F; and often distinctly divided into two or more joints. This form of antenna is found in the series Aschiza and Schizophora; it is well exemplified in the common house-fly, where the organs in question hang from the forehead, and are placed in a hollow formed for their reception on the front of the head. Flies with this form of antennae are called Athericerous. Between the two forms of antennae we have mentioned there exists what may, speaking roughly, be called an intermediate condition, or rather a variety of intermediate conditions, associated in the series Brachycera (Fig. 213, B to D).[347] Here there are three (sometimes one or two) segments and a terminal appendage, but the appendage is usually compound (often so distinctly compound that it is evidently a series of partially, or even completely, separate joints, Fig. 213, B): the appendage in these cases is terminal, that is to say it is placed, not as in the Eumyiidae on the front of the joint that bears it, but (in the great majority of Brachycera) at the tip thereof; this appendage is often conical and pointed, often hair-like. Exceptional forms of antenna are found in the parasitic flies of the series Pupipara. In the Order generally the two basal joints of the antennae are evidently distinct in function from the others, and form the "scape"; the {442}part of the antenna beyond the scape is called the "flagellum"; an appendage of the flagellum is called "arista" when bristle-like, when thicker "style." In the basal joint of the antenna there is a complex nervous structure known as Johnston's organ. It is specially well developed in _Culex_ and _Chironomus_, and is larger in the male than it is in the female. Child has found something of the kind present in all the Diptera he has examined, and he considers that an analogous structure exists in Insects of other Orders. He thinks it is concerned with the perception of vibration, there being no sharp distinction between auditory and tactile sensation.[348]

About one-half of the Diptera possess a peculiar structure in the form of a head-vesicle called "ptilinum." In the fly emerging from the pupa this appears as a bladder-like expansion of the front of the head; being susceptible of great distension, it is useful in rupturing the hard shell in which the creature is then enclosed. In the mature fly the ptilinum is completely introverted, and can be found only by dissection; a little space, the "lunula," just under an arched suture, extending over the point of insertion of the antennae remains, however, and offers evidence of the existence of the ptilinum. This structure is also of importance in classification, though, unfortunately, it is difficult to verify.[349]

No point of Insect morphology has given rise to more difference of opinion than the mouth of Diptera; and the subject is still very far from being completely understood. The anatomy and morphology of the mandibulate Insect-mouth are comparatively simple (though not without greater difficulties than are usually appreciated); and it has been the desire of morphologists to homologise the sucking mouth of Diptera with the biting mouth; hence the view that the appendages of three segments are separate and distinct in the fly's mouth is taken for granted, and it is further assumed that some of the secondary parts of the appendages of the biting mouth can also be recognised in the sucking mouth. The anatomy of the mouth-parts is, however, {443}subject to great diversity of structure within the limits of the Order itself, even the two sexes in some species differing profoundly in this respect.[350] In the majority of the family Oestridae the mouth-parts are practically absent, and no definite entry to the alimentary canal can be perceived (Fig. 245). Besides this condition and its antithesis (Fig. 214), the complex assemblage of lancets seen in the Breeze-flies that draw blood, there is a great variety of other anatomical conditions.

[Illustration: Fig. 214.—Mouth-parts of the common blood-sucking fly, _Haematopota pluvialis_ ♀. A, Viewed from beneath, the proboscis removed; _a_, labrum; _b_, _b_, cultelli (mandibles of other anatomists); _c_, _c_, scalpella (maxillae of other anatomists); _d_, part of ventral scutum of second metamere; _e_, _e_, _f_, _f_, parts of palpi; _g_, hypopharynx and pellucid salivary duct; _h_, salivary receptacle; _i_, salivary duct; _k_, membranous part of second metamere; _l_, pharynx: B, labrum, pharynx, hypopharynx, separated, seen from beneath; _a_, labrum; _b_, hypopharynx; _c_, salivary duct; _d_, pharynx; _e_, protractor muscles: C, proboscis (labium) from beneath; _a_, scutum proboscidis; _c_, _c_, labella; _d_, _d_, retractor muscles. (After Meinert).]

Although, as we have said, great diversity of opinion exists, yet on the whole the majority of Dipterologists accept a view something to the following effect:—the labrum, or the labrum combined with the epipharynx, is frequently much prolonged; the tongue—hypopharynx—may also be much prolonged, and may form, in apposition with the labrum, a more or less imperfect tube for ingestion of the nutriment; the labium is more or less membranous or fleshy, and acts as a sheathing organ, its tips—called labella—-being in some cases developed to a quite extraordinary extent. As to the other parts of the mouth there is less agreement; the pointed organs (Fig. 214, A, _b_ _b_) are by {444}many identified as mandibles, while another pair of pointed processes (_c_ _c_) are considered to be parts of a maxilla, and the palpi (_f_ _f_) are by some considered to be maxillary palps. The Danish entomologist, Meinert, has published the best anatomical description of many of the diverse kinds of Dipterous mouth.[351] He, however, takes a different view of the morphology; he considers that not only may parts of the appendages of the mouth be much modified during the early stages of the individual development, but that they may be differently combined, even parts of the appendages of two segments being brought together in intimate combination. He has also pointed out that the mandibulate and sucking mouth are mechanical implements constructed on opposed principles; the main object of a biting mouth being the fixing and perfecting of the articulations of the mouth, so that great power of holding may be attained with a limited but definite power of movement. In the sucking mouth the parts are intimately associated for simple protrusion. Hence the two kinds of mouth must have been distinguished very early in the phylogeny, so that we must not expect to find a great correspondence between the parts of biting and sucking mouths. He apparently also considers that not only the appendages of a head-segment, but also part of the body of the segment, may be used in the construction of the mouth-organs. Meinert's views allow a much greater latitude of interpretation of the parts of the Dipterous mouth; had he contented himself with enunciating them in the manner we have followed him in summarily describing, they would have been recognised as a formidable obstacle to the facile adoption of the ordinary views. He has, however, accompanied his general statement with a particular interpretation and a distinct nomenclature, neither of which is it possible to adopt at present, as they have no more justification than the ordinary view. So that instead of one set of doubtful interpretations we have two.[352] In so difficult a question as homologising the trophi of different Orders of Insects we ought to use {445}exhaustively every method of inquiry: and from this point of view the development is of great importance. This has, however, as yet thrown but little light on the subject, this study being a very difficult one owing to the profound changes that take place during metamorphosis, the diversity of the parts in the early stages of Diptera, and the possibility that the larval conditions may themselves have been greatly changed in the course of the phylogeny. Miall informs us, however, that in _Chironomus_ as well as in _Corethra_ the new parts of the mouth of the imago are developed within those of the larva.[353] This may permit of an identification of the main divisions of the mouth, at any rate in these cases. Lowne has to some extent traced the development in the blowfly, and he does not agree with the usual interpretation of the parts in the adult.

The mouth is of considerable importance in the classification of Diptera. The Nemocera are remarkable from the linear development and flexibility of the palpi, which are nearly always at least three- or four-jointed; this condition occurring in no other Diptera. The palpi attain an extraordinary development in some Culicidae; in the genus _Megarrhina_ they are nearly as long as the body, and project in front of the head after the fashion of the palpi of Lepidoptera. In the Brachycera the sclerites or hard parts of the mouth reach a maximum of development, and in Tabanidae (Fig. 214), Nemestrinidae and Bombyliidae are often quite disproportionate to the size of the Insect. In many of the Eumyiid flies the soft parts are greatly developed, and capable of a variety of movement, the proboscis as a whole being protrusible, and having an elbow-joint in the middle.

The thorax is remarkable from the absence of distinct separation into the three divisions that may usually be so easily distinguished in Insects. The perfect combination of the three segments adds much to the difficulty of arriving at general conclusions as to the identification of the parts; hence considerable difference of opinion still prevails. It was formerly supposed that a segment from the abdomen was added to the thorax of Diptera as it is in Hymenoptera, but this has been shown by Brauer to be erroneous. Indeed, according to Lowne, the abdominal cavity is increased by the addition of the small posterior area of the thorax; it being the mesophragma that separates the {446}second and third great divisions of the body-cavity. The prothorax is always small, except in a few of the abnormal wingless forms (_Melophagus_); in _Nycteribia_ (Fig. 248) the mesothorax forms the anterior part of the body; the head and such parts of the prothorax as may be subsequently discovered to exist being placed entirely on the dorsum of the body. The mesothorax in all the winged Diptera forms by far the larger portion of the thoracic mass, the prominent part of it, that projects backwards to a greater or less extent over the base of the abdomen, being the scutellum. The first or prothoracic stigma is remarkably large and distinct, and is by some called mesothoracic. Another large stigma is placed very near to the halter (or balancer); the metathorax being very small. An imperfect stigma is said by Lowne to exist in the blowfly near the base of the wing. The number of abdominal segments externally visible is very diverse; there may be as many as nine (in the male _Tipula_), or as few as five, or even four, when the basal segment is much concealed; the diminution is due to certain segments at the extremity being indrawn and serving as a sort of tubular ovipositor in the female, or curled under the body and altered in form in the other sex, so as to constitute what is called a "hypopygium." In the female of Tipulidae the body is terminated by some horny pieces forming an external ovipositor. In nearly all Diptera the feet are five-jointed; the claws are well developed, there being placed under each of them a free pad or membrane, the "pulvillus"; there may be also a median structure between each pair of claws, of diverse form, the "empodium."

On the surface of the body of many flies there will be seen an armature of pointed bristles; these flies are called "chaetophorous"; where no regularly arranged system of such bristles exists the fly is "eremochaetous." In some families the arrangement of these bristles is of importance in classification, and a system of description has been drawn up by Baron Osten Sacken: this branch of descriptive entomology is known as chaetotaxy.[354]

The wings are of great importance in classifying Diptera; but unfortunately, like the other parts, they have not received an exhaustive anatomical study, and Dipterologists are not agreed as to the names that should be applied to their parts.

{447}[Illustration: Fig. 215—Nervuration of Dipterous wing. A, Wing of a Tipulid, according to Loew, who uses the following nomenclature: _a_, costal nervure; _b_, mediastinal; _c_, subcostal; _d_, radial; _e_, cubital; _f_, discoidal; _g_, postical; _h_, anal; _i_, axillar; _x_, transverse, _y_, posterior transverse, nervure; 1, 2, mediastinal areas; 3, subcostal; 4, cubital; 5, anterior basal; 6, posterior basal; 7, anal; 8, posterior marginal; 9, discoidal. B, Wing of an Acalypterate Muscid (_Ortalis_), according to Schiner, who uses the following nomenclature: (nervures, small letters; cells, capital letters): _a_, transverse shoulder; _b_, auxiliary; _c_ to _h_, first to sixth longitudinal; _i_, middle transverse; _k_, posterior transverse; _l_, _m_, _n_, _o_, costa; _p_, anterior basal transverse; _q_, posterior basal transverse; _r_, rudiment of a fourth nervure; _s_, axillary incision: A, B, C, first, second, and third costal cells; D, marginal; E, sub-marginal; F, G, H, first, second, and third posterior; I, discal; K, L, M, first, second, and third basal cells; N, anal angle; O, alula.]

We give below figures of two systems that have been used by eminent Dipterologists for the description of the nervures and cells. The comprehension of these features of the Dipterous wing will be facilitated by noticing that the wing—being extended at right angles to the body—is divided by the longitudinal nervures into two great fields, anterior and posterior, with an interval between them: this interval is traversed only by a short cross-vein (marked x in Fig. 215 A, and i in B). This cross-vein may be placed near the base or nearer to the tip of the wing; it is of importance because no nervure in front of the median area traversed by it can correspond with a nervure placed behind it in another wing. The very different nature of the nervuration in the two wings we have figured will readily be appreciated by an inspection of the parts posterior to the little cross-vein. On the hind margin of the wing, near the base, there is often a more or less free lobe (Fig. 215, B, O) called the "alula": still nearer to the base, or placed on the side of the body, may be seen one or two other lobes, of which the one nearer the alula is called the "tegula," or (when a lobe behind it is also present) the "upper tegula," (the "antitegula" of Osten Sacken); the other being the "lower tegula." These two terms are erroneous, the word tegula being definitely applied to another part of the Insect-body. In speaking of this structure in the following pages, we have preferred to call it the {448}"squama."[355] Those Muscidae in which the squama covers the halter like a hood are called "calypterate." In Fig. 216, we represent these structures, and in the explanation have mentioned the synonyms. The terms we think most applicable to the three lobes are alula, antisquama, squama. The squama may be called "calypter" when it covers the halter.

[Illustration: Fig. 216—Parts at the base of the wing in _Calliphora_. _a_, Anal angle or lobe of the wing; _b_, alula; _c_, antisquama, squama alaris, or antitegula; _d_, squama, squama thoracicalis, tegula, calypter, or calyptron; _e_, posterior extremity (scutellum) of the mesothorax; _f_, scutum of mesothorax.]

The halteres—commonly called balancers or poisers—are perhaps the most characteristic of all the Dipterous structures, though they are absent in most of the few wingless forms of the Order. Outside the Diptera similar organs appear to exist only in male Coccidae. The pair of halteres is placed on the metathorax, one on each of the pleural regions. They are believed to be the homologues of the hind wings; Weinland states[356] that certain canals existing in the interior of the halter correspond to wing-nervures. The halter may be described as a small rod-like body with a head like a pin, this terminal part being, however, rather variable in form. We have already stated that in many Diptera the squama forms a hood, the position of which leads to the belief that it is an important adjunct to the halter. Although the exact functions of the halteres are far from clear, it is certain that they are highly complex bodies, of extremely delicate structure: they are doubtless sense-organs, possessing as they do, groups of papillae on the exterior and a chordotonal organ (a structure for assisting the perception of sound) in the basal part; each halter is provided with four muscles at the base, and can, like the wings, execute most rapid vibrations. Seeing that they are the homologues of wings, it is a remarkable fact that in no Diptera are they replaced by wings, or by structures intermediate between these two kinds of organs.

INTERNAL STRUCTURE.—Information about the internal anatomy {449}is by no means extensive. The tracheal system is highly developed, and has air-sacs connected with it; a large pair at the base of the abdomen being called aërostats by Dufour. Inside the thoracic spiracles there are peculiar structures supposed by some to be voice-organs, while the abdominal spiracles are said to be remarkably simple in structure. Lowne says that there are ten or eleven pairs of spiracles in the Blow-fly; one of these, near the base of the wing, is peculiar in structure, and may not be a true stigma; he calls it a tympanic spiracle; it seems doubtful whether there are more than seven abdominal pairs. The alimentary canal is very elongate, and is provided with a diverticulum, the crop; this is usually called the sucking stomach, though its function is extremely doubtful. The Malpighian tubes are four in number, and are very elongate; in several groups of Nemocera there are, however, five Malpighian tubes, a number known to occur in only very few other Insects. The nervous system is remarkable on account of the concentration of ganglia in the thorax, so as to form a thoracic, in addition to the usual cephalic, brain. For particulars as to the positions of the ganglia and the great changes that occur in the lifetime, the student should refer to Brandt, to Künckel, and to Brauer.[357] Much information as to the internal anatomy of the Blowfly is given by Lowne, but it is doubtful to what extent it is applicable to Diptera in general.[358]

[Illustration: Fig. 217—Acephalous larva or maggot of the blow-fly, with the head, _a_, extended. (After Lowne.)]

The LARVAE of Diptera are—so far as the unaided eye is concerned—without exception destitute of any kind of adornment, the vast majority of them being of the kind known as maggots. None of them have true thoracic legs; though in the earlier groups, pseudopods or protuberances of the body that serve as aids in locomotion are common. Unlike what occurs in other Orders the arrangement of these pseudopods on the body differs greatly in various forms; in a few cases they are surmounted by {450}curved hairs. The most important distinction in external form in Dipterous larvae is that while those that are thorough maggots possess no visible head, others have a well-marked one (Fig. 225); these are therefore called "eucephalous": they have a mouth of the mandibulate type. In some other Dipterous larvae the head is more or less reduced in size, and in the acephalous forms there is only a framework of a few chitinous rods to represent it. The nervous system in the most completely headless larvae is very remarkable, all the ganglia being concentrated in a single mass placed in the thorax. The tracheal system exhibits a great variety; some larvae have stigmata arranged along the sides of the body after the fashion normal in Insect-larvae; these are called "peripneustic"; as many as ten pairs of stigmata may be present in these cases, but nine pairs is much more common. Other larvae have a pair of stigmata placed at the termination of the body, and another pair near the anterior extremity, the two pairs communicating by large tracheal trunks extending the length of the body; these larvae are said to be "amphipneustic": this is the condition usual in the more completely acephalous larvae. Others have only the terminal pair of spiracles, and are styled "metapneustic." Some begin life in the metapneustic state and afterwards become amphipneustic. In the aquatic larva of _Corethra_ there are no spiracles, though there is an imperfect tracheal system. Many Dipterous larvae that live in water or in conditions that prevent access of air to the body have remarkable arrangements for keeping the tip of the body in communication with the atmosphere. The stigmata in metapneustic and amphipneustic larvae are very remarkable compound structures, exhibiting however great diversity; their peculiarities and uses are not well understood; it appears very doubtful whether some of them have any external opening. Reference may be made, as to the variety of structure, to Meijere's paper[359] from which we take the accompanying figure of a posterior stigmatic apparatus in _Lipara lucens_. It appears that there is a compound chamber—"Filzkammer"—terminating externally in lobes or fingers—"Knospen" and appearing as marks on the outer surface: this chamber is seated on a tracheal tube, and is, Meijere thinks, probably a secondary growth of the trachea coming to the outer surface. It is traversed by what may be {451}considered the original tracheal tube, opening externally as an external stigmatic scar—"Stigmennarbe"—and with a second or inner scar placed internally. We may conclude from what is already known that these structures will be found to differ in the same larva according to the stage of its development.

[Illustration: Fig. 218—The posterior stigma of the larva of _Lipara lucens_. _a_, One of the three "Knospen" or lobes; _b_, external stigmatic scar; _c_, internal scar; _d_, stigmatic chamber (Filzkammer); _e_, trachea. (After Meijere.)]

An extremely valuable summary of the characters and variety of Dipterous larvae has been given by Brauer,[360] from which it appears that the larvae of the first half of the family exhibit great variety and have been much studied, while the more purely maggot-like forms of the Muscidae have, with one or two exceptions, been little investigated.

The PUPAL instar is of two distinct kinds. First, we meet with a pupa like that of Lepidoptera, viz. a mummy-like object, or pupa obtecta, in which there is a crisp outer shell, formed in part by the adherent cases of the appendages of the future imago. This condition, with a few exceptions to be subsequently noticed, obtains in the Nemocera and Brachycera. It is exhibited in various degrees of perfection, being most complete in Tipulidae; in other forms the shell is softer and the appendages more protuberant. The second kind of pupa is found in the Cyclorrhaphous flies; it has externally no marks except some faint circular rings and, frequently, a pair of projections from near one extremity of the body; occasionally there is a single prominence at the other extremity of the body. This condition is due to the fact that the larva does not escape from the skin at the last ecdysis, but merely shrinks within it, so that the larval skin, itself contracted and altered by an excretion of chitin, remains and forms a perfect protection to the included organism. This kind of pupa looks like a seed, and is well exemplified by the common Blow-fly. The capacity for entering on such a condition is evidently correlative with the absence of a larval head. The metamorphosis in this curious little barrel goes on in a different manner to what it does in the pupa {452}obtecta. A good name for the whole structure of this instar has not been found. Older authors called it "pupa coarctata," or "nympha inclusa"; Brauer speaks of it as a "compound pupa"; ordinarily in our language it is called a "puparium," a term which is more applicable to the case alone.

In species having a pupa obtecta the larval skin is cast after the chief processes of the external metamorphosis have occurred, and then an exudation of chitin hardens the general surface. In the "compound pupa" of the Blow-fly there is for a considerable period no formed pupa at all, but merely a shell or case containing the results of histolysis and the centres for regeneration of new organs; the chitin-exudation to the exterior of the larval skin occurs in the early part of the series of metamorphic changes, and the organism breaks down to a cream within the shell thus formed, and then gradually assumes therein the condition of a soft, nymphoid pupa. The exceptional conditions previously referred to as exhibited by a few forms are certain cases in which a more or less perfect pupa obtecta is found within the last larval skin, as is the case in _Stratiomys_. Another highly remarkable condition exists in the Hessian fly, and a few other Cecidomyiids, where the Insect apparently makes an exudation which it uses as a covering case, independent of the larval skin; this latter being subsequently shed inside the case, so that this condition of coarctate pupa differs from that we have described as existing in Cyclorrhaphous flies, although the two are superficially similar. In the Pupipara the larval stage is passed in the body of the mother, which produces a succession of young, nourished one at a time by the secretion of glands; this young is born as a full-grown larva that becomes at once a pupa.

METAMORPHOSIS.—As it is in Diptera that the phenomena of Insect-metamorphosis have reached their highest development we endeavoured to give some idea of their nature in the previous volume, therefore we need give only a brief sketch of the chief features of Dipterous metamorphosis. The Blow-fly undergoes a rapid embryonic development, the later stages of which are, on the whole, of a retrogressive nature. On the emergence of the young maggot it feeds up rapidly, the rapidity varying greatly according to circumstances, and then when full-grown rests. While resting, a process of internal liquefaction, called histolysis, is going on, and the maggot contracts and exudes an excretion {453}that hardens its skin. At the time this hard skin has become complete, or soon after, the maggot inside has dissolved into a cream contained in a sac inside the shell; this cream becomes reconstituted into a fly by a gradual process of growth and development of certain minute portions of the body—the imaginal discs or folds, the histoblasts and neuroblasts that were exempt from the histolytic process: in the early stages of the reconstitution the general structure is, of course, altogether vague, and this condition—purely one of transition—is called the pronymph; the nymph becomes gradually developed: it corresponds vaguely with the pupa obtecta of the early groups of Diptera, but is soft like the pupa of Hymenoptera. This nymph gradually develops into the fly itself, the external parts being first completed and the internal organs elaborated subsequently. The sexual organs do not undergo metamorphosis like other internal organs, there being a gradual (though irregular or interrupted) growth of them in the young larva, till they are completed some time after the emergence of the perfect fly. The processes in the Blow-fly have been studied by numerous able histologists of various nationalities, and have recently been described by Lowne in our own language.[361] Comparatively little has been done in studying the corresponding phenomena in other Diptera. Weismann has investigated the development of _Corethra_, and Miall that of _Chironomus_. These two flies belong to a division of Diptera different from that which includes the Blow-fly, and they display a condition of the metamorphic processes allied to what occurs in Lepidoptera, as well as to that which takes place in the Blow-fly. Imaginal folds are formed, but they only appear much later in the life, and they are much less distant from the positions they will, when developed, occupy in the imago. In _Chironomus_, according to Miall, the imaginal folds only appear in the last larval instar, but they grow with such rapidity that the legs and wings of the future fly can be distinguished in the larva, even before pupation; thus when the activity of the larva ceases but little change is required to complete the obtected pupa. In the Blow-fly some of the imaginal folds have been {454}traced back to the embryo; how many centres for the new growth there may be is uncertain, for though there are upwards of sixty for the outer body, the number of regenerative centres for the internal organs is not ascertained. The peculiar central nervous mass, mentioned in our remarks on the larva, consists of two kinds of tissue mixed together in a complex manner; one of these kinds is functionally active during the larval life and at the metamorphosis undergoes histolysis, while the other, or embryonic, portion develops into the nervous system of the fly.

It forms no part of our task to deal with general subjects, but we may be pardoned for calling attention to the bearing the metamorphosis of the higher Diptera has on our ideas of heredity in Insects. The fly bears no resemblance whatever to the larva, and is only obtained by the organic destruction of the latter, which occurs before the perfection of the sexual organs takes place, and yet the fly reproduces itself only secondarily, but primarily gives rise to the totally different larva. It is supposed that the larval structures have been gradually acquired, and yet they are transmitted with the utmost faithfulness by the totally different fly. We can only conclude that that which is bequeathed in each species is the early state of a particular process of development from which the subsequent stages follow necessarily if the developing organism be placed in conditions having on it influences like to those that influenced the ancestors.

CLASSIFICATION.—The classification of Diptera is as yet very imperfect. Formerly they were divided into two great groups, Nemocera and Brachycera, according to the structure of the antennae, as previously mentioned. This division has been abandoned, and the term Brachycera is now applied to only a small part of the old section that bore the name. The primary division usually adopted at present is into Orthorrhapha and Cyclorrhapha. The characters of these two groups are based on the nature of the metamorphosis, and have been gradually elaborated by Brauer in various memoirs.[362] The Orthorrhapha includes the forms with obtected pupae, the Cyclorrhapha those with a nymph-compound, as previously described. This distinction is of great importance, but unfortunately it is difficult to apply to the fly itself; the only character that can be used in {455}connection with the imago is the existence of a suture over the insertion of the antennae in a portion, but not all, of the Cyclorrhapha.[363] The next set of divisions used by Brauer divides the Order into four sections, viz. 1. Orthorrhapha Nematocera, 2. O. Brachycera, 3. Cyclorrhapha Aschiza, 4. C. Schizophora. As these four groups are recognised more readily than the two major groups the student will do well at first to disregard the primary division and consider the Diptera as divisible into four great groups. To these four divisions we, however, add temporarily a fifth, viz. Pupipara. This is included by Brauer in Schizophora, but it appears to be really an unnatural complex, and had better be kept separate till it has been entirely reconsidered. These great sections may be thus summarised:—

Series 1. _Orthorrhapha Nemocera._—Antennae with more than 6 segments, not terminated by an arista; with the segments of the flagellum more or less similar to one another. Palpi slender and flexible, four- or five-jointed.[364]

Series 2. _Orthorrhapha Brachycera._—Antennae variable, but never truly Nemocerous nor like those of Cyclorrhapha; when an arista is present it is usually placed terminally, not superiorly; when an arista is not present the flagellum terminates as an appendage consisting of a variable number of indistinctly separated segments; thus the flagellum is not composed of similar joints; [rarely are the antennae as many as seven-jointed]. Palpi only one- or two-jointed.[365] Around the insertion of the antennae there is no definite arched suture enclosing a small depressed space. The nervuration of the wings is usually more complex than in any of the other divisions.

Series 3. _Cyclorrhapha Aschiza._—Antennae composed of not more than three joints and an arista; the latter is not terminal. Front of head without definite arched suture over the antennae, but frequently with a minute area of different colour or texture there. This group consists of the great family Syrphidae, and of four small families, viz. Conopidae, Pipunculidae, Phoridae, and Platypezidae. The section is supposed to be justified by its being Cyclorrhaphous in pupation, and by the members not possessing a ptilinum (or having no trace of one when quite mature). The Syrphidae are doubtless {456}a natural group, but the association with them of the other families mentioned is a mere temporary device. The greatest difficulty is experienced in deciding on a position for Phoridae, as to which scarcely any two authorities are agreed.

Series 4. _Cyclorrhapha Schizophora_, or Eumyiid flies. The antennae consist of three joints and an arista. In the Calyptratae the frontal suture, or fold over the antennae, is well marked and extends downwards along each side of the face, leaving a distinct lunule over the antennae. In the Acalyptrate Muscids the form of the head and of the antennae vary much and are less characteristic, but the wings differ from those of Brachycera by their much less complex nervuration.

Series 5. _Pupipara._ These are flies of abnormal habits, and only found in connection with living Vertebrates, of which they suck the blood (one species, _Braula caeca_, lives on bees). Many are wingless, or have wings reduced in size. The young are produced alive, full grown, but having still to undergo a metamorphosis. This group consists of a small number of flies of which some are amongst the most aberrant known. This is specially the case with the Nycteribiidae. This Section will probably be greatly modified, as it is far from being a natural assemblage.[366]

The Sub-Order _Aphaniptera_, or Fleas, considered a distinct Order by many entomologists, may for the present be placed as a part of Diptera.

It must be admitted that these sections are far from satisfactory. Brauer divides them into Tribes, based on the nature of the larvae, but these tribes are even more unsatisfactory than the sections, hosts of species being entirely unknown in the larval state, and many of those that are known having been very inadequately studied. We must admit that the classification of Diptera has at present advanced but little beyond the stage of arranging them in natural families capable of exact definition. We may, however, draw attention to the attempt that is being made by Osten Sacken to remodel the classification of the Nemocera and Brachycera by the combination of families into super-families.[367] He proposes to divide the Nemocera into two super-families: 1. Nemocera Vera, including all the families from Cecidomyiidae to Tipulidae; 2. Nemocera Anomala, consisting of the small families Bibionidae, Simuliidae, Blepharoceridae, Rhyphidae and Orphnephilidae.

For Orthorrhapha Brachycera he adopts the following {457}arrangement: 1. Super-family Eremochaeta, for Stratiomyidae, Tabanidae, Acanthomeridae and Leptidae; 2. Tromoptera, for Nemestrinidae, Acroceridae, Bombyliidae, Therevidae, and Scenopinidae; 3. Energopoda, for Asilidae, Dolichopidae, Empidae and Lonchopteridae, Phoridae being included with doubt; 4. Mydaidae remains isolated.

This classification is based on the relations of the eyes and bristles of the upper surface, and on the powers of locomotion, aërial or terrestrial. At present it is not sufficiently precise to be of use to any but the very advanced student.

BLOOD-SUCKING DIPTERA.—The habit of blood-sucking from Vertebrates is, among Insects, of course confined to those with suctorial mouth, and is exhibited by various Diptera. It is, however, indulged in by but a small number of species, and these do not belong to any special division of the Order. It is remarkable that as a rule the habit is confined to the female sex, and that a large proportion of the species have aquatic larvae. This subject has many points of interest, but does not appear to have yet received the attention it merits. We give below a brief summary of the facts as to blood-sucking Diptera.

Series I. Nemocera.—In this section the habit occurs in no less than five families, viz.:

Blepharoceridae. _Curupira_; in the female only; larva aquatic.

Culicidae. _Culex_, Mosquitoes; in the female only; other genera, with one or two exceptions, do not suck blood; larvae aquatic.

Chironomidae. _Ceratopogon_, Midge; in the female only; exceptional even in the genus, though the habit is said to exist in one or two less known, allied genera; larval habits not certain; often aquatic; in _C. bipunctatus_ the larva lives under moist bark.

Psychodidae. _Phlebotomus_: in the female only (?); quite exceptional in the family; larva aquatic or in liquid filth.

Simuliidae. _Simulium_, sand-flies; general in the family (?), which, however, is a very small one; larva aquatic, food probably mixed vegetable and animal microscopic organisms.

Series II. Brachycera. Tabanidae. Gad-flies: apparently general in the females of this family; the habits of the exotic forms but little known; in the larval state, scarcely at all known; some are aquatic.

{458}Series IV. Cyclorrhapha Schizophora: _Stomoxys_, _Haematobia_; both sexes (?); larvae in dung. [The Tse-tse flies, _Glossina_, are placed in this family, though their mode of parturition is that of the next section].

Series V. Pupipara. The habit of blood-sucking is probably common to all the group and to both sexes. The flies, with one exception, frequent Vertebrates; in many cases living entirely on their bodies, and apparently imbibing much blood; the larvae are nourished inside the flies, not on the imbibed blood, but on a milky secretion from the mother.

Sub-Order Aphaniptera. Fleas. The habit of blood-sucking is common to all the members and to both sexes. The larvae live on dried animal matter.

FOSSIL DIPTERA.—A considerable variety of forms have been found in amber, and many in the tertiary beds; very few members of the Cyclorrhaphous Sections are, however, among them; the Tipulidae, on the other hand, are richly represented. In the Mesozoic epoch the Order is found as early as the Lias, the forms being exclusively Orthorrhaphous, both Nemocera and Brachycera being represented. All are referred to existing families. Nothing has been found tending to connect the Diptera with other Orders. No Palaeozoic Diptera are known.

SERIES 1. ORTHORRHAPHA NEMOCERA

FAM. 1. CECIDOMYIIDAE.—_An extensive family of very minute and fragile flies, the wings of which have very few nervures; the antennae are rather long, and are furnished with whorls of hair._ In the case of some species the antennae are beautiful objects; in _Xylodiplosis_ some of the hairs have no free extremities, but form loops (Fig. 220). In the males of certain species the joints appear to be double, each one consisting of a neck and a body. Although comparatively little is known as to the flies themselves, yet these Insects are of importance on account of their preparatory stages. The larvae have very diverse habits; the majority live in plants and form galls, or produce deformations of the leaves, flowers, stems, buds, or roots in a great variety of ways; others live under bark or in animal matter; some are predaceous, killing Aphidae or Acari, and even other Cecidomyiids.

{459}[Illustration: Fig. 219—_Cecidomyia_ (_Diplosis_) _buxi_. Britain. A, Larva, magnified; B, pupa; C, imago; D, portion of antenna. (After Laboulbène.)]

[Illustration: Fig. 220—One segment of antenna of _Xylodiplosis_ sp.; _a_, Tip of one segment; _b_, base of another. (After Janet.)]

The North American _Diplosis resinicola_ lives in the resin exuded as the results of the attacks of a caterpillar. The larva burrows in the semi-liquid resin, and, according to Osten Sacken,[368] is probably amphipneustic. Cecidomyiid larvae are short maggots, narrowed at the two ends, with a very small head, and between this and the first thoracic segment (this bears a stigma), a small supplementary segment; the total number of segments is thirteen, besides the head; there are eight pairs of stigmata on the posterior part of the body. Brauer defines the Cecidomyiid larva thus, "peripneustic, with nine pairs of stigmata, the first on the second segment behind the head; two to nine on fifth to twelfth segments; body as a whole fourteen-segmented without a fully-formed head." The most remarkable peculiarity of Cecidomyiid larvae is that those of many species possess a peculiar organ—called breast-bone, sternal spatula, or anchor-process—projecting from the back of the lower face of the prothoracic segment. The use of so peculiar a structure has been much discussed. According to Giard,[369] in addition to the part {460}that protrudes externally, as shown in Fig. 219, A, there is a longer portion concealed, forming a sort of handle, having muscles attached to it. Some of these larvae have the power of executing leaps, and he states that such larvae are provided on the terminal segment with a pair of corneous papillae; bending itself almost into a circle, the larva hooks together the breast-bone and the papillae, and when this connection is broken the spring occurs. This faculty is only possessed by a few species, and it is probable that in other cases the spatula is used as a means for changing the position or as a perforator. Some of the larvae possess false feet on certain of the segments. Williston says they probably do not moult. In the pupal instar (Fig. 219, B), the Cecidomyiid greatly resembles a minute Lepidopterous pupa. The Hessian fly, _Cecidomyia destructor_, is frequently extremely injurious to crops of cereals, and in some parts of the world commits serious depredation. The larva is lodged at the point where a leaf enwraps the stem; it produces a weakness of the stem, which consequently bends. This Insect and _C. tritici_ (the larva of which attacks the flowers of wheat) pupate in a very curious manner: they form little compact cases like flax-seeds; these have been supposed to be a form of pupa similar to what occurs in the Blow-fly; but there are important distinctions. The larva, when about to undergo its change, exudes a substance from its skin, and this makes the flax-seed; the larval skin itself does not form part of this curious kind of cocoon, for it may be found, as well as the pupa, in the interior of the "flax-seed." Other Cecidomyiids form cocoons of a more ordinary kind; one species, described by Perris as living on _Pinus maritima_, has the very remarkable faculty of surrounding itself, by some means, with a cocoon of resin. Walsh describes the cocoon-forming process of certain Cecidomyiids as one of exudation and inflation; Williston as somewhat of the nature of crystallisation. Some Cecidomyiids are said to possess, in common with certain other Diptera, the unusual number of five Malpighian tubes; and Giard says that in the larva there is only a pair of these tubes, and that their extremities are united so as to form a single tube, which is twisted into an elegant double loop.

Thirty years or more ago the Russian naturalist, Wagner, made the very remarkable discovery that the larva of a Cecidomyiid produces young; and it has since been found by Meinert and {461}others that this kind of paedogenesis occurs in several species of the genera _Miastor_ and _Oligarces_. The details are briefly as follows:—A female fly lays a few, very large, eggs, out of each of which comes a larva, that does not go on to the perfect state, but produces in its interior young larvae that, after consuming the interior of the body of the parent larva, escape by making a hole in the skin, and thereafter subsist externally in a natural manner. This larval reproduction may be continued for several generations, through autumn, winter, and spring till the following summer, when a generation of the larvae goes on to pupation and the mature, sexually perfect fly appears. Much discussion has taken place as to the mode of origination of the larvae; Carus and others thought they were produced from the rudimental, or immature ovaries of the parent larva. Meinert, who has made a special study of the subject,[370] finds, however, that this is not the case; in the reproducing larva of the autumn there is no ovary at all; in the reproducing larvae of the spring-time rudimentary ovaries or testes, as the case may be, exist; the young are not, however, produced from these, but from germs in close connection with the fat-body. In the larvae that go on to metamorphosis the ovaries continue their natural development. It would thus appear that the fat-body has, like the leaf of a _Begonia_, under certain circumstances, the power, usually limited to the ovaries, of producing complete and perfect individuals.

Owing to the minute size and excessive fragility of the Gall-midge flies it is extremely difficult to form a collection of them; and as the larvae are also very difficult of preservation, nearly every species must have its life-history worked out as a special study before the name of the species can be ascertained. Notwithstanding the arduous nature of the subject it is, however, a favourite one with entomologists. The number of described and named forms cannot be very far short of 1000, and each year sees some 20 or 30 species added to the list. The number of undescribed forms is doubtless very large. The literature of the subject is extensive and of the most scattered and fragmentary character.

The Cecidomyiidae have but little relation to other Nemocera, and are sometimes called Oligoneura, on account of the reduced number of wing-nervures. Their larvae are of a peculiar type {462}that does not agree with the larvae of the allied families having well-marked heads (and therefore called Eucephala), nor with the acephalous maggots of Eumyiidae.

FAM. 2. MYCETOPHILIDAE.—_These small flies are much less delicate creatures than the Cecidomyiidae, and have more nervures in the wings; they possess ocelli, and frequently have the coxae elongated, and in some cases the legs adorned with complex arrangements of spines: their antennae have not whorls of hair._ Although very much neglected there are probably between 700 and 1000 species known; owing to many of their larvae living in fungoid matter the flies are called Fungus-gnats. We have more than 100 species in Britain. _Epidapus_ is remarkable, inasmuch as the female is entirely destitute of wings and halteres, while the male has the halteres developed but the wings of very reduced size. _E. scabiei_ is an excessively minute fly, smaller than a common flea, and its larva is said to be very injurious to stored potatoes. The larvae of Mycetophilidae are usually very elongate, worm-like maggots, but have a distinct, small head; they are peripneustic, having, according to Osten Sacken, nine pairs of spiracles, one pair prothoracic, the others on the first eight abdominal segments. They are usually worm-like, and sometimes seem to consist of twenty segments. Some of them have the faculty of constructing a true cocoon by some sort of spinning process, and a few make earthen cases for the purpose of pupation. The pupae themselves are free, the larval skin having been shed. The Mycetophilidae are by no means completely fungivorous, for many live in decaying vegetable, some even in animal, matter.

[Illustration: Fig. 221—_Mycetobia pallipes._ Britain. A, Larva; B, pupa; C, imago. (After Dufour.)]

The habits of many of the larvae are very peculiar, owing to their spinning or exuding a mucus, that reminds one of snail-slime; they are frequently gregarious, and some of them have likewise, as we shall subsequently mention, migratory habits. Perris has described the very curious manner in which _Sciophila {463}unimaculata_ forms its slimy tracks;[371] it stretches its head to one side, fixes the tip of a drop of the viscous matter from its mouth to the surface of the substance over which it is to progress, bends its head under itself so as to affix the matter to the lower face of its own body; then stretches its head to the other side and repeats the operation, thus forming a track on which it glides, or perhaps, as the mucus completely envelops its body, we should rather call it a tunnel through which the maggot slips along. According to the description of Hudson[372] the so-called New Zealand Glow-worm is the larva of _Boletophila luminosa_; it forms webs in dark ravines, along which it glides, giving a considerable amount of light from the peculiarly formed terminal segment of the body. This larva is figured as consisting of about twenty segments. The pupa is provided with a very long, curiously-branched dorsal structure: the fly issuing from the pupa is strongly luminous, though no use can be discovered for the property either in it or in the larva. The larva of the Australian _Ceroplatus mastersi_ is also luminous. Another very exceptional larva is that of _Epicypta scatophora_; it is of short, thick form, like Cecidomyiid larvae, and has a very remarkable structure of the dorsal parts of the body; by means of this its excrement, which is of a peculiar nature, is spread out and forms a case for enveloping and sheltering the larva. Ultimately the larval case is converted into a cocoon for pupation. This larva is so different from that of other Mycetophilidae, that Perris was at first unable to believe that the fly he reared really came from this unusually formed larva. The larva of _Mycetobia pallipes_ (Fig. 221) offers a still more remarkable phenomenon, inasmuch as it is amphipneustic instead of peripneustic (that is to say, it has a pair of stigmata at the termination of the body and a pair on the first thoracic segment instead of the lateral series of pairs we have described as normal in Mycetophilidae). This larva lives in company with the amphipneustic larva of _Rhyphus_, a fly of quite another family, and the _Mycetobia_ larva so closely resembles that of the _Rhyphus_, that it is difficult to distinguish the two. This anomalous larva gives rise, like the exceptional larva of _Epicypta_, to an ordinary Mycetophilid fly.[373]

{464}But the most remarkable of all the Mycetophilid larvae are those of certain species of _Sciara_, that migrate in columns, called by the Germans, Heerwurm. The larva of _Sciara militaris_ lives under layers of decomposing leaves in forests, and under certain circumstances, migrates, sometimes perhaps in search of a fresh supply of food, though in some cases it is said this cannot be the reason. Millions of the larvae accumulate and form themselves by the aid of their viscous mucus into great strings or ribbons, and then glide along like serpents: these aggregates are said to be sometimes forty to a hundred feet long, five or six inches wide, and an inch in depth. It is said that if the two ends of one of these processions be brought into contact, they become joined, and the monstrous ring may writhe for many hours before it can again disengage itself and assume a columnar form. These processional maggots are met with in Northern Europe and the United States, and there is now an extensive literature about them.[374] Though they sometimes consist of almost incredible numbers of individuals, yet it appears that in the Carpathian mountains the assemblages are usually much smaller, being from four to twenty inches long. A species of _Sciara_ is the "Yellow-fever fly" of the Southern United States. It appears that it has several times appeared in unusual numbers and in unwonted localities at the same time as the dreaded disease, with which it is popularly supposed to have some connection.

FAM. 3. BLEPHAROCERIDAE.[375]—_Wings with no discal cell, but with a secondary set of crease-like lines._ The flies composing this small family are very little known, and appear to be obscure Insects with somewhat the appearance of Empidae, though with strongly iridescent wings; they execute aerial dances, after the manner of midges, and are found in Europe (the Pyrenees, Alps and Harz mountains) as well as in North and South America. Their larvae are amongst the most remarkable of Insect forms; indeed, no entomologist recognises them as belonging to a Hexapod Insect when he makes a first {465}acquaintance with them. The larva of _Curupira_ (Fig. 222) lives in rapid streams in Brazil, fixed by its suckers to stones or rocks. It consists only of six or seven divisions, with projecting side-lobes; the usual segmentation not being visible. There are small tracheal gills near the suckers, and peculiar scale-like organs are placed about the edges of the lobes. Müller considers that the first lobe is "cephalothorax," corresponding to head, thorax and first abdominal segment of other larvae, the next four lobes he considers to correspond each to an abdominal segment, and the terminal mass to four segments. He also says that certain minute points existing on the surface, connected with the tracheal system by minute strings, represent nine pairs of spiracles. These larvae and their pupae can apparently live only a short time after being taken out of the highly aërated water in which they exist, but Müller succeeded in rearing several flies from a number of larvae and pupae that he collected, and, believing them to be all one species, he announced that the females exhibited a highly developed dimorphism, some of them being blood-suckers, others honey-suckers. It is however, more probable that these specimens belonged to two or three distinct species or even genera. This point remains to be cleared up. The larva we have figured is called by Müller _Paltostoma torrentium_. It is certain, however, that the Brazilian Insect does not belong to the genus _Paltostoma_, and it will no doubt bear the name used by Osten Sacken, viz. _Curupira_.

[Illustration: Fig. 222—Under surface of the larva of _Curupira_ (_Paltostoma_) _torrentium_, showing the suckers along the middle of the body, much magnified. Brazil. (After Fritz Müller.)]

The metamorphoses of the European _Liponeura brevirostris_ have been

## partially examined by Dewitz, who found the Insects in the valley of the

Ocker in September.[376] He does not consider the "cephalothorax" to include an abdominal segment; and he found that two little, horn-like projections from the thorax of the {466}pupa are really each four-leaved. The pupa is formed within the larval skin, but the latter is subsequently cast so that the pupa is exposed; its dorsal region is horny, but the under surface, by which it clings firmly to the stones of the rapid brook, is white and scarcely chitinised, and Dewitz considers that the chitinous exudation from this part is used as a means of fastening the pupa to the stones. Blepharoceridae possess, in common with _Culex_, _Psychoda_ and _Ptychoptera_, the peculiar number of five Malpighian tubes, and it has been proposed by Müller to form these Insects into a group called Pentanephria.

FAM. 4. CULICIDAE (_Mosquitoes_, _Gnats_).—_Antennae with whorls of hair or plumes, which may be very dense and long in the male, though scanty in the female; head with a long, projecting proboscis._ Although there are few Insects more often referred to in general literature than Mosquitoes, yet the ideas in vogue about them are of the vaguest character. The following are the chief points to be borne in mind as to the prevalence of Mosquitoes:—The gently humming Gnat that settles on us in our apartments, and then bites us, is a Mosquito; there are a large number of species of Mosquitoes; in some countries many in one locality; in Britain we have ten or a dozen; notwithstanding the multiplicity of species, certain Mosquitoes are very widely diffused; the larvae are all aquatic, and specially frequent stagnant or quiet pools; they are probably diffused by means of the water in ships, it being known that Mosquitoes were introduced for the first time to the Hawaiian Islands by a sailing vessel about the year 1828. Hence it is impossible to say what species the Mosquitoes of a given locality may be without a critical examination. No satisfactory work on the Mosquitoes of the world exists. Urich states that he is acquainted with at least ten species in Trinidad. The species common in our apartments in Central and Southern England is _Culex pipiens_, Linn., and this species is very widely distributed, being indeed one of the troublesome Mosquitoes of East India. The term Mosquito is a Spanish or Portuguese diminutive of Mosca. It is applied to a variety of small flies of other families than Culicidae, but should be restricted to these latter. The irritation occasioned by the bites of Mosquitoes varies according to several circumstances, viz. the condition of the biter, the condition or constitution of the bitten, and also the species of Mosquito. Réaumur and {467}others believed that some irritating fluid is injected by the Mosquito when it bites. But why should it want to irritate as well as to bite? Macloskie, considering that the Mosquito is really a feeder on plant-substances, suggests that the fluid injected may be for the purpose of preventing coagulation of the plant-juices during the process of suction. It is a rule that only the female Mosquito bites, the male being an inoffensive creature, and provided with less effectual mouth-organs; it has, however, been stated by various authors that male Mosquitoes do occasionally bite. It is difficult to understand the blood-sucking propensities of these Insects; we have already stated that it is only the females that suck blood. There is reason to suppose that it is an acquired habit; and it would appear that the food so obtained is not essential to their existence. It has indeed been asserted that the act is frequently attended with fatal consequences to the individual that does it. The proper method of mitigating their nuisance is to examine the stagnant waters in localities where they occur, and deal with them so as to destroy the larvae. These little creatures are remarkable from the heads and thorax being larger and more distinct than in other Dipterous larvae. Their metamorphoses have been frequently described, and recently the numerous interesting points connected with their life-histories have been admirably portrayed by Professor Miall,[377] in an accessible form, so that it is unnecessary for us to deal with them. _Corethra_ is placed in Culicidae, but the larva differs totally from that of _Culex_; it is predaceous in habits, is very transparent, has only an imperfect tracheal system, without spiracles, and has two pairs of air-sacs (perhaps we should rather say pigmented structures possibly for aerostatic purposes, but not suppliers of oxygen). The kungu cake mentioned by Livingstone as used on Lake Nyassa is made from an Insect which occurs in profusion there, and is compressed into biscuit form. It is believed to be a _Corethra_. One of the peculiarities of this family is the prevalence of scales on various parts of the body, and even on the wings: the scales are essentially similar to those of Lepidoptera. Though Mosquitoes are generally obscure plain Insects, there are some—in the South American genus _Megarrhina_—that are elegant, beautifully adorned creatures. Swarms of various species of Culicidae, consisting sometimes of almost incalculable numbers {468}of individuals, occur in various parts of the world; one in New Zealand is recorded as having been three-quarters of a mile long, twenty feet high, and eighteen inches thick. There is good reason for supposing that Mosquitoes may act as disseminators of disease, but there is no certain evidence on the subject. The minute _Filaria_ that occurs in great numbers in some patients, is found in the human body only in the embryonic and adult conditions. Manson considers that the intermediate stages are passed in the bodies of certain Mosquitoes.[378]

FAM. 5. CHIRONOMIDAE (_Gnats_, _Midges_).—_Small or minute flies of slender form, with narrow wings, without projecting rostrum, usually with densely feathered antennae in the male, and long slender legs._ The flies of this family bear a great general resemblance to the Culicidae. They are much more numerous in species, and it is not improbable that we have in this country 200 species of the genus _Chironomus_ alone. They occur in enormous numbers, and frequently form dancing swarms in the neighbourhood of the waters they live in. The species are frequently extremely similar to one another, though distinguished by good characters; they are numerous about Cambridge. Many of them have the habit of using the front legs as feelers rather than as means of support or locomotion. This is the opposite of what occurs in Culicidae, where many of the species have a habit of holding up the hind legs as if they were feelers. The eggs of _Chironomus_ are deposited as strings surrounded by mucus, and are many of them so transparent that the development of the embryo can be directly observed with the aid of the microscope. They are said to possess a pair of air-sacs. The larvae, when born, are aquatic in habits, and are destitute of tracheal system. They subsequently differ greatly from the larvae of _Culex_, inasmuch as the tracheal system that develops is quite closed, and in some cases remains rudimentary. There is, however, much diversity in the larvae and also in the pupae. The little Blood-worms, very common in many stagnant and dirty waters, and used by anglers as bait, are larvae of _Chironomus_. They are said to be αἱ Ἐμπίδες of Aristotle. The red colour of these larvae is due to haemoglobin, a substance which has the power of attracting and storing oxygen, and giving it off to the tissues as they require it. Such larvae are able to live in burrows they construct {469}amongst the mud. Some of them, provided plentifully with haemoglobin, are in consequence able to live at great depths, it is said even at 1000 feet in Lake Superior, and come to the surface only occasionally. A few are able even to tolerate salt water, and have been fished up from considerable depths in the sea. It is a remarkable fact that these physiological capacities differ greatly within the limits of the one genus, _Chironomus_, for some of these species are destitute of haemoglobin, and have to live near the surface of the water; these have a superior development of the tracheal system. The pupae of _Chironomus_ have the legs coiled, and the thorax, instead of being provided with the pair of tubes or trumpets for breathing that is so common in this division of Diptera, have a pair of large tufts of hair-like filaments.[379] A very curious form of parthenogenesis has been described by Grimm[380] as existing in an undetermined species of _Chironomus_, inasmuch as the pupa deposits eggs. Although this form of parthenogenesis is of much interest, it is not in any way to be compared with the case, already referred to, of _Miastor_ (p. 461). The "pupa" is at the time of oviposition practically the imago still covered by the pupal integument; indeed Grimm informs us that in some cases, after depositing a small number of ova, the pupa became an imago. This parthenogenesis only occurs in the spring-generation; in the autumn the development goes on in the natural manner. The case is scarcely entitled to be considered as one of paedogenesis.

Gnats of this family, and believed to be a variety of _Chironomus plumosus_, are subject to a curious condition, inasmuch as individuals sometimes become luminous or "phosphorescent"; this has been noticed more specially in Eastern Europe and Western Asia. The whole of the body and legs may exhibit the luminous condition, but not the wings. It has been suggested by Schmidt that this condition is a disease due to bacteria in the body of the gnat.[381]

_Ceratopogon_ is a very extensive genus, and is to some extent anomalous as a member of Chironomidae. The larvae exhibit considerable variety of form. Some of them are aquatic {470}in habits, but the great majority are terrestrial, frequenting trees, etc. The former larvae are very slender, and move after the manner of leeches; they give rise to imagos with naked wings, while the terrestrial larvae produce flies with hairy wings. There are also important distinctions in the pupae of the two kinds; the correlation between the habits, and the distinctions above referred to, is, however, far from being absolutely constant.[382] Certain species of midges are in this country amongst the most annoying of Insects; being of very minute size, scarcely visible, they settle on the exposed parts of the body in great numbers, and by sucking blood create an intolerable irritation. _Ceratopogon varius_ is one of the most persistent of these annoyers in Scotland, where this form of pest is much worse than it is in England; in Cambridgeshire, according to Mr. G. H. Verrall, the two troublesome midges are the females of _C. pulicaris_ and _C. bipunctatus_.

FAM. 6. ORPHNEPHILIDAE.—_Small, brown or yellowish flies, bare of pubescence, with very large eyes contiguous in both sexes, and with antennae composed of two joints and a terminal bristle; both the second joint and the bristle are, however, really complex._ One of the smallest and least known of the families of Diptera, and said to be one of the most difficult to classify. The nervures of the wings are very distinct. Nothing is known of the habits and metamorphoses; there is only one genus—_Orphnephila_; it is widely distributed; we have one species in Britain.

FAM. 7. PSYCHODIDAE (_Moth-flies_).—_Extremely small, helpless flies, usually with thickish antennae, bearing much hair, with wings broader than is usual in small flies, and also densely clothed with hair, giving rise to a pattern more or less vague._ These flies are very fragile creatures, and are probably numerous in species. In Britain forty or fifty species have been recognised.[383] A South European form is a blood-sucker, and has received the appropriate name of _Phlebotomus_. The life-history of _Pericoma canescens_ has recently been studied by Professor Miall.[384] The larva is of aquatic habits, but is amphibious, being capable of existing in the air; it has a pair of anterior spiracles, by means {471}of which it breathes in the air, and a pair at the posterior extremity of the body, surrounded by four ciliated processes, with which it forms a sort of cup for holding air when it is in the water. The favourite position is amongst the filaments of green algae on which it feeds. A much more extraordinary form of larva from South America, doubtless belonging to this family, has recently been portrayed by Fritz Müller, under the name of _Maruina_.[385] These larvae live in rapid waters in company with those of the genus _Curupira_, and like the latter are provided with a series of suctorial ventral discs. Fritz Müller's larvae belong to several species, and probably to more than one genus, and the respiratory apparatus at the extremity of the body exhibits considerable diversity among them.

FAM. 8. DIXIDAE.—The genus _Dixa_ must, it appears, form a distinct family allying the Culicid series of families to the Tipulidae. The species are small, gnat-like Insects, fond of damp places in forests. We have four British species (_D. maculata_, _D. nebulosa_, _D. aestivalis_, _D. aprilina_). The genus is very widely distributed, occurring even in Australia. The larvae are aquatic, and have been described by Réaumur, Miall, and Meinert. The pupa has the legs coiled as in the Culicidae.

FAM. 9. TIPULIDAE (_Daddy-long-legs, or Crane-flies_).—_Slender Insects with elongate legs, a system of wing-nervures, rather complex, especially at the tip; an angulate, or open V-shaped, suture on the dorsum of the thorax in front of the wings: the female with the body terminated by a pair of hard, pointed processes, concealing some other processes, and forming an ovipositor._ The curious, silly Insects called daddy-long-legs are known all over the world, the family being a very large one, and found everywhere, some of its members extending their range even to the most inclement climates. It includes a great variety of forms that would not be recognised by the uninitiated, but can be readily distinguished by the characters mentioned above. It is impossible to assign any reason of utility for the extreme elongation of the legs of these Insects; as everyone knows, they break off with great ease, and the Insect appears to get on perfectly well without them. It is frequently {472}the case that they are much longer in the males than in the females. Other parts of the body exhibit a peculiar elongation; in some forms of the male the front of the head may be prolonged into a rostrum. In a few species the head is separated by a great distance from the thorax, the gap being filled by elongate, hard, cervical sclerites; indeed it is in these Insects that the phenomenon, so rare in Insect-structure, of the elongation of these sclerites and their becoming a part of the actual external skeleton, reaches its maximum. In several species of _Eriocera_ the male has the antennae of extraordinary length, four or five times as long as the body, and, strange to say, this elongation is accompanied by a reduction in the number of the segments of which the organ is composed, the number being in the male about six, in the female ten, in place of the usual fourteen or sixteen. In _Toxorrhina_ and _Elephantomyia_ the proboscis is as long as the whole body. In other forms the wings become elongated to an unusual extent by means of a basal stalk. It is probable that the elongation of the rostrum may be useful to the Insects. Gosse,[386] indeed, describes _Limnobia intermedia_ as having a rostrum half as long as the body, and as hovering like a Syrphid, but this is a habit so foreign to Tipulidae, that we may be pardoned for suspecting a mistake. The larvae exhibit a great variety of form, some being terrestrial and others aquatic, but the terrestrial forms seem all to delight in damp situations, such as shaded turf or rotten tree-stems. They are either amphipneustic or metapneustic, that is, with a pair of spiracles placed at the posterior extremity of the body; the aquatic species frequently bear appendages or projections near these spiracles. The pupae in general structure are very like those of Lepidoptera, and have the legs extended straight along the body; they possess a pair of respiratory processes on the thorax in the form of horns or tubes.

There are more than 1000 species of these flies known, and many genera. They form three sub-families, which are by some considered distinct families, viz.: Ptychopterinae, Limnobiinae or Tipulidae Brevipalpi, Tipulinae or Tipulidae Longipalpi.

The Ptychopterinae are a small group in which the angulate {473}suture of the mesonotum is indistinct; the larvae are aquatic and have the head free, the terminal two segments of the body enormously prolonged (Fig. 223), forming a long tail bearing, in the North American _Bittacomorpha_, two respiratory filaments. Hart[387] describes this tail as possessing a stigmatal opening at the extremity; no doubt the structure is a compounded pair of spiracles. The pupa (Fig. 223, B) has quite lost the respiratory tube at the posterior extremity of the body, but has instead quite as long a one at the anterior extremity, due to one tube of the pair normal in Tipulidae being enormously developed, while its fellow remains small. This is a most curious departure from the bilateral symmetry that is so constantly exhibited in Insect-structure. Our British species of _Ptychoptera_ have the pupal respiratory tube as extraordinary as it is in _Bittacomorpha_, though the larval tail is less peculiar.[388] This group should perhaps be distinguished from the Tipulidae as a separate family, but taxonomists are not yet unanimous as to this. Brauer considers that the head of the larva, and the condition of five Malpighian tubules in the imago, require the association of Ptychopterinae with the preceding families (Chironomidae, etc.), rather than with the Tipulidae.

[Illustration: Fig. 223.—_Bittacomorpha clavipes._ North America x 2/1. (After Hart.) A, Larva; B, pupa: _l_, the left, _r_, the right respiratory tube.]

The great majority of the Tipulidae are comprised in the sub-family Limnobiinae—the Tipulidae Brevipalpi of Osten Sacken:[389] in them the last joint of the palpi is shorter or not much longer than the two preceding together. They exhibit great variety, and many of them are types of fragility. The common winter gnats of the genus _Trichocera_ are a fair sample of this sub-family. The species of this genus mostly inhabit high latitudes, and delight in {474}a low temperature; it has been said that they may be seen on the wing in the depth of winter when the temperature is below freezing, but it is pretty certain that the spots chosen by the Insects are above that temperature, and Eaton states that the usual temperature during their evolutions is about 40° or 45° Fahr. They often appear in the damp conditions of a thaw when much snow is on the ground. _T. simonyi_ was found at an elevation of 9000 feet in the Tyrol, crawling at a temperature below the freezing-point, when the ground was deeply covered with snow. _T. regelationis_ occurs commonly in mines even when they are 500 feet or more deep. The most extraordinary of the Limnobiinae is the genus _Chionea_, the species of which are totally destitute of wings and require a low temperature. _C. araneoides_ inhabits parts of northern Europe, but descends as far south as the mountains near Vienna; it is usually said to be only really active in the depth of winter and on the surface of the snow. More recently, however, a large number of specimens were found by Professor Thomas in the month of October in his garden in Thuringia; they were caught in little pit-falls constructed to entrap snails. The larva of this Insect is one of the interesting forms that display the transition from a condition with spiracles at the sides of the body to one where there is only a pair at the posterior extremity.

A very peculiar Fly, in which the wings are reduced to mere slips, _Halirytus amphibius_, was discovered by Eaton in Kerguelen Land, where it is habitually covered by the rising tide. Though placed in Tipulidae, it is probably a Chironomid.

The group Cylindrotomina is considered by Osten Sacken[390] to be to some extent a primitive one having relationship with the Tipulinae; it was, he says, represented by numerous species in North America during the Oligocene period. It is of great interest on account of the larvae, which are in several respects similar to caterpillars of Lepidoptera. The larva of _Cylindrotoma distincta_ lives upon the leaves of plants—_Anemone_, _Viola_, _Stellaria_—almost like a caterpillar; it is green with a crest along the back consisting of a row of fleshy processes. Though this fly is found in Britain the larva has apparently not been observed here. The life-history of _Phalacrocera replicata_ has been recently published by Miall and Shelford.[391] The larva eats {475}submerged mosses in the South of England, and bears long forked filaments, reminding one of those of caterpillars. This species has been simultaneously discussed by Bengtsson, who apparently regards these Tipulids with caterpillar-like larvae—he calls them Erucaeformia[392]—as the most primitive form of existing Diptera.

The Tipulinae—Tipulidae Longipalpi, Osten Sacken[393]—have the terminal joint of the palpi remarkably long, longer than the three preceding joints together. The group includes the largest forms, and the true daddy-long-legs, a Chinese species of which, _Tipula brobdignagia_, measures four inches across the expanded wings. The group contains some of the finest Diptera. Some of the exotic forms allied to _Ctenophora_ have the wings coloured in the same manner as they are in certain Hymenoptera, and bear a considerable resemblance to members of that Order.

[Illustration: Fig. 224—Head of _Bibio_. × 10. A, Of male, seen from the front; C, from the side; _a_, upper, _b_, lower eye; B, head of female.]

FAM. 10. BIBIONIDAE.—_Flies of moderate or small size, sometimes of different colours in the two sexes, with short, thick, straight, antennae; front tibiae usually with a long pointed process; coxae not elongate. Eyes of male large, united, or contiguous in front._ The flies of the genus _Bibio_ usually appear in England in the spring, and are frequently very abundant; they are of sluggish habits and poor performers on the wing.

{476}[Illustration: Fig. 225—Larva of _Bibio_ sp. Cambridge. × 5.]

The difference in colour of the sexes is very remarkable, red or yellow predominating in the female, intense black in the male; and it is a curious fact that the same sexual distinction of colour reappears in various parts of the world—England, America, India, and New Zealand; moreover, this occurs in genera that are by no means closely allied, although allied species frequently have concolorous sexes. The eyes of the males are well worth study, there being a very large upper portion, and, abruptly separated from this, a smaller, differently faceted lower portion, practically a separate eye; though so largely developed the upper eye is in some cases so hairy that it must greatly interfere with the formation of a continuous picture. Carrière considers that the small lower eye of the male corresponds to the whole eye of the female. The larvae of _Bibio_ (Fig. 225) are caterpillar-like in form, have a horny head, well developed, biting mouth-organs, and spine-like processes on the body-segments. They are certified by good authorities[394] to possess the extremely unusual number of ten pairs of spiracles; a larva found at Cambridge, which we refer to _Bibio_ (Fig. 225) has nine pairs of moderate spiracles, as well as a large terminal pair separated from the others by a segment without spiracles. The genus _Dilophus_ is closely allied to _Bibio_, the larvae of which (and those of Bibionidae in general) are believed to feed on vegetable substances; the parasitism of _Dilophus vulgaris_ on the larva of a moth, _Epinotia_ (_Chaetoptria_) _hypericana_, as recorded by Meade,[395] must therefore be an exceptional case. In the genus _Scatopse_ there is a very important point to be cleared up as to the larval respiratory system; it is said by Dufour and Perris[396] to be amphipneustic; there are, however, nine projections on each side of the body that were considered by Bouché, and probably with good reason, to {477}be spiracles. The food of _Scatopse_ in the larval state is principally vegetable. The larva of _Scatopse_ changes to a pupa inside the larval skin; the pupa is provided on the thorax with two branched respiratory processes that project outside the larval skin.[397] Lucas has given an interesting account of the occurrence of the larva of _Bibio marci_ in enormous numbers at Paris; they lived together in masses, there being apparently some sort of connection between the individuals.[398] In the following year the fly was almost equally abundant.

[Illustration: Fig. 226—Portion of integument of _Bibio_ sp. Cambridge. _p_, Intersegmental processes; _s_, spiracle.]

Owing to the great numbers in which the species of Bibionidae sometimes appear, these Insects have been supposed to be very injurious. Careful inquiry has, however, generally exculpated them as doers of any serious injury, though _Dilophus febrilis_—a so-called fever-fly—appears to be really injurious in this country when it multiplies excessively, by eating the roots of the hop-plant.

FAM. 11. SIMULIIDAE (_Sand-flies_, _Buffalo-gnats_).—_Small obese flies with humped back, rather short legs and broad wings, with short, straight antennae destitute of setae; proboscis not projecting._ There is only one genus, _Simulium_, of this family, but it is very widely spread, and will probably prove to be nearly cosmopolitan. Some of the species are notorious from their blood-sucking habits, and in certain seasons multiply to an enormous extent, alight in thousands on cattle, and induce a disease that produces death in a few hours; it is thought as the result of an instilled poison. _S. columbaczense_ has occasioned great losses amongst the herds near the Danube; in North America the Buffalo- and Turkey-gnats attack a variety of mammals and birds. In Britain {478}and other parts of the world they do not increase in numbers to an extent sufficient to render them seriously injurious: their bite is however very annoying and irritating to ourselves. In their early stages they are aquatic and require well aërated waters: the larvae hold themselves erect, fixed to a stone or some other object by the posterior extremity, and have on the head some beautiful fringes which are agitated in order to bring food within reach; the pupae are still more remarkable, each one being placed in a pouch or sort of watch-pocket, from which projects the upper part of the body provided with a pair of filamentous respiratory processes. For an account of the interesting circumstances connected with the metamorphoses of this species the reader should refer to Professor Miall's book; and for the life-history of the American Buffalo-gnat to Riley.[399]

FAM. 12. RHYPHIDAE.—This is another of the families that have only two or three genera, and yet are very widely distributed. These little flies are distinguished from other Nemocera Anomala (cf. p. 456) by _the presence of a discal cell; the empodia of the feet are developed as if they were pulvilli, while the true pulvilli remain rudimentary_. The larvae are like little worms, being long and cylindric; they are amphipneustic, and have been found in decaying wood, in cow-dung, in rotten fruits, and even in dirty water. The "petite tipule," the metamorphoses of which were described and figured by Réaumur, is believed to be the common _Rhyphus fenestralis_.[400] _R. fenestralis_ is often found on windows, as its name implies.

SERIES 2. ORTHORRHAPHA BRACHYCERA

FAM. 13. STRATIOMYIDAE.—_Antennae with three segments and a terminal complex of obscure joints, frequently bearing an arista: tibiae not spined; wings rather small, the anterior nervures usually much more strongly marked than those behind. The median cell small, placed near to the middle of the wing. Scutellum frequently spined; terminal appendages of the tarsi small, but pulvilli and a pulvilliform empodium are {479}present._ This is a large family, whose members are very diversified, consequently definition of the whole is difficult. The species of the typical sub-family Stratiomyinae generally have the margins of the body prettily marked with green or yellow, and the scutellum spined. In the remarkable American genus, _Hermetia_, the abdomen is much constricted at the base, and the scutellum is not spined; in the division Sarginae the body is frequently of brilliant metallic colours. The species all have an only imperfect proboscis, and are not blood-suckers. The larvae are also of diverse habits; many of those of the Stratiomyinae are aquatic, and are noted for their capacity of living in salt, alkaline, or even very hot water. Mr. J. C. Hamon found some of these larvae in a hot spring in Wyoming, where he could not keep his hand immersed, and he estimated the temperature at only 20° or 30° Fahr. below the boiling-point. The larva of _Stratiomys_ is of remarkably elongate, strap-like, form, much narrowed behind, with very small head; the terminal segment is very long and ends in a rosette of hairs which the creature allows to float at the surface. After the larval skin is shed the pupa, though free, is contained therein; the skin alters but little in form, and has no organic connection with the pupa, which merely uses the skin as a shield or float. These larvae have been very frequently described; they can live out of the water. Brauer describes the larvae of the family as "peripneustic, some perhaps amphipneustic." Miall says there are, in _Stratiomys_, nine pairs of spiracles on the sides of the body which are not open, though branches from the longitudinal air-tubes pass to them. There are probably upwards of 1000 species of Stratiomyidae known, and in Britain we have 40 or 50 kinds. The American genus _Chiromyza_, Wied., was formerly treated by Osten Sacken as a separate family, Chiromyzidae, but Williston places it in Stratiomyidae.

FAM. 14. LEPTIDAE, including XYLOPHAGIDAE and COENOMYIIDAE.—_The Leptidae proper are flies of feeble build; antennae with three joints and a terminal bristle; in the Xylophagidae the antennae are longer, and the third joint is complex. The wings have five posterior cells, the middle tibiae are spined. Pulvilli and a pulvilliform empodium present._ The three families are considered distinct by most authors, but there has always been much difficulty about the Xylophagidae and Coenomyiidae, we therefore treat them as sub-families.

{480}[Illustration: Fig. 227—_Atherix ibis._ A, The fly, nat. size; B, mass of dead flies overhanging water, much reduced.]

The Xylophaginae are a small group of slender Insects, perhaps most like the short-bodied kinds of Asilidae; the third joint of the antenna is vaguely segmented, and there is no terminal bristle. _Rhachicerus_ is a most anomalous little fly with rather long stiff antennae of an almost nemocerous character, the segments of which give off a short thick prolongation on each side, reminding one of a two-edged saw. The three or four British species of Xylophaginae are forest Insects, the larvae of which live under bark, and are provided with a spear-like head with which they pierce other Insects. The Coenomyiinae consist of the one genus _Coenomyia_, with two or three European and North American species. They are remarkably thick-bodied, heavy flies, reminding one somewhat of an imperfect Stratiomyid destitute of ornamentation. The metamorphosis of _C. ferruginea_ has been described by Beling.[401] The larva is not aquatic, but lives in burrows or excavations in the earth where there are, or have recently been, rotten logs; it is probably predaceous. It is cylindric, with an extremely small head and eleven other segments, the stigma on the first thoracic segment distinct; the terminal segment is rather broad, and the structures surrounding the stigma are complex. The pupa has stigmata on each of abdominal segments 2 to 8. Notwithstanding that the fly is so different to _Xylophagus_, the larvae indicate the two forms as perhaps really allied. One of the Leptinae, _Atherix ibis_, has a singular mode of oviposition (Fig. 227), the females of the species deposit their eggs in common, and, dying as they do so, add their bodies to the common mass, which becomes an agglomeration, it may be of thousands of individuals, and of considerable size. The mass is attached to a branch of a bush or to a plant overhanging water, into which it {481}ultimately falls. These curious accumulations are occasionally found in England as well as on the Continent, but no reason for so peculiar a habit is at present forthcoming. Still more remarkable are the habits of some European Leptids of the genera _Vermileo_ (_Psammorycter_ of some authors) and _Lampromyia_, slender rather small flies of Asilid-like appearance, the larvae of which form pit-falls after the manner of the Ant-lion. According to Beling[402] the larva of _Leptis_ is very active, and is distinguished by having the stigmatic orifice surrounded by four quite equal, quadrangularly placed prominences; and at the other extremity of the body a blackish, naked, triangular plate; on the under side of each of seven of the abdominal segments there is a band of spines. The larva of _Atherix_ has seven pairs of abdominal feet. Altogether there are some two or three hundred known species of Leptidae; our British species scarcely reach a score. They are destitute of biting-powers and are harmless timid creatures. _Leptis scolopacea_, the most conspicuous of our native species, a soft-bodied fly of rather large size, the wings much marked with dark colour, and the thick, pointed body yellowish, marked with a row of large black spots down the middle, is a common Insect in meadows.

[Illustration: Fig. 228.—Larva of _Vermileo degeeri_ (_Psammorycter vermileo_). A, lateral, B, dorsal view: _p_, an abdominal pseudopod; _st_, stigma. Europe. (After Réaumur and Brauer.)]

FAM. 15. TABANIDAE (_Breeze-flies_, _Cleggs_, or _Horse-flies_, also frequently called _Gad-flies_).—_Proboscis fleshy, distinct, enwrapping pointed horny processes, palpi distinct, terminal joint inflated, pendent in front of proboscis. Antennae projecting, four-jointed, second joint very short, third variable in form, fourth forming an indistinctly segmented continuation of the third, but not ending in a bristle. A perfect squama in front of the halter. Eyes large, very large in the males, but laterally extending, rather than globose._

{482}[Illustration: Fig. 229—_Pangonia longirostris._ x 1. Nepal. (After Hardwicke.)]

This large and important family of flies, of which Williston states that 1400 or 1500 species are named, is well known to travellers on account of the blood-sucking habits of its members; they have great powers of flight, and alight on man and animals, and draw blood by making an incision with the proboscis; only the females do this, the males wanting a pair of the lancets that enable the other sex to inflict their formidable wounds. They are comparatively large Insects, some of our English species of _Tabanus_ attaining an inch in length. The smaller, grey _Haematopota_, is known to every one who has walked in woods or meadows in the summer, as it alights quietly on the hands or neck and bites one without his having previously been made aware of its presence. The larger Tabani hum so much that one always knows when an individual is near. The species of _Chrysops_, in habits similar to _Haematopota_, are remarkable for their beautifully coloured golden-green eyes. In Brazil the Motuca fly, _Hadrus lepidotus_, Perty, makes so large and deep a cut that considerable bleeding may follow, and as it sometimes settles in numbers on the body, it is deservedly feared. The most remarkable forms of Tabanidae are the species of the widely distributed genus _Pangonia_ (Fig. 229). The proboscis in the females of some of the species is three or four times the length of the body, and as it is stiff and needle-like the creature can use it while hovering on the wing, and will pierce the human body even through clothing of considerable thickness. The males suck the juices of flowers. The Seroot fly, that renders some of the districts of Nubia uninhabitable for about three months of the year, appears, from the figure and description given by Sir Samuel Baker, to be a _Pangonia_. Tabanidae are a favourite food of the fossorial wasps of the family Bembecidae. These wasps are apparently aware of the blood-sucking habits of their favourites, and attend on travellers and pick up the flies as they are about to settle down to their phlebotomic operations. The larvae of the Tabanidae are some {483}of them aquatic, but others live in the earth or in decaying wood; they are of predaceous habits, attacking and sucking Insect-larvae, or worms. Their form is cylindric, attenuate at the two extremities; the slender small head is retractile, and armed with a pair of conspicuous, curved black hooks. The body is surrounded by several prominent rings. The breathing apparatus is apparently but little developed, and consists of a small tube at the extremity of the body, capable of being exserted or withdrawn; in this two closely approximated stigmata are placed. In a larva, probably of this family, found by the writer in the shingle of a shallow stream in the New Forest, the annuli are replaced by seven circles of prominent pseudopods, on the abdominal segments about eight in each circle, and each of these feet is surmounted by a crown of small hooks, so that there are fifty or sixty feet distributed equally over the middle part of the body without reference to upper or lower surface. The figures of the larva of _T. cordiger_, by Brauer, and of _Haematopota pluvialis_, by Perris, are something like this, but have no setae on the pseudopods. The metamorphoses of several Tabanidae are described and figured by Hart;[403] the pupa is remarkably like a Lepidopterous pupa. We have five genera and about a score of species of Tabanidae in Britain.

[Illustration: Fig. 230—Larva of a Tabanid. [? _Atylotus fulvus._] A, the larva, × 3; B, head; C, end of body; D, one of the pseudopods. New Forest.]

FAM. 16. ACANTHOMERIDAE.—A very small family of two genera (_Acanthomera_ and _Rhaphiorhynchus_) confined to America, and including the largest Diptera, some being two inches long. The antenna is terminated by a compound of seven segments and a style; the proboscis is short, and the squama rudimentary. The general form reminds one of Tabanidae or Oestridae. A dried larva exists in the Vienna collection; it is amphipneustic, and very remarkable on account of the great size of the anterior stigma.

{484}FAM. 17. THEREVIDAE.—_Moderate-sized flies, with somewhat the appearance of short Asilidae. They have, however, only a feeble fleshy proboscis, and minute claws, with pulvilli but no empodium; the antennae project, are short, three-jointed, pointed._—The flies of this family are believed to be predaceous like the Robber-flies, but they appear to be very feebly organised for such a life. We have about ten species in Britain, and there are only some 200 known from all the world. But little is known as to the metamorphoses. Meigen found larvae of _T. nobilitata_ in rotten stumps, but other larvae have been recorded as devouring dead pupae or larvae of Lepidoptera. The larvae are said to be elongate, very slender, worm-like, and to have nineteen body-segments, the posterior pair of spiracles being placed on what looks like the seventeenth segment, but is really the eighth of the abdomen. The pupa is not enclosed in the larval skin; that of _Psilocephala_ is armed with setae and spinous processes, and was found in rotten wood by Frauenfeld.

[Illustration: Fig. 231—_Thereva (Psilocephala) confinis._ A, Pupa; B, larva. Europe. (After Perris.)]

FAM. 18. SCENOPINIDAE.—_Rather small flies, without bristles. Antennae three-jointed, the third joint rather long, without appendage. Proboscis not projecting. Empodium absent._ These unattractive flies form one of the smallest families, and are chiefly found on windows. _S. fenestralis_ looks like a tiny Stratiomyid, with a peculiar, dull, metallic surface. The larva of this species has been recorded as feeding on a variety of strange substances, but Osten Sacken is of opinion[404] that it is really predaceous, and frequents these substances in order to find the larvae that are developing in them. If so, _Scenopinus_ is useful in a small way by destroying "moth," etc. The larva is a little slender, cylindrical, hard, pale worm of nineteen segments, with a small brown head placed like a hook at one extremity of the body and with two short, divergent processes at the other extremity, almost exactly like the larva of _Thereva_. Full references to the literature about this Insect are given by Osten Sacken.

FAM. 19. NEMESTRINIDAE.—These Insects appear to be allied to the Bombyliidae. _They are of medium size, often pilose, and {485}sometimes with excessively long proboscis; antennae short, with a simple third joint, and a jointed, slender, terminal appendage; the tibiae have no spurs, the empodium is pulvillus-like. The wing-nervuration is perhaps the most complex found in Diptera, there being numerous cells at the tip, almost after the fashion of Neuroptera._ With this family we commence the aerial forms composing the Tromoptera of Osten Sacken. Nemestrinidae is a small family of about 100 species, but widely distributed. _Megistorhynchus longirostris_ is about two-thirds of an inch long, but has a proboscis at least four times as long as itself. In South Africa it may be seen endeavouring to extract, with this proboscis, the honey from the flower of a _Gladiolus_ that has a perianth just as long as its own rostrum; as it attempts to do this when it is hovering on the wing, and as the proboscis is, unlike that of the Bombylii, fixed, the Insect can only succeed by controlling its movements with perfect accuracy; hence it has great difficulty in attaining its purpose, especially when there is much wind, when it frequently strikes the earth instead of the flower. M. Westermann thinks[405] the life of the Insect and the appearance and duration of the flower of the _Gladiolus_ are very closely connected. The life-history of _Hirmoneura obscura_ has recently been studied in Austria by Handlirsch and Brauer.[406] The larva is parasitic on the larva of a Lamellicorn beetle (_Rhizotrogus solstitialis_); it is metapneustic, and the head is highly modified for predaceous purposes. The young larva apparently differs to a considerable extent from the matured form. The most curious fact is that the parent fly does not oviposit near the Lamellicorn-larva, but places her eggs in the burrows of some wood-boring Insect in logs; the larvae when hatched come to the surface of the log, hold themselves up on their hinder extremity and are carried away by the wind; in what manner they come into contact with the Lamellicorn larva, which feeds in turf, is unknown. The pupa is remarkable on account of the prominent, almost stalked stigmata, and of two pointed divergent processes at the extremity of the body. This life-history is of much interest, as it foreshadows to some extent the complex parasitic life-histories of Bombyliidae. The Nemestrinidae are not represented in the British fauna.

{486}[Illustration: Fig. 232—_Argyromoeba trifasciata._ A, Young larva; B, adult larva; C, pupa. France. (After Fabre.) A, Greatly, B, C, slightly magnified.]

FAM. 20. BOMBYLIIDAE.—_Body frequently fringed with down, or covered in large part with hair. Legs slender, claws small, without distinct empodium, usually with only minute pulvilli. Proboscis very long or moderate, antennae three-jointed, terminal joint not distinctly divided, sometimes large, sometimes hair-like._ This is a very large family, including 1500 species, and is of great importance to both naturalist and economist. Two well-marked types, formerly treated as distinct families, are included in it—(1) the Bombyliides with very long exserted rostrum, and humped thorax; and (2) Anthracides, with a short beak, and of more slender and graceful form. None of these flies are bloodsuckers, they frequent flowers only, and use their long rostrums in a harmless manner. The members of both of these groups usually have the wings ornamented with a pattern, which in _Anthrax_ is frequently very remarkable; in both, the clothing of the body is frequently variegated. Their powers of flight are very great, and the hovering _Bombylius_ of early spring is endowed with an unsurpassed capacity for movement, remaining perfectly still on the wing, and darting off with lightning rapidity; _Anthrax_ is also most rapid on the wing. In Britain we have but few species of Bombyliidae, but in warm and dry climates they are very numerous. The life-history of these Insects was till recently unknown, but that of _Argyromoeba_ (_Anthrax_) _trifasciata_ has been described by the French naturalist, Fabre, who ascertained that the species is parasitic on the Mason-bee, _Chalicodoma muraria_, that forms nests of solid masonry. He endeavoured to discover the egg, but failed; the parent-fly oviposits, it appears, by merely dropping a minute egg while flying over the surface of the mass of masonry by which the grubs of the _Chalicodoma_ are protected. From this egg there is hatched a minute delicate vermiform larva (Fig. 232, A). In order to obtain its food, it is necessary for this feeble creature to penetrate the masonry; apparently a hopeless task, the animal {487}being scarcely a twentieth of an inch long and very slender; it is, however, provided with a deflexed horny head, armed in front with some stiff bristles, while on the under surface of the body there are four pairs of elongate setae serving as organs of locomotion; thus endowed, the frail creature hunts about the surface of the masonry, seeking to find an entrance; frequently it is a long time before it is successful; but though it has never taken any food it is possessed of great powers of endurance. Usually, after being disclosed from the egg, it remains about fifteen days without stirring; and even after it commences its attempts to enter the nest it is still capable of a long life without taking any food. Possibly its organisation will not permit it to feed (supposing any food were obtainable by it) without its growing somewhat thereafter, and if so, its chance of obtaining entrance through the masonry would be diminished. Abstention, it would appear, is the best policy, whether inevitable or not; so the starving little larva continues its endeavours to find a chink of entrance to the food contained in the interior of the masonry. It has plenty of time for this, because it is better for it not to get into the cell of the bee until the grub is quite full grown, and is about to assume the pupal form, when it is quite incapable of self-defence. Finally, after greater or less delay, the persevering little larva succeeds in finding some tiny gap in the masonry through which it can force itself. M. Fabre says that the root of a plant is not more persistent in descending into the soil that is to support it than is this little _Anthrax_ in insinuating itself through some crack that may admit it to its food. Having once effected an entrance the organisation that has enabled it to do so is useless; this primary form of the larva has, in fact, as its sole object to enable the creature to penetrate to its food. Having penetrated, it undergoes a complete change of form, and appears as a creature specially fitted for feeding on the quiescent larva of the bee without destroying it. To accomplish this requires an extreme delicacy of organisation and instinct; to bite the prey would be to kill it, and if this were done, the _Anthrax_ would, Fabre supposes, ensure its own death, for it cannot feed on the dead and putrefying grub; accordingly, the part of its body that does duty as a mouth is merely a delicate sucker which it applies to the skin of the _Chalicodoma_-grub; and thus without inflicting any perceptible wound it sucks day {488}after day, changing its position frequently, until it has completely emptied the pupa of its contents, nothing being left but the skin. Although this is accomplished without any wound being inflicted, so effectual is the process that all the _Chalicodoma_ is gradually absorbed. The time requisite for completely emptying the victim is from twelve to fifteen days; at the end of this time the _Anthrax_-larva is full grown, and the question arises, how is it to escape from the cell of solid masonry in which it is imprisoned? It entered this cell as a tiny, slender worm through a minute orifice or crack, but it has now much increased in size, and exit for a creature of its organisation is not possible. For some months it remains a quiescent larva in the cell of the _Chalicodoma_, but in the spring of the succeeding year it undergoes another metamorphosis, and appears as a pupa provided with a formidable apparatus for breaking down the masonry by which it is imprisoned. The head is large and covered in front with six hard spines, to be used in striking and piercing the masonry, while the other extremity of the body bears some curious horns, the middle segments being armed with rigid hairs directed backwards, and thus facilitating movement in a forward direction and preventing slipping backwards. The pupa is strongly curved, and fixes itself by the aid of the posterior spines; then, unbending itself, it strikes with the armour of the other extremity against the opposing wall, which is thus destroyed piecemeal until a gallery of exit is formed; when this is completed the pupa-skin bursts and the perfect fly emerges, leaving the pupa-case still fixed in the gallery. Thus this species appears in four consecutive forms—in addition to the egg—each of which is highly specialised for the purposes of existence in that stage.

The habits of our British _Bombylius major_ have been partially observed by Dr. Chapman,[407] and exhibit a close analogy with those of _Anthrax trifasciata_. The bee-larva that served as food was in this case _Andrena labialis_, and the egg was deposited by the fly, when hovering, by jerking it against the bank in which the nest of the bee was placed.

It has recently been discovered that the larvae of various species of Bombyliidae are of great service by devouring the eggs of locusts. Riley found that the egg-cases of _Caloptenus {489}spretus_ are emptied of their contents by the larvae of _Systoechus oreas_ and _Triodites mus_. A similar observation has been made in the Troad by Mr. Calvert, who found that the Bombyliid, _Callostoma fascipennis_, destroys large quantities of the eggs of _Caloptenus italicus_. Still more recently M. Künckel d'Herculais has discovered that the destructive locust _Stauronotus maroccanus_ is kept in check in Algeria in a similar manner, as many as 80 per cent of the eggs of the locust being thus destroyed in certain localities. He observes that the larva of the fly, after being full fed in the autumn, passes the winter in a state of lethargy—he calls it "hypnody"—in the egg-case of the locust, and he further informs us that in the case of _Anthrax fenestralis_, which devours the eggs of the large _Ocnerodes_, the lethargy may be prolonged for a period of three years. After the pupa is formed it works a way out of the case by means of its armature, and then again becomes for some days immobile before the perfect fly appears. Lepidopterous larvae are also attacked by Bombyliid flies. A species of _Systropus_ has been recorded as destroying the larva of _Limacodes_. Several of the Bombyliids of the genus just mentioned are remarkable for the great resemblance they display to various Hymenoptera, some of them being very slender flies, like the thin bodied fossorial Hymenoptera. The difference between the pupa and imago in this case is very remarkable (Fig. 233).

[Illustration: Fig. 233—_Systropus crudelis._ South Africa. A, Pupa; B, imago, appendages of the left side removed. (After Westwood.)]

FAM. 21. ACROCERIDAE or CYRTIDAE.—_Flies of the average size, of peculiar form, the small head consisting almost entirely of the eyes, and bent down under the humped thorax: wings small, halteres entirely concealed by the very large horizontal squamae; antennae very diverse._ The peculiar shape of these flies is an exaggeration of that we have already noticed in _Bombylius_. The mouth in Acroceridae is very variable; there may be a very long, slender proboscis (_Acrocera_), or the mouth-parts may be so atrophied that it is doubtful whether even an orifice exists (_Ogcodes_). There are but {490}few species known, and all of them are rare;[408] in Britain we have but two (_Ogcodes gibbosus_, _Acrocera globulus_). The genus _Pterodontia_, found in North America and Australia, an inflated bladder-like form with a minute head, is amongst the most extraordinary of all the forms of Diptera. The habits are very peculiar, the larvae, so far as known, all living as parasites within the bodies of spiders or in their egg-bags. It appears, however, that the flies do not oviposit in appropriate places, but place their eggs on stems of plants, and the young larvae have to find their way to the spiders. Brauer has described the larva of the European _Astomella lindeni_,[409] which lives in the body of a spider, _Cteniza ariana_; it is amphipneustic and maggot-like, the head being extremely small. The larva leaves the body of the spider for pupation; the pupa is much arched, and the head is destitute of the peculiar armature of the Bombyliidae, but has a serrate ridge on the thorax. Emerton found the larvae of an _Acrocera_ in the webs of a common North American spider, _Amaurobius sylvestris_, they having eaten, it was supposed, the makers of the cobwebs.

[Illustration: Fig. 234—_Megalybus gracilis._ × 4. (Acroceridae.) Chili. (After Westwood.)]

FAM. 22. LONCHOPTERIDAE.—_Small, slender flies, with pointed wings, short, porrect antennae, with a simple, circular third joint, bearing a bristle; empodium very small, pulvilli absent._—Only one genus of these little flies is known, but it is apparently widely distributed, and its members are common Insects. They have the appearance of Acalyptrate Muscidae, and the nervuration of the wing is somewhat similar, the nervures being simple and parallel, and the minute cross-nervures placed near the base. The systematic position is somewhat doubtful, and the metamorphoses are but incompletely known, very little having been added to what was discovered by Sir John Lubbock in 1862.[410] The larva lives on the earth under vegetable matter; it is very transparent, amphipneustic, with a peculiar head, and with fringes on the margins. This larva changes to a semi-pupa or apterous maggot-like form, within the larval skin; the true pupa was {491}not noticed by Lubbock, but Frauenfeld[411] has since observed it, though he only mentions that it possesses differentiated limbs and segments. The metamorphoses appear to be very peculiar. This fly requires a thorough study.

FAM. 23. MYDAIDAE.—_Large flies of elongate form; the hind femora long and toothed beneath; the antennae knobbed at the tip, projecting, rather long, the basal joint definite, but the divisions of the subsequent joints more or less indistinct. Empodium small. Wings frequently heavily pigmented; with a complex nervuration._ These fine flies are exotic; a few species occur in the Mediterranean region, even in the South of Europe; the chief genus, _Mydas_, is South American, but most of the other genera are Australian or African. But little is known as to the life-histories. The larvae are thought to live in wood, and to prey on Coleopterous larvae.

FAM. 24. ASILIDAE (_Robber-flies_).—_Mouth forming a short, projecting horny beak, the palpi usually only small; the feet generally largely developed; the claws large, frequently thick and blunt, the pulvilli generally elongate, the empodium a bristle; halteres free; no squama._ The Asilidae is one of the largest families of flies, and probably includes about 3000 described species: as will readily be believed, there is much variety of form; some are short and thick and extremely hairy, superficially resembling hairy bees, but the majority are more or less elongate, the abdomen being specially long, and having eight segments conspicuously displayed. The antennae are variable, but are three-jointed with a terminal appendage of diverse form and structure. They belong to the super-family Energopoda of Osten Sacken, but the association of Empidae and Dolichopidae with them does not seem to be very natural. In their perfect state these flies are most voracious, their prey being Insects, which they seize alive and impale with the rostrum. They are amongst the most formidable of foes and fear nothing, wasps or other stinging Insects being attacked and mastered by the stronger species without difficulty. They have been observed to capture even dragon-flies and tiger-beetles. As is the case with so many other Insects that prey on living Insects, the appetite in the Asilidae seems to be insatiable; a single individual has been observed to kill eight moths in twenty minutes. They have {492}been said to suck blood from Vertebrates, but this appears to be erroneous. The metamorphoses of a few species have been observed. Perris has called attention to the close alliance between the larvae of Tabanidae and of Asilidae,[412] and it seems at present impossible to draw a line of distinction between the two. So far as is known, the larvae of Asilidae are terrestrial and predaceous, attacking more particularly the larvae of Coleoptera, into which they sometimes bore; in _Laphria_ there are numerous pseudopods, somewhat of the kind shown in Fig. 230, but less perfect and without hairs; the head and breathing organs appear to be very different. According to Beling's descriptions of the larvae of _Asilus_, the head in this case is more like that of the figure, but there are no pseudopods. The flies of Asilidae and Tabanidae are so very distinct that these resemblances between their larvae are worthy of note.

FAM. 25. APIOCERIDAE.—_Moderate-sized flies marked with black and white, with an appearance like that of some Muscidae and Asilidae; with clear wings, the veins not deeply coloured; antennae short, with a short, simple appendage; no empodium._ But little is known as to the flies of this family, of which only two genera, consisting of about a dozen species, are found in North America, Chili, and Australia. Osten Sacken is inclined to treat them as an aberrant division of Asilidae. Brauer looks on them as primitive or synthetic forms of much interest, and has briefly described a larva which he considers may be that of _Apiocera_, but this is doubtful; it is a twenty-segmented form, and may be that of a _Thereva_.[413]

FAM. 26. EMPIDAE.—_Small or moderate-sized flies of obscure colours, grey, rusty, or black, with small head, somewhat globular in form, with three-jointed antennae, the terminal joint long and pointed; usually there is a long slender beak; the legs are elongate, frequently hairy; the tarsi bear long pulvilli and a small empodium._ The Empidae are an extensive family of flies, with predaceous habits, the rostrum being used by the female as an instrument for impaling and sucking other flies. They are occasionally very numerous in individuals, especially in wooded districts. There is great variety; there are nearly 200 species in Britain. The forms placed in the sub-family Hybotinae are curious slender little Insects, with very convex thorax and {493}large hind legs. In _Hemerodromia_ the front legs are raptorial, the femora being armed with spines on which the tibiae close so as to form a sort of trap. Many Empidae execute aërial dances, and some of the species of the genus _Hilara_ are notorious for carrying veils or nets in the form of silken webs more or less densely woven. This subject is comparatively new, the fact having been discovered by Baron Osten Sacken in 1877,[414] and it is not at all clear what purpose these peculiar constructions serve; it appears probable that they are carried by means of the hind legs, and only by the males. Mik thinks that in _H. sartor_ the veil acts as a sort of parachute, and is of use in carrying on the aërial performance, or enhancing its effect; while in the case of other species, _H. maura_ and _H. interstincta_, the object appears to be the capture or retention of prey, after the manner of spiders. The source of the silk is not known, and in fact all the details are insufficiently ascertained. The larvae of Empidae are described as cylindrical maggots, with very small head, and imperfect ventral feet; the stigmata are amphipneustic, the thoracic pair being, however, excessively small; beneath the posterior pair there is nearly always a tooth- or spine-like prominence present.

[Illustration: Fig. 235—A, Larva, B, pupa of _Medeterus ambiguus_. France. (After Perris.)]

FAM. 27. DOLICHOPIDAE.—_Graceful flies of metallic colours, of moderate or small size, and long legs; usually with bristles on the thorax and legs, the halteres exposed, squamae being quite absent; antennae of two short stout joints (of which the second is really two, its division being more or less distinct), with a thread-like or hair-like appendage. Proboscis short, fleshy. Claws, pulvilli, and empodium small; wings with a simple system of nervures, those on the posterior part of the wing are but few, there is no anterior basal cross-vein between the discal and second basal cells, which therefore form but one cell._ This is also a very extensive family of flies, of which we have probably about 200 species in Britain. They are conspicuous on account of their golden, or golden-green colours, only a few being yellow or black. {494}The males are remarkable for the curious special characters they possess on the feet, antennae, face, or wings. These characters are not alike in any two species; they are believed to be of the nature of ornaments, and according to Professor Aldrich and others are used as such in courtship.[415] This family of flies approaches very closely to some of the Acalyptrate Muscidae in its characters. It is united by Brauer with Empidae to form the tribe Orthogenya. Although the species are so numerous and abundant in Europe, little is known as to their metamorphoses. Some of the larvae frequent trees, living under the bark or in the overflowing sap, and are believed to be carnivorous; they are amphipneustic; a cocoon is formed, and the pupa is remarkable on account of the existence of two long horns, bearing the spiracles, on the back of the thorax; the seven pairs of abdominal spiracles being excessively minute.[416]

[Illustration: Fig. 236—Wing of _Trineura aterrima_, one of the Phoridae. Britain.]

SERIES 3. CYCLORRHAPHA ASCHIZA

FAM. 28. PHORIDAE.—_Small flies, with very convex thorax, small head, very small two-jointed antennae, bearing a long seta; femora more or less broad; wings with two dark, thick, approximate veins, meeting on the front margin near its middle, and besides these, three or four very fine veins, that run to the margins in a sub-parallel manner without forming any cells or forks._ This obscure family of flies is of small extent, but its members are extremely common in Europe and North America, where they often occur in numbers running on the windows of houses. It is one of the most isolated groups of Diptera, and great difference of opinion prevails as to its classification. The wing-nervuration is peculiar (but varies somewhat in the species), the total absence of any cross-veins even on the basal part of the wing being remarkable. There are bristles on the head and thorax, but they are not arranged in a regular manner. The larvae live in a great variety of animal and vegetable decaying matter, {495}and attack living Insects, and even snails, though probably only when these are in a sickly or diseased condition. The metamorphoses of several species have been described.[417] The larvae are rather slender, but sub-conical in form, with eleven segments and a very small head, amphipneustic, the body behind terminated by some pointed processes. The pupa is remarkable; it is contained in a case formed by the contracted and hardened skin of the larva; though it differs much in form from the larva the segmentation is distinct, and from the fourth segment there project two slender processes. These are breathing organs, attached to the prothorax of the imprisoned pupa; in what manner they effect a passage through the hardened larval skin is by no means clear. Perris supposes that holes for them pre-exist in the larval skin, and that the newly-formed pupa by restless movements succeeds in bringing the processes into such a position that they can pass through the holes. The dehiscence of the puparium seems to occur in a somewhat irregular manner, as in _Microdon_; it is never Cyclorrhaphous, and according to Perris is occasionally Orthorrhaphous; probably there is no ptilinum.

[Illustration: Fig. 237—_Aenigmatias blattoides._ × 27. Denmark. (After Meinert.)]

The Insect recently described by Meinert as _Aenigmatias blattoides_,[418] is so anomalous, and so little is known about it, that it cannot at present be classified. It is completely apterous; the arrangement of the body-segments is unlike that of Diptera, but the antennae and mouth-parts are said to be like those of Phoridae. The Insect was found near Copenhagen under a stone in the runs of _Formica fusca_. Meinert thinks it possible that the discovery of the male may prove _Aenigmatias_ to be really allied to Phoridae, and Mik suggests that it may be the same as _Platyphora lubbocki_, Verrall, known to be parasitic on ants. Dahl recently described a wingless Dipteron, found living as a parasite on land-snails in the Bismarck archipelago, under the name of _Puliciphora lucifera_, {496}and Wandolleck has recently made for this and some allies the new family Stethopathidae. It seems doubtful whether these forms are more than wingless Phoridae.

FAM. 29. PLATYPEZIDAE.—_Small flies, with porrect three-jointed antennae, first two joints short, third longer, with a terminal seta; no bristles on the back; hind legs of male, or of both sexes, with peculiar, broad, flat tarsi; the middle tibiae bear spurs; there is no empodium._ Platypezidae is a small family of flies, the classification of which has always been a matter of considerable difficulty, and is still uncertain. The larvae are broad and flat, fringed at the margin with twenty-six spines; they live between the lamellae of Agaric fungi. At pupation the form alters but little; the imago emerges by a horizontal cleft occurring at the margins of segments two and four.[419] We have four genera (_Opetia_, _Platycnema_, _Platypeza_, _Callomyia_), and nearly a score of species of Platypezidae in our British list, but very little seems to be known about them. There is much difference in the eyes of the sexes, in some at any rate of the species, they being large and contiguous in the male, but widely separated in the female.

[Illustration: Fig. 238—Head of _Pipunculus_ sp. A, Seen from in front; B, side view, showing an antenna magnified. Pyrenees.]

FAM. 30. PIPUNCULIDAE.[420]—_Small flies, with very short antennae bearing a long seta that is not terminal; head almost globular, formed, except at the back, almost entirely by the large conjoined eyes; the head is only slightly smaller in the female, but in the male the eyes are more approximate at the top._ This is another of the small families of flies, that seems distinct from any other, though possessing no very important characters. In many of the flies that have very large eyes, the head is either flattened (_i.e._ compressed from before backwards, as in Tabanidae, Asilidae), or forced beneath the humped thorax (as in Acroceridae), but neither of these conditions exists in _Pipunculus_; in them the head extends far forwards, so that the area of the {497}eye compared with the size of the body is perhaps greater than in any other Diptera. The general form is somewhat that of _Anthrax_, but the venation on the hind part of the wing is much less complex. There is a remarkable difference between the facets on the front and the back of these great eyes. We have three genera and about a dozen species of Pipunculidae in Britain but apparently they are far from common Insects. What is known about the life-history is almost confined to an imperfect observation by Boheman, who found the larva of _P. fuscipes_ living after the manner of a Hymenopterous parasite in the body of a small Homopterous Insect.[421] The pupa seems to be of the type of that of Syrphidae.

FAM. 31. CONOPIDAE.—_Elegant flies of moderate size, of varied colours, with abdomen slender at the base, at the tip strongly incurved and thicker; antennae inserted close together on a prominence, three-jointed, first joint sometimes very short. The upper surface of the body without bristles or with but few. There is a slender, elongate proboscis, which is retractile and usually invisible._ This rather small family of flies includes some of the most remarkable forms of Diptera; it includes two divisions, the Conopinae with long antennae terminated by a very minute pointed process, and Myopinae with shorter antennae bearing a hair that is not placed at the end of the third joint. The former are the more wasp-like and elegant; the Myopinae being much more like ordinary flies, though they frequently have curious, inflated heads, with a white face. The mode of life of the larva of _Conops_ is peculiar, it being parasitic in the interior of _Bombus_, or other Hymenoptera. They have been found to attack _Bombus_, _Chalicodoma_, _Osmia_, _Vespa_, _Pompilus_, and other Aculeates. Williston says that Orthoptera are also attacked. _Conops_ has been seen to follow Bumble-bees and alight on them, and Williston says this act is accompanied by oviposition, the larva that is hatched boring its way into the body of the bee. Others have supposed that the flies enter the bees' nests and place their eggs in the larvae or pupae; but this is uncertain, for _Conops_ has never been reared from a bee-larva or pupa, though it has frequently been procured from the imago: cases indeed having been recorded in which _Conops_ has emerged from the body {498}of a _Bombus_ several months after the latter had been killed and placed in an entomologist's collection. The larva is broad, and when full grown apparently occupies nearly all the space of the interior of the abdomen of the bee; it has very peculiar terminal stigmata. The pupa is formed in the larval skin, which is greatly shortened and indurated for the purpose; this instar bears, in addition to the posterior stigmata, a pair of slightly projecting, anterior stigmata. We have several species of Conopidae in Britain; those belonging to the division Conopinae are all rare Insects, but the Myopinae are not so scarce; these latter are believed to be of similar habits with the Conopinae, though remarkably little is known about them. This is another of the numerous families, the relations of which are still a subject for elucidation. Brauer places the Conopidae in his section Schizophora away from Syrphidae, but we do not comprehend on what grounds; an inspection of the head shows that there is no frontal lunule as there is in Eumyiidae; both _Myopa_ and _Conops_ agreeing fairly well with _Syrphus_ as to this. We therefore place the family in its old position near _Syrphus_ till the relations with Acalypterate Muscidae shall be better established.

FAM. 32. SYRPHIDAE (_Hover-flies_).—_Of moderate or rather large size, frequently spotted or banded with yellow, with a thick fleshy proboscis capable of being withdrawn into a cleft on the under side of the head; antennae not placed in definite cavities, three-jointed (usually very short), and leaving a seta that is not terminal in position, and may be feathered. Squama variable, never entirely covering the halteres; the chief (third to fifth) longitudinal veins of the wings connected near their termination by cross-veins and usually thus forming a sort of short margin parallel with the hind edge of the wing; a more or less imperfect false nervure running between the third and fourth longitudinal nervures; no empodium and generally no distinct system of bristles on the back of the body._ The Syrphidae (Fig. 212) form one of the largest and best known of all the families of flies; they abound in our gardens where, in sunny weather, some species may be nearly always seen hovering over flowers, or beneath trees in places where the rays of the sun penetrate amidst the shade. There are two or three thousand species known, so that of course much variety exists; some are densely covered with hair (certain _Volucella_ and others), many are of elegant form, and some bear a {499}considerable resemblance to Hymenoptera of various groups. The peculiar veining of the wings permits of their easy identification, the line of two nervules, approximately parallel with the margin of the distal part of the wing (Fig. 212, D), and followed by a deep bay, being eminently characteristic, though there are some exceptions; there are a few forms in which the antennae are exceptional in having a terminal pointed process. The proboscis, besides the membranous and fleshy lips, consists of a series of pointed slender lancets, the use of which it is difficult to comprehend, as the Insects are not known to pierce either animals or vegetables, their food being chiefly pollen; honey is also doubtless taken by some species, but the lancet-like organs appear equally ill-adapted for dealing with it. The larvae are singularly diversified; first, there are the eaters of Aphidae, or green-fly; some of these may be generally found on our rose-bushes or on thistles, when they are much covered with Aphids; they are soft, maggot-like creatures with a great capacity for changing their shape and with much power of movement, especially of the anterior part of the body, which is stretched out and moved about to obtain and spear their prey: some of them are very transparent, so that the movements of the internal organs and their vivid colours can readily be seen: like so many other carnivorous Insects, their voracity appears to be insatiable. The larvae of many of the ordinary Hover-flies are of this kind. _Eristalis_ and its allies are totally different, they live in water saturated with filth, or with decaying vegetable matter (the writer has found many hundreds of the larvae of _Myiatropa florea_ in a pool of water standing in a hollow beech-tree). These rat-tailed maggots are of great interest, but as they have been described in almost every work on entomology, and as Professor Miall[422] has recently given an excellent account of their peculiarities, we need not now discuss them. Some of the flies of the genus _Eristalis_ are very like honey-bees, and appear in old times to have been confounded with them; indeed, Osten Sacken thinks this resemblance gave rise to the "Bugonia myth," a fable of very ancient origin to the effect that Honey-bees could be procured from filth, or even putrefying carcases, by the aid of certain proceedings that savoured slightly of witchcraft, and may therefore have increased the belief of the operator in the {500}possibility of a favourable result. It was certainly not bees that were produced from the carcases, but Osten Sacken suggests that _Eristalis_-flies may have been bred therein.

In the genus _Volucella_ we meet with a third kind of Syrphid larva. These larvae are pallid, broad and fleshy, surrounded by numerous angular, somewhat spinose, outgrowths of the body; and have behind a pair of combined stigmata, in the neighbourhood of which the outgrowths are somewhat larger; these larvae live in the nests of Bees and Wasps, in which they are abundant. Some of the _Volucella_, like many other Syrphidae, bear a considerable resemblance to Bees or Wasps, and this has given rise to a modern fable about them that appears to have no more legitimate basis of fact than the ancient Bees-born-of-carcases myth. It was formerly assumed that the _Volucella_-larvae lived on the larvae of the Bees, and that the parent flies were providentially endowed with a bee-like appearance that they might obtain entrance into the Bees' nests without being detected, and then carry out their nefarious intention of laying eggs that would hatch into larvae and subsequently destroy the larvae of the Bees. Some hard-hearted critic remarked that it was easy to understand that providence should display so great a solicitude for the welfare of the _Volucella_, but that it was difficult to comprehend how it could be, at the same time, so totally indifferent to the welfare of the Bees. More recently the tale has been revived and cited as an instance of the value of deceptive resemblance resulting from the action of natural selection, without reference to providence. There are, however, no facts to support any theory on the subject. Very little indeed is actually known as to the habits of _Volucella_ in either the larval or imaginal instars; but the little that is known tends to the view that the presence of the _Volucella_ in the nests is advantageous to both Fly and Bee. Nicolas has seen _Volucella zonaria_ enter the nest of a Wasp; it settled at a little distance and walked in without any fuss being made. Erné has watched the _Volucella_-larvae in the nests, and he thinks that they eat the waste or dejections of the larvae. The writer kept under observation _Volucella_-larvae and portions of the cells of _Bombus_, containing some larvae and pupae of the Bees and some honey, but the fly-larvae did not during some weeks touch any of the Bees or honey, and ultimately died, presumably of starvation. Subsequently, he experimented with _Volucella_-larvae and a portion {501}of the comb of wasps containing pupae, and again found that the flies did not attack the Hymenoptera; but on breaking a pupa of the Wasp in two, the fly-larvae attacked it immediately and eagerly; so that the evidence goes to show that the _Volucella_-larvae act as scavengers in the nests of the Hymenoptera. Künckel d'Herculais has published an elaborate work on the European _Volucella_; it is remarkable for the beauty of the plates illustrating the structure, anatomy and development, but throws little direct light on the natural history of the Insects. _V. bombylans_, one of the most abundant of our British species, appears in two forms, each of which has a considerable resemblance to a _Bombus_, and it has been supposed that each of the two forms is specially connected with the Bee it resembles, but there is no evidence to support this idea; indeed, there is some little evidence to the contrary. The genus _Merodon_ has larvae somewhat similar to those of _Volucella_, but they live in bulbs of _Narcissus_; _M. equestris_ has been the cause of much loss to the growers of Dutch bulbs; this Fly is interesting on account of its great variation in colour; it has been described as a whole series of distinct species.

The most remarkable of the numerous forms of Syrphid larvae are those of the genus _Microdon_ (Fig. 239), which live in ants' nests. They have no resemblance to Insect-larvae, and when first discovered were not only supposed to be little Molluscs, but were actually described as such under the generic names of _Parmula_ and _Scutelligera_. There is no appearance of segmentation of the body; the upper surface is covered by a sort of network formed by curved setae, which help to retain a coating of dirt; there is no trace externally of any head, but on the under surface there is a minute fold in which such mouth-organs as may be present are probably concealed; the sides of the body project so as to form a complex fringing arrangement; the terminal stigmata are very distinct, the lateral processes connected with them (the "Knospen" of Dr. Meijere), are, however, very irregular and placed at some distance from the stigmatic scar. Pupation occurs by the induration of the external covering and the growth from it, or rather through it, of two short horns in front. Inside this skin there is formed a soft pupa, of the kind usual in Cyclorrhaphous flies; the dehiscence of the external covering is, however, of unusual nature, three little pieces being {502}separated from the anterior part of the upper surface, while the lower face remains intact. The account of the pupation given by Elditt[423] is not complete: the two horns that project are, it would appear, not portions of the larval skin, but belong to the head of the pupa, and according to Elditt are used to effect the dehiscence of the case for the escape of the fly; there does not appear to be any head-vesicle. Nothing is known as to the details of the life of these anomalous larvae. M. Poujade has described two species found in France in the nests of the ant _Lasius niger_.[424] The larva we figure was found by Colonel Yerbury in nests of an _Atta_ in Portugal, and an almost identical larva was recently found by Mr. Budgett in Paraguay. The flies themselves are scarce, _Microdon mutabilis_ (formerly called _M. apiformis_) being one of the rarest of British flies. They have the antennae longer than is usual in Syrphidae, and the cross-veins at the outside of the wing are irregularly placed, so that the contour is very irregular: the resemblance to bees is very marked, and in some of the South American forms the hind legs are flattened and hairy like those of bees. The oviposition of _Microdon_ has been observed by Verhoeff;[425] he noticed that the fly was frequently driven away by the ants—in this case, _Formica sanguinea_—but returned undiscouraged to its task.

[Illustration: Fig. 239—Larva of _Microdon_ sp. Portugal. A, Dorsal view of the larva, × 4; 1, the stigmatic structure; B, posterior view of stigmatic structure; C, a portion of the marginal fringe of the body.]

A brief résumé of the diverse modes of life of Syrphid larvae has been given by Perris,[426] and he also gives some information as to the curious horns of the pupae, but this latter point much {503}wants elucidation. Whether the Syrphidae, or some of them, possess a ptilinum that helps them to emerge from the pupa is more than doubtful, though its existence has been affirmed by several authors of good repute.[427]

[Illustration: Fig. 240.—_Diopsis apicalis._ Natal. A, The fly; B, extremity of cephalic protuberance, more magnified. _a_, The eye; _b_, the antenna; C, middle of head, front view; _c_, ocelli.]

SERIES 4. CYCLORRHAPHA SCHIZOPHORA

FAM. 33. MUSCIDAE ACALYPTRATAE.—This group of flies has been the least studied of all the Diptera; it is generally treated as composed of twenty or thirty different families distinguished by very slight characters. It is, however, generally admitted by systematists that these assemblages have not the value of the families of the other divisions of Diptera, and some even go so far as to say that they are altogether only equivalent to a single family. We do not therefore think it necessary to define each one _seriatim_; we shall merely mention their names, and allude to certain points of interest connected with them. Taken collectively they may be defined as very _small flies, with three-jointed antennae (frequently looking as if only two-jointed), bearing a bristle that is not terminally placed; frequently either destitute of squamae or having these imperfectly developed so as not to cover the halteres; and possessing a comparatively simple system of nervuration, the chief nervures being nearly straight, so that consequently few cells are formed_. These characters will distinguish the group {504}from all the other Diptera except from forms of Aschiza, and from certain Anthomyiidae, with both of which the Acalyptratae are really intimately connected. Considerable difference of opinion prevails as to the number of these divisions, but the families usually recognised are:—

1. Doryceridae. 2. Tetanoceridae. 3. Sciomyzidae. 4. Diopsidae. 5. Celyphidae. 6. Sepsidae incl. Piophilidae. 7. Chloropidae (= Oscinidae). 8. Ulidiidae. 9. Platystomidae. 10. Ephydridae. 11. Helomyzidae. 12. Dryomyzidae. 13. Borboridae. 14. Phycodromidae. 15. Thyreophoridae. 16. Scatophagidae. (= Scatomyzidae). 17. Geomyzidae incl. Opomyzidae. 18. Drosophilidae; incl. Asteidae. 19. Psilidae. 20. Tanypezidae (= Micropezidae). 21. Trypetidae. 22. Sapromyzidae incl. Lonchaeidae. 23. Rhopalomeridae. 24. Ortalidae. 25. Agromyzidae incl. Phytomyzidae. 26. Milichiidae. 27. Octhiphilidae. 28. Heteroneuridae. 29. Cordyluridae.

Brauer associates Conopidae with Acalyptrate Muscids, and calls the Group Holometopa; applying the term Schizometopa to the Calyptrate Muscidae.

No generalisation can yet be made as to the larvae of these divisions, neither can any characters be pointed out by which they can be distinguished from the larvae of the following families. In their habits they have nothing specially distinctive, and may be said to resemble the Anthomyiidae, vegetable matter being more used as food than animal; many of them mine in the leaves or stems of plants; in the genus _Dorycera_ the larva is aquatic, mining in the leaves of water-plants, and in Ephydridae several kinds of aquatic larvae are found, some of which are said to resemble the rat-tailed larvae of Syrphidae; certain of these larvae occur in prodigious quantities in lakes, and the Insects in some of their early stages serve the Mexicans as food, the eggs being called Ahuatle, the larvae Pusci, the pupae Koo-chah-bee. Some of the larvae of the Sciomyzidae are also aquatic: that of _Tetanocera ferruginea_ is said by Dufour to consist only of eight segments, and to be metapneustic; Brauer considers the Acalyptrate larvae to be, however, in general, amphipneustic, like those of Calyptratae. The Chloropidae are a very important family owing to their occasional excessive multiplication, and to their living on cereals and other grasses, various parts of which they attack, sometimes causing great losses to the agriculturist. The species of the genus _Chlorops_ are {505}famous for the curious habit of entering human habitations in great swarms: frequently many millions being found in a single apartment. Instances of this habit have been recorded both in France and England, Cambridge being perhaps the place where the phenomenon is most persistently exhibited. In the year 1831 an enormous swarm of _C. lineata_ was found in the Provost's Lodge at King's College and was recorded by Leonard Jenyns; in 1870 another swarm occurred in the same house if not in the same room.[428] Of late years such swarms have occurred in certain apartments in the Museums (which are not far from King's College), and always in the same apartments. No clue whatever can be obtained as to their origin; and the manner in which these flies are guided to a small area in numbers that must be seen to be believed, is most mysterious. These swarms always occur in the autumn, and it has been suggested that the individuals are seeking winter quarters.

[Illustration: Fig. 241—_Celyphus_ (_Paracelyphus_) sp. West Africa. A, The fly seen from above; _a_, scutellum; _b_, base of wing: B, profile, with tip of abdomen bent downwards; _a_, scutellum; _b_, _b_, wing; _c_, part of abdomen.]

Several members of the Acalyptratae have small wings or are wingless, as in some of our species of _Borborus_. The Diopsidae—none of which are European—have the sides of the head produced into long horns, at the extremity of which are placed the eyes and antennae; these curiosities (Fig. 240) are apparently common in both Hindostan and Africa. In the horned flies of the genus _Elaphomyia_, parts of the head are prolonged into horns of very diverse forms according to the species, but bearing on the whole a great resemblance to miniature stag-horns. A genus (_Giraffomyia_) with a long neck, and with partially segmented appendages, instead of horns on the head, has been recently discovered by Dr. Arthur Willey in New Britain. Equally remarkable are the species of _Celyphus_; they do not look like flies at all, owing to the scutellum being inflated and enlarged so as to cover all the posterior parts {506}of the body as in the Scutellerid Hemiptera: the wings are entirely concealed, and the abdomen is reduced to a plate, with its orifice beneath, not terminal; the surface of the body is highly polished and destitute of bristles. Whether this is a mimetic form, occurring in association with similar-looking Bugs is not known. The North American genus _Toxotrypana_ is furnished with a long ovipositor; and in this and in the shape of the body resembles the parasitic Hymenoptera. This genus was placed by Gerstaecker in Ortalidae, but is considered by later writers to be a member of the Trypetidae. This latter family is of considerable extent, and is remarkable amongst the Diptera for the way in which the wings of many of its members are ornamented by an elaborate system of spots or marks, varying according to the species.

FAM. 34. ANTHOMYIIDAE.—_Flies similar in appearance to the House-fly; the main vein posterior to the middle of the wing (4th longitudinal) continued straight to the margin, not turned upwards. Eyes of the male frequently large and contiguous, bristle of antenna either feathery or bare._ This very large family of flies is one of the most difficult and unattractive of the Order. Many of its members come close to the Acalyptrate Muscidae from which they are distinguished by the fact that a well-developed squama covers the halteres; others come quite as close to the Tachinidae, Muscidae and Sarcophagidae, but may readily be separated by the simple, not angulate, main vein of the wing. The larval habits are varied. Many attack vegetables, produce disintegration in them, thus facilitating decomposition. _Anthomyia brassicae_ is renowned amongst market gardeners on account of its destructive habits. _A. cana_, on the contrary, is beneficial by destroying the migratory Locust _Schistocerca peregrina_; and in North America, _A. angustifrons_ performs a similar office with _Caloptenus spretus_. One or two species have been found living in birds; in one case on the head of a species of _Spermophila_, in another case on a tumour of the wing of a Woodpecker. _Hylemyia strigosa_, a dung-frequenting species, has the peculiar habit of producing living larvae, one at a time; these larvae are so large that it would be supposed they are full grown, but this is not the case, they are really only in the first stage, an unusual amount of growth being accomplished in this stadium. _Spilogaster angelicae_, on the other hand, according to {507}Portschinsky, lays a small number of very large eggs, and the resulting larvae pass from the first to the third stage of development, omitting the second stage that is usual in Eumyiid Muscidae.[429]

[Illustration: Fig. 242—_Ugimyia sericariae._ A, The perfect fly, × 3/2; B, tracheal chamber of a silkworm, with body of a larva of _Ugimyia_ projecting; _a_, front part of the maggot; _b_, stigmatic orifice of the maggot; _c_, stigma of the silkworm. (After Sasaki.)]

FAM. 35. TACHINIDAE.—_First posterior cell of wing nearly or quite closed. Squamae large, covering the halteres: antennal arista bare: upper surface of body usually bristly._ This is an enormous family of flies, the larvae of which live parasitically in other living Insects, Lepidopterous larvae being especially haunted. Many have been reared from the Insects in which they live, but beyond this little is known of the life-histories, and still less of the structure of the larvae of the Tachinidae, although these Insects are of the very first importance in the economy of Nature. The eggs are usually deposited by the parent-flies near or on the head of the victim; Riley supposed that the fly buzzes about the victim and deposits an egg with rapidity, but a circumstantial account given by Weeks[430] discloses a very different process: the fly he watched sat on a leaf quietly facing a caterpillar of _Datana_ engaged in feeding at a distance of rather less than a quarter of an inch. "Seizing a moment when the head of the larva was likely to remain stationary, the fly stealthily and rapidly bent her abdomen downward and extended from the last segment what proved to be an ovipositor. This passed forward beneath her body and between the legs until it projected beyond and nearly on a level with the head of the fly and came in contact with the eye of the larva upon which an egg was deposited," making an addition to five already there. _Ugimyia sericariae_ does great {508}harm in Japan by attacking the silkworm, and in the case of this fly the eggs are believed to be introduced into the victim by being laid on mulberry leaves and swallowed with the food; several observers agree as to the eggs being laid on the leaves, but the fact that they are swallowed by the silkworm is not so certain. Sasaki has given an extremely interesting account of the development of this larva.[431] According to him, the young larva, after hatching in the alimentary canal, bores through it, and enters a nerve-ganglion, feeding there for about a week, after which the necessity for air becoming greater, as usual with larvae, the maggot leaves the nervous system and enters the tracheal system, boring into a tube near a stigmatic orifice of the silkworm, where it forms a chamber for itself by biting portions of the tissues and fastening them together with saliva. In this it completes its growth, feeding on the interior of the silkworm with its anterior part, and breathing through the stigmatic orifice of its host; after this it makes its exit and buries itself deeply in the ground, where it pupates. The work of rupturing the puparium by the use of the ptilinum is fully described by Sasaki, and also the fact that the fly mounts to the surface of the earth by the aid of this same peculiar air-bladder, which is alternately contracted and distended. Five, or more, of the _Ugimyia_-maggots may be found in one caterpillar, but only one of them reaches maturity, and emerges from the body. The Tachinid flies appear to waste a large proportion of their eggs by injudicious oviposition; but they make up for this by the wide circle of their victims, for a single species has been known to infest Insects of two or three different Orders.

[Illustration: Fig. 243—Diagrammatic section of silkworm to show the habits of _Ugimyia_. _a_, Young larva; _b_, egg of _Ugimyia_ in stomach of the silkworm; _c_, larva in a nerve-ganglion; _d_, larva entering a ganglion; _e_, larva embedded in tracheal chamber, as shown in Fig. 242, B. (After Sasaki.)]

The species of _Miltogramma_—of which there are many in Europe and two in England—live at the expense of Fossorial {509}Hymenoptera by a curious sort of indirect parasitism. They are obscure little flies, somewhat resembling the common House-fly, but they are adepts on the wing and have the art of ovipositing with extreme rapidity; they follow a Hymenopteron as it is carrying the prey to the nest for its young. When the wasp alights on the ground at the entrance to the nest, the _Miltogramma_ swoops down and rapidly deposits one or more eggs on the prey the wasp designs as food for its own young. Afterwards the larvae of the fly eat up the food, and in consequence of the greater rapidity of their growth, the young of the Hymenopteron perishes. Some of them are said to deposit living larvae, not eggs. Fabre has drawn a very interesting picture of the relations that exist between a species of _Miltogramma_ and a Fossorial Wasp of the genus _Bembex_[432]. We may remind the reader that this Hymenopteron has not the art of stinging its victims so as to keep them alive, and that it accordingly feeds its young by returning to the nest at proper intervals with a fresh supply of food, instead of provisioning the nest once and for all and then closing it. This Hymenopteron has a habit of catching the largest and most active flies—especially Tabanidae—for the benefit of its young, and it would therefore be supposed that it would be safe from the parasitism of a small and feeble fly. On the contrary, the _Miltogramma_ adapts its tactics to the special case, and is in fact aided in doing so by the wasp itself. As if knowing that the wasp will return to the carefully-closed nest, the _Miltogramma_ waits near it, and quietly selects the favourable moment, when the wasp is turning round to enter the nest backwards, and deposits eggs on the prey. It appears from Fabre's account that the _Bembex_ is well aware of the presence of the fly, and would seem to entertain a great dread of it, as if conscious that it is a formidable enemy; nevertheless the wasp never attacks the little fly, but allows it sooner or later to accomplish its purpose, and will, it appears, even continue to feed the fly-larvae, though they are the certain destroyers of its own young, thus repeating the relations between cuckoo and sparrow. Most of us think the wasp stupid, and find its relations to the fly incredible or contemptible. Fabre takes a contrary view, and looks on it as a superior Uncle Toby. We sympathise with the charming French naturalist, without forming an opinion.

{510}Doubtless there are many other interesting features to be found in the life-histories of Tachinidae, for in numbers they are legion. It is probable that we may have 200 species in Britain, and in other parts of the world they are even more abundant, about 1000 species being known in North America.[433] The family Actiidae is at present somewhat doubtful. According to Karsch,[434] it is a sub-family of Tachinidae; but the fourth longitudinal vein, it appears, is straight.

FAM. 36. DEXIIDAE.—_These Insects are distinguished from Tachinidae by the bristle of the antennae being pubescent, and the legs usually longer._ The larvae, so far as known, are found in various Insects, especially in Coleoptera, and have also been found in snails. There are eleven British genera, and about a score of species.

FAM. 37. SARCOPHAGIDAE.—_Distinguished from Muscidae and Tachinidae by little more than that the bristle of the antennae is feathery at the base but hair-like and very fine at the tip._—_Sarcophaga carnaria_ is one of the commonest British Insects; it is like the Blow-fly, though rather longer, conspicuously grey and black, with the thorax distinctly striped, and the pulvilli very conspicuous in the live fly. _Cynomyia mortuorum_ is a bright blue fly rather larger than the Blow-fly, of which it is a competitor; but in this country an unsuccessful one. The larvae of the two Insects are found together, and are said to be quite indistinguishable. _Cynomyia_ is said to lay only about half the number of eggs that the Blow-fly does, but it appears earlier in the year, and to this is attributed the fact that it is not altogether crowded out of existence by the more prolific _Calliphora_. The species of Sarcophagidae are usually viviparous, and one of them, _Sarcophila magnifica_ (_wohlfahrti_), has the habit of occasionally depositing its progeny in the nostrils of mammals, and even of human beings, causing horrible sufferings and occasionally death: it is said to be not uncommon in Europe but does not occur in Britain. The genus _Sarcophaga_ is numerous in species, and many of them are beneficial. Sir Sidney Saunders found in the Troad that Locusts were destroyed by the larvae of a _Sarcophaga_ living in their bodies; and {511}Künckel has recently observed that in Algeria several species of this genus attack Locusts and destroy large quantities by depositing living larvae in the Orthoptera. In North America the Army-worm is decimated by species of _Sarcophaga_.

Many of these Insects, when food is scarce, eat their own species with eagerness, and it seems probable that this habit is beneficial to the species. The parent-fly in such cases usually deposits more eggs than there is food for, thus ensuring that every portion of the food will be rapidly consumed, after which the partially-grown larvae complete their development by the aid of cannibalism. It is thus ensured that the food will raise up as many individuals as possible.

FAM. 38. MUSCIDAE.—_Bristle of antennae feathered._ This family contains many of the most abundant flies, including the House-fly, Blue-bottles or Blow-flies, Green-bottles, and other forms which, though very common, are perhaps not discriminated from one another by those who are not entomologists. The larvae live on carrion and decaying or excrementitious matters. The common House-fly, _Musca domestica_, runs through its life-history in a very short time. It lays about 150 very small eggs on dung or any kind of soft damp filth; the larvae hatch in a day or two and feed on the refuse; they may be full-grown in five or six days, and, then pupating, may in another week emerge as perfect flies. Hence it is no wonder that they increase to enormous numbers in favourable climates. They are thought to pass the winter chiefly in the pupal state. The House-fly is now very widely distributed over the world; it sometimes occurs in large numbers away from the dwellings of man. Of Blow-flies there are two common species in this country, _Calliphora erythrocephala_ and _C. vomitoria_. The Green-bottle flies, of which there are several species, belonging to the genus _Lucilia_, have the same habits as Blow-flies, though they do not commonly enter houses. The larvae are said to be indistinguishable from those of _Calliphora_.

The larvae of Eumyiid Muscidae are, when first hatched, metapneustic, but subsequently an anterior pair of stigmata appears, so that the larva becomes amphipneustic. They usually go through three stages, distinguished by the condition of the posterior stigmata. In the early instar these have a single heart-shaped fissure, in the second stage two fissures exist, {512}while in the third instar there is a greater diversity in the condition of the breathing apertures.

The various forms of Muscidae show considerable distinctions in the details of their natural history, and these in certain species vary according to the locality. This subject has been chiefly studied by Portschinsky, a Russian naturalist, and a very interesting summary of his results has been given by Osten Sacken,[435] to which the student interested in the subject will do well to refer.

A few years ago a great deal of damage was caused in the Netherlands by _Lucilia sericata_, a Green-bottle-fly, extremely similar to our common _L. caesar_, which deposited its eggs in great quantities on sheep amongst their wool. This epidemic was attributed to the importation of sheep from England; but, according to Karsch, there is reason to suppose that the fly was really introduced from Southern Europe or Asia Minor.[436]

The larvae of species of the genus _Lucilia_ sometimes attack man and animals in South America, but fortunately not in this country. The larva of _Lucilia_ (_Compsomyia_) _macellaria_ is called the screw-worm, and is the best known of the forms that infest man, the larvae living in the nasal fossae and frontal sinuses, and causing great suffering. The fly is common in North America, but is said never to attack man farther north than in Kansas. A little fly (_Stomoxys calcitrans_), very like the common house-fly though rather more distinctly spotted with grey and black, and with a fine, hard, exserted proboscis, frequently enters our houses and inflicts a bite or prick on us. It is commonly mistaken for an ill-natured house-fly that has taken to biting. It is frequently a source of irritation to cattle. A closely allied fly, _Haematobia serrata_, is very injurious to cattle in North America, but the same species causes no serious annoyance in England. We may mention that the various attacks of Dipterous larvae on man have received the general name "myiasis."

The Tse-tse fly (_Glossina morsitans_), another ally of _Stomoxys_, is not very dissimilar in size and shape to the blow-fly.[437] {513}It bites man and animals in South Africa, and if it have previously bitten an animal whose blood was charged with the Haematozoa that really constitute the disease called Nagana (fly-disease), it inoculates the healthy animal with the disease; fortunately only some species are susceptible, and man is not amongst them. It has recently been shown by Surgeon Bruce[438] that this fly multiplies by producing, one at a time, a full-grown larva, which immediately changes to a pupa, as do the members of the series Pupipara. There are already known other Muscid flies with peculiarities in their modes of reproduction, so that it is far from impossible that the various conditions between ordinary egg-laying and full-grown larva- or pupa-production may be found to exist. Although it has been supposed that the Tse-tse fly is a formidable obstacle to the occupation of Africa by civilised men, there is reason to suppose that this will not ultimately prove to be the case. It only produces disease when this pre-exists in animals in the neighbourhood; only certain species are liable to it; and there is some evidence to the effect that even these may in the course of a succession of generations become capable of resisting the disease inoculated by the fly. As long ago as 1878 Dr. Drysdale suggested[439] that this fly only produces disease by inoculating a blood-parasite, and all the evidence that has since been received tends to show that his idea is correct.

[Illustration: Fig. 244—The Tse-tse fly (_Glossina morsitans_). A, The fly with three divisions of the proboscis projecting; B, adult larva; C, pupa.]

Although the facts we have mentioned above would lead to the supposition that Muscidae are unmitigated nuisances, yet it is probable that such an idea is the reverse of the truth, and that on the whole their operations are beneficial. It would be difficult to overestimate their value as scavengers. And in addition to this they destroy injurious creatures. Thus in Algeria _Idia fasciata_, a fly like the House-fly, destroys the {514}dreaded migratory Locust _Schistocerca peregrina_ in great quantities, by the larvae eating the eggs of the Locust. The female of this fly, in order to reach the desired food, penetrates from one to three inches below the surface of the ground.

FAM. 39. OESTRIDAE (_Bot-flies_).—_Rather large or very large flies, with extremely short antennae, bearing a segmented arista, the front of the head prominent, the posterior part of the wings frequently rough, and with but few veins: the mouth usually atrophied, the trophi being represented only by tubercles; larvae living in Vertebrates, usually Mammals, though it is possible that a few occur in Birds and even in Reptiles._ This is a family of small extent, less than 100 species being known from all the world, yet it is of much interest on account of the habits of its members, which, though of large size, live entirely at the expense of living Vertebrates, to the viscera or other structures of which they have definite relations, varying according to the species. Some (_Gastrophilus_, etc.) live in the alimentary canal; others (_Hypoderma_, etc.) are encysted in or under the skin; while others (_Oestrus_, etc.) occupy the respiratory passages. As many of them attack the animals used by man, and some of them do not spare man himself, they have attracted much attention, and there is an extensive literature connected with them; nevertheless the life-histories are still very incompletely known. Indeed, the group is from all points of view a most difficult one, it being almost impossible to define the family owing to the great differences that exist in important points. Some think the family will ultimately be dismembered; and Girschner has recently proposed to treat it as a division of Tachinidae. The chief authority is Brauer, in whose writings the student will find nearly all that is known about Oestridae.[440] Some of them exist in considerable numbers (it is believed that they are now not so common as formerly), and yet the flies are but rarely met with, their habits being in many respects peculiar. Some of them, for purposes of repose, frequent the summits of mountains, or towers, or lofty trees. Some have great powers of humming; none of them are known to bite their victims, indeed the atrophied mouth of most of the Oestridae forbids such a proceeding.

{515}[Illustration: Fig. 245—_Cephalomyia maculata_, a Bot-fly of the camel. Arabia. A, The fly with extended wings; B, under aspect of the head: _a_, antenna; _b_, the obsolete mouth-parts.]

Some deposit their eggs on the hairs of the beasts from which the larvae are to draw their nutriment, but others place their larvae, already hatched, in the entrances of the nasal passages. They do not feed on the blood or tissues of their victims, but on the secretions, and these are generally altered or increased by the irritation induced by the presence of the unwelcome guests. It would appear, on the whole, that their presence is less injurious than would be expected, and as they always quit the bodies of their hosts for the purposes of pupation, a natural end is put to their attacks. We have ten species in Britain, the animals attacked being the ox, the horse, the ass, the sheep, and the red deer; others occasionally occur in connexion with animals in menageries. The eggs of _Gastrophilus equi_ are placed by the fly, when on the wing, on the hair of horses near the front parts of the body, frequently near the knee, and, after hatching, the young larvae pass into the stomach of the horse either by being licked off, or by their own locomotion; in the stomach they become hooked to the walls, and after being full grown pass out with the excreta: the Bots—as these larvae are called—are sometimes very numerous in the stomach, for a fly will lay as many as four or five hundred eggs on a single horse: in the case of weakly animals, perforation of the stomach has been known to occur in consequence of the habit of the Bot of burying itself to a greater or less extent in the walls of the stomach. _Hypoderma bovis_ and _H. lineata_ attack the ox, and the larvae cause tumours in the skin along the middle part of the back. It was formerly {516}inferred from this that the fly places its eggs in this situation, and as the cattle are known to dread and flee from the fly, it was supposed to be on account of the pain inflicted when the egg was thrust through the skin. Recent observations have shown that these views are erroneous, but much still remains to be ascertained. The details of oviposition are not yet fully known, but it appears that the eggs are laid on the lower parts of the body, especially near the heels, and that they hatch very speedily.[441] As the imago of _Hypoderma_ appears for only a very short period in the summer, the time of the oviposition is certain. The newly-disclosed larva is considerably different from the more advanced instar found in the skin of the back; moreover, a long period of many months intervenes between the hatching of the larva and its appearance in the part mentioned. Brauer has shown that when the grub is first found in that situation it is entirely subcutaneous. Hence it would be inferred that the newly-hatched larva penetrated the skin probably near the spot it was deposited on, and passed a period in subcutaneous wandering, on the whole going upwards till it arrived at the uppermost part: that after moulting, and in consequence of greater need for air, it then pierced the skin, and brought its breathing organs into contact with the external air; that the irritation caused by the admission of air induced a purulent secretion, and caused the larva to be enclosed in a capsule. Dr. Cooper Curtice has however found, in the oesophagus of cattle, larvae that he considers to be quite the same as those known to be the young of _Hypoderma_; and if this prove to be correct, his inference that the young larvae are licked up by the cattle and taken into the mouth becomes probable. The larva, according to this view, subsequently pierces the oesophagus and becomes subcutaneous by passing through the intervening tissues. The later history of the grub is briefly, that when full grown it somewhat enlarges the external orifice of its cyst, and by contractions and expansions of the body, passes to the surface, falls to the ground, buries itself and becomes a pupa. If Dr. Curtice be correct, there should, of course, be as many, if not more, larvae found in the oesophagus as in the back of the animal; but, so far as is known, this is not the case, and we shall not be surprised if the normal course of development be found different from what Dr. Curtice supposes it to be. His {517}observations relate to _Hypoderma lineata_. Our common British species is usually supposed to be _H. bovis_; but from recent observations it seems probable that most of the "Ox-warbles" of this country are really due to the larvae of _H. lineata_.

The history of _Oestrus ovis_, which attacks the sheep, is also incompletely known, but appears to be much simpler. This fly is viviparous, and deposits its young larvae at the entrance of the nasal passages of the sheep, thereby causing extreme annoyance to the animal. The larvae penetrate to the frontal sinuses to complete their growth. The duration of their lives is unknown, for it is commonly the case that larvae of various sizes are found together. _Cephenomyia rufibarbis_ has recently been found in Scotland. It attacks the Red deer, and its life-history is similar to that of _Oestrus ovis_, though the larvae apparently prefer to attain their full growth in the pharynx of the deer.

In reference to the Oestridae that attack man, we may merely mention that the larva of the _Hypoderma_ of the ox is occasionally found in Europe infesting human beings, but only as an extremely rare and exceptional event; and that only those engaged in attending on cattle are attacked; from which it is inferred that the flies are deceived by an odour emanating from the garments. In America numerous cases are known of Oestrid larvae being taken from the body of man, but information about them is very scanty. It appears, however, that there are at least four species, one of which, _Dermatobia noxialis_, is known as a fly as well as a larva. Whether any of these are peculiar to man is uncertain.[442] There are several larvae of Muscidae that have similar habits to the Oestridae; hence the statements that exist as to larvae being found in birds and reptiles cannot be considered to apply to members of the latter family until the larvae have been studied by an expert.

The family Ctenostylidae has been established by Bigot for a South American Insect, of which only a single individual exists in collections. It is doubtful whether it can be referred to Oestridae.[443]

SERIES V. PUPIPARA

The four families included in this Series are, with the exception of the Hippoboscidae, very little known. Most of {518}them live by sucking blood from Mammals and Birds, and sometimes they are wingless parasites. The single member of the family Braulidae lives on bees. The term Pupipara is erroneous, and it would be better to revert to Réaumur's prior appellation Nymphipara. Müggenburg has suggested that the division is not a natural one, the points of resemblance that exist between its members being probably the results of convergence. Recent discoveries as to the modes of bringing forth of Muscidae give additional force to this suggestion. A satisfactory definition of the group in its present extent seems impossible.

FAM. 40. HIPPOBOSCIDAE.—_Wings very variable, sometimes present and large, then with waved surface and thick nervures confined to the anterior and basal part; sometimes mere strips, sometimes entirely absent._ Certain members of this family are well known, the Forest-fly, or Horse-fly, and the Sheep-tick belonging to it. The proboscis is of peculiar formation, and not like that of other flies. Seen externally it consists of two elongate, closely adapted, hard flaps; these are capable of diverging laterally to allow an inner tube to be exserted from the head. The details and morphology of the structure have recently been discussed by Müggenburg.[444] _Melophagus ovinus_, commonly called the Sheep-tick, is formed for creeping about on the skin of the sheep beneath the wool, and may consequently be procured with ease at the period of sheep-shearing: it has no resemblance to a fly, and it is difficult to persuade the uninitiated that it is such. _Hippobosca equina_ (called in this country the Forest-fly, perhaps because it is better known in the New Forest than elsewhere), looks like a fly, but will be readily recognised by the two little cavities on the head, one close to each eye, in which the antennae are concealed, only the fine bristle projecting. Very little seems to be known as to the Natural History of this fly. _Lipoptena cervi_ lives on the Red deer; the perfect Insect has apparently a long life, and both sexes may be found in a wingless state on the deer all through the winter. When first disclosed in the summer they are however provided with wings, but when they have found a suitable host they bite off, or cast, the wings. The female, it appears, does this more promptly than the male, so that it is difficult to get winged individuals of the former sex.[445]

{519}[Illustration: Fig. 246.—Diagrammatic section of the larva of _Melophagus ovinus_. (After Pratt.) _a_, mouth; _b_, suctorial pouch; _c_, imaginal disc for adult head; _d_, meso- and meta-notal discs; _e_, anterior tracheal anastomosis; _f_, first muscular belt; _g_, transverse tracheal branch; _h_, the dorsal tracheal tube; _i_, sex-organ; _k_, Malpighian tube; _l_, terminal part of intestine; _m_, terminal chamber of tracheal tube; _n_, stigmatic fossa; _o_, terminal part of intestine; _p_, anus; _q_, anal disc; _r_, ventral tracheal tube; _s_, stomach; _t_, nervous system; _u_, discs for the three pairs of legs of the imago; _v_, ventral pouch; _w_, pharynx; _x_, suctorial lip.]

Most of the known Hippoboscidae live on birds, and are apparently specially fond of the Swallow tribe. They are all winged, though in some species the wings are very small. The bird-infesting Hippoboscidae have been very little studied, and will probably form a distinct family; the antennae of _Stenopteryx hirundinis_ are quite different from those of _Hippobosca_. The development is remarkable, and has been studied by Leuckart[446] and by Pratt[447] in the case of _Melophagus ovinus_. The ovaries are peculiarly formed, and produce one large egg at a time; this passes into the dilated oviduct, and there goes through its full growth and a certain amount of development; it is then extruded, and undergoing little or no change of form becomes externally hardened by the excretion of chitin, passing thus into the condition of the Eumyiid pupa. Dufour thought that there is no larval stage in this Insect, but it is quite clear from later researches that he was wrong, and that a larval stage of a peculiar kind, but in some respects resembling that of the Eumyiid Muscidae, occurs. The larva has no true head, but the anterior part of the body is invaginated, and the most anterior part again protrudes in the invagination, so that two little passages appear on section (Fig. 246); the upper one leads to the stomach, which is of very large size. The tracheal system is peculiar; it is metapneustic, there being neither anterior nor lateral spiracles. Pratt says that there is at first a single pair of terminal spiracles, and subsequently three pairs, hence he considers that the terminal part of the body corresponds to three segments. This is however probably a mistaken view; it appears more probable that the so-called three pairs of stigmata really correspond with the complex {520}condition of the stigmata in the later instars of certain other Dipterous larvae. The _Melophagus_-larva is nourished by secretion from certain glands of the mother-fly; this is swallowed and the stomach is greatly distended by this milky fluid. Probably it was this condition that induced Dufour to suppose the larva to be only an embryo.

Some of the Hippoboscidae that live on birds take to the wing with great readiness, and it is probable that these bird-parasites will prove more numerous than is at present suspected.

We may here notice an animal recently described by Dr. Adensamer and called _Ascodipteron_.[448] He treats it as the female imago of a Pupiparous Dipteron. It was found buried in the skin of the wing of a bat of the genus _Phyllorhina_, in the Dutch East Indies, only one individual being known. It is entirely unsegmented, and externally without head. If Dr. Adensamer should prove to be correct in his surmise the creature can scarcely be inferior in interest to the Strepsiptera.

[Illustration: Fig. 247.—_Braula coeca._ × 18/1. (After Meinert.)]

FAM. 41. BRAULIDAE.—This consists only of a minute Insect that lives on bees. The antennae are somewhat like those of the sheep-tick, though they are not so completely concealed in the cavities in which they are inserted. According to Müggenburg[449] a ptilinum exists, and he is also of opinion that although the parts of the mouth differ very much from those of Hippoboscidae they are essentially similar. Lucas says that _Braula_ specially affects the thorax of the bee: Müggenburg, that it is fond of the queen-bee because of the exposed membranes between the body-segments that exist in that sex. Whether this Insect is truly Pupiparous is unknown, though Boise states that a pupa is deposited in the cell of the bee by the side of the young larva of {521}the bee, and appears as the perfect Insect in about twenty-one days. Müggenburg suggests that _Braula_ may be oviparous, as he has never found a larva in the abdomen. Packard says that on the day the larva hatches from the egg it sheds its skin and turns to an oval puparium of a dark brown colour. The Insect is frequently though inappropriately called bee-louse; notwithstanding its name it is not quite blind, though the eyes are very imperfect.

FAM. 42. STREBLIDAE.— _Winged; possessing halteres; the head small, narrow and free._ These very rare Diptera are altogether problematic. According to Kolenati the larvae live in bats' excrement and the perfect Insects on the bats.[450] If the former statement be correct the Insects can scarcely prove to be Pupipara. The wing-nervuration is, in the figures of the Russian author, quite different from that of Hippoboscidae. The Streblidae have been associated by some entomologists with Nycteribiidae, and by Williston with Hippoboscidae.

[Illustration: Fig. 248.—_Nycteribia_ sp., from _Xantharpyia straminea_. Aden. A, Upper surface of female, with head in the position of repose; B, under surface of male. x 12/1.]

FAMILY 43. NYCTERIBIIDAE.—The species of this family are found on bats; they are apparently rare, and we have been able to examine only one species. The form is very peculiar, the {522}Insects looking as if the upper were the under surface. They are wingless, with a narrow head, which reposes on the back of the thorax. The prothorax appears to be seated on the dorsum of the mesothorax. According to Müggenburg there is no trace of a ptilinum. A brief note on the metamorphosis[451] by Baron Osten Sacken indicates that the mature larva differs from that of _Melophagus_ in the arrangement of the stigmata; they appear to be dorsal instead of terminal. There are apparently no characters of sufficient importance to justify the association of these Insects with the other divisions of Pupipara; the sole ground for this connection being the supposed nature of the life-history of the larva.

[Illustration: Fig. 249—Anterior part of the body of _Nycteribia_ sp., found on _Xantharpyia straminea_ by Colonel Yerbury at Aden. A, Upper surface of female, with head extended; B, under surface of male, with head extended; C, claws of a foot.]

SUB-ORDER APHANIPTERA or SIPHONAPTERA (_Fleas_)

FAM. PULICIDAE.—_Wingless, with the body laterally compressed, so that the transverse diameter is small, the vertical one great. The head indistinctly separated from the body, small, with short thick antennae placed in depressions somewhat behind and above the unfaceted eyes. These are always minute, and sometimes wanting._

{523}[Illustration: Fig. 250—_Hystrichopsylla talpae._ Britain. (After Ritsema.)]

[Illustration: Fig. 251.—Mouth-parts of a flea, _Vermipsylla alakurt_ ♂. _H._ Unpaired pricking organ; _Lp._ labial palp; _Md._ mandible; _Mx._ maxilla; _Mxp._ maxillary palp. (After Wagner.)]

We all know that the Flea is so flat, or compressed sideways, that it does not mind the most severe squeeze. This condition is almost peculiar to it; a great flattening of the body is common in Insects—as is seen in another annoying Insect, the bed-bug—but the compression, in the flea, is in the reverse direction. In other respects the external anatomy of the flea shows several peculiarities, the morphological import of which has not yet been elucidated. The head is of very peculiar shape, small, with the antennae placed in an unusual position; the clypeus is said to be entirely absent, the front legs are articulated in such a manner that they have a large additional basal piece—called by some anatomists the ischium—and in consequence appear to be placed far forwards, looking as if they were attached to the head; the meso- and meta-thorax have certain flaps that have been considered to be homologues of wings; and the maxillary palpi are attached to the head in such a way that they appear to play the part of the antennae of other Insects (Fig. 250), and were actually considered to be the antennae by Linnaeus, as well as others; the mouth-parts themselves are differently constructed from those of any other Insects.[452] The maxillae and labium are considered to be not only present, but well developed, the former possessing palpi moderately well developed, while the labial palps are very large and of highly peculiar form, being imperfectly transversely jointed and acting as sheaths; the mandibles are present in the form of a pair of elongate, slender organs, with serrated edges; and there is an unpaired, elongate pricking-organ, thought by some to be a hypopharynx, and by others a labrum.

{524}[Illustration: Fig. 252—Larva of _Pulex serraticeps_, the dog- and cat-flea. (After Künckel.)]

The antennae are of unusual form, consisting of two basal joints, and, loosely connected therewith, a terminal mass of diverse form and more or less distinctly, though irregularly, segmented. The full number of ten stigmata exists, Wagner giving three thoracic, with seven abdominal, placed on segments 2-8 of the abdomen; but Packard thinks the supposed metathoracic stigma is really the first abdominal. Fleas undergo a very complete metamorphosis; the larvae are wormlike, resembling those of Mycetophilid Diptera (Fig. 252). The egg of the cat's flea is deposited among the fur of the animal, but (unlike the eggs of other parasites) apparently is not fastened to the hair, for the eggs fall freely to the ground from infested animals; the young larva when hatched bears on the head a curious structure for breaking the egg-shell. It has the mouth-parts of a mandibulate Insect and is peripneustic, having ten pairs of stigmata. It subsequently becomes of less elongate form. Flea-larvae are able to nourish themselves on almost any kind of refuse animal matter, Laboulbène having reared them on the sweepings of apartments; they may perhaps sometimes feed on blood; at any rate the contents of the alimentary canal appear red through the transparent integuments. When full grown the larva makes a cocoon, and frequently covers it with pieces of dust. The perfect flea appears in a week or two thereafter; the pupa has the members free. The food of the larvae of fleas has been much discussed and a variety of statements made on the subject. It has been stated that the mother-flea after being gorged with blood carries some of it to the young, but Künckel has shown that there is very little foundation for this tale. Enormous numbers of fleas are sometimes found in uninhabited apartments to which animals have previously had access, and these fleas will attack in numbers and with great eagerness any unfortunate person who may enter {525}the apartment. The cat-flea can pass through its growth and metamorphosis with excessive rapidity, the entire development of a generation in favourable conditions extending but little beyond a fortnight.[453]

About a hundred kinds of fleas are known, all of which live on mammals or birds. _Hystrichopsylla talpae_ (Fig. 250) is one of the largest, it occurs on the Mole. It was found by Ritsema in the nests of _Bombus subterraneus_ (and was described under the name of _Pulex obtusiceps_). As these nests are known to be harried by Voles, and as this flea has also been found on Field-mice, it is probable that the parasites are carried to the nests by the Voles. The species that chiefly infests man is _Pulex irritans_, an Insect that is nearly cosmopolitan, though arid desert regions are apparently unsuitable to it. _Pulex avium_ occurs on a great variety of birds. _P. serraticeps_ infests the dog and the cat, as well as a variety of other Mammals. It is a common opinion that each species of Mammal has its own peculiar flea, but this is far from correct. Fleas pass readily from one species of animal to another; the writer formerly possessed a cat that was a most determined and successful hunter of rabbits, and she frequently returned from her excursions swarming with fleas that she had become infested with when in the rabbits' burrows; her ears were on some occasions very sore from the flea-bites. Some of the fleas of other animals undoubtedly bite man. There appears, however, to be much difference in the liability of different individuals of our own species to the bites of fleas. _Sarcopsylla penetrans_ differs in habits from other fleas, as the female buries the anterior parts of her body in the flesh of man or other Vertebrates, and the abdomen then becomes enormously enlarged and distended and undergoes a series of changes that are of much interest.[454] While in this position the Insect discharges a number of eggs. This species multiplies sufficiently to become a serious pest in certain regions, the body of one man having been known to be affording hospitality to 300 of these fleas. _Sarcopsylla penetrans_ is known as the Sand-flea, or chigger, and by numerous other names. Originally a native of tropical America it has been carried to other parts of the world. Another _Sarcopsylla_, _S. gallinacea_, attaches itself to the eyelids {526}of the domestic fowl in Ceylon, and an allied form, _Rhynchopsylla pulex_, fastens itself to the eyelids and other parts of the body of birds and bats in South America. In Turkestan _Vermipsylla alakurt_ attacks cattle—ox, horse, camel, sheep—fastening itself to the body of the animal after the fashion of a tick. Retaining this position all through the winter, it becomes distended somewhat after the manner of the Sand-flea, though it never forms a spherical body. The parts of the mouth in this Insect (Fig. 251) are unusually long, correlative with the thickness of the skins of the animals on which it lives. Grassi considers that the dog's flea, _Pulex serraticeps_, acts as the intermediate host of Taenia.

Great difference of opinion has for long prevailed as to whether fleas should be treated as a Sub-Order of Diptera or as a separate Order of Insects. Wagner and Künckel, who have recently discussed the question, think they may pass as aberrant Diptera, while Packard,[455] the last writer on the subject, prefers to consider them a separate Order more closely allied to Diptera than to any other Insects. Although widely known as Aphaniptera, several writers call them Siphonaptera, because Latreille proposed that name for them some years before Kirby called them Aphaniptera. Meinert considers them a separate Order and calls it Suctoria, a most unfortunate name.

ORDER VIII. THYSANOPTERA.

_Small Insects, with a palpigerous mouth placed on the under side of the head and apposed to the sternum so as to be concealed. With four slender wings, fringed with long hairs on one or both margins, or with rudiments of wings, or entirely apterous. Tarsi of one or two joints, terminated by a vesicular structure. The young resemble the adult in general form, but there is a pupal stadium in which the Insect is quiescent and takes no food._

The tiny Insects called Thrips are extremely abundant and may often be found in profusion in flowers. Their size is only from 1/50 to ⅓ of an inch in length; those of the latter magnitude are in fact giant species, and so far as we know at present are found only in Australia (Fig. 253). As regards the extent {527}of the Order it would appear that Thysanoptera are insignificant, as less than 150 species are known. Thrips have been, however, very much neglected by entomologists, so it will not be a matter for surprise if there should prove to be several thousand species. These Insects present several points of interest; their mouth-organs are unique in structure; besides this, they exhibit so many points of dissimilarity from other Insects that it is impossible to treat them as subdivisions of any other Order. They have, however, been considered by some to be aberrant Pseudoneuroptera (cf. Vol. V.), while others have associated them with Hemiptera. Both Brauer and Packard have treated Thysanoptera as a separate Order, and there can be no doubt that this is correct. Thysanoptera have recently been monographed by Uzel in a work that is, unfortunately for most of us, in the Bohemian language.[456]

[Illustration: Fig. 253—_Idolothrips spectrum._ Australia.]

The antennae are never very long, and are 6 to 9-jointed. The head varies much, being sometimes elongate and tubular, but sometimes short; it has, however, always the peculiarity that the antennae are placed quite on its front part, and that the mouth appears to be absent, owing to its parts being thrust against the under side of the thorax and concealed. Their most remarkable peculiarity is that some of them are asymmetrical: Uzel looks on the peculiar structure, the "Mundstachel," _m_, _m_ {528}(Fig. 254) found on the left side of the body, as probably an enormous development of the epipharynx. Previous to the appearance of Uzel's work, Garman had, however, correctly described the structure of the mouth;[457] he puts a different interpretation on the parts; he points out that the mandibles (_j_), so-called by Uzel, are attached to the maxillae, and he considers that they are really jointed, and that they are lobes thereof; while the Mundstachel or piercer is, he considers, the left mandible; the corresponding structure of the other side being nearly entirely absent. He points out that the labrum and endocranium are also asymmetrical. We think Garman's view a reasonable one, and may remark that dissimilarity of the mandibles of the two sides is usual in Insects, and that the mandibles may be hollow for sucking, as is shown by the larvae of Hemerobiides. There are usually three ocelli, but they are absent in the entirely apterous forms.

[Illustration: Fig. 254—Face (with base of the antennae) of _Aeolothrips fasciata_. (After Uzel.) _a_, Labrum; _b_, maxilla with its palp (_c_); _bl_, terminal part of vertex near attachment of month-parts; _d_, membrane between maxilla and mentum; _e_, mentum ending in a point near _f_; _g_, membrane of attachment of the labial palp _h_; _i_, ligula; _j_, _j_ the bristle-like mandibles; _k_, the thicker base of mandible; _l_, chitinous lever; _m_, mouth-spine, with its thick basal part _n_, and _o_, its connection with the forehead, _r_, _r_; _p_, foramen of muscle; _s_ and _t_, points of infolding of vertex; _u_, a prolongation of the gena.]

The wings appear to spring from the dorsal surface of the body, not from the sides; the anterior pair is always quite separated from the posterior; the wings are always slender, sometimes very slender; in other respects they exhibit considerable variety; sometimes the front pair are different in colour and consistence from the other pair. The abdomen has ten segments, the last of which is often tubular in form. The peculiar vesicular structures by which the feet are terminated are, during movement, alternately distended and emptied, and have two hooks or claws on the sides. The stigmata are extremely peculiar, there being four pairs, the first being the mesothoracic, 2nd {529}metathoracic, 3rd on the second abdominal segment, 4th on the eighth abdominal segment.[458] There are four Malpighian tubes, and two or three pairs of salivary glands. The dorsal vessel is said to be a short sack placed in the 7th and 8th abdominal segments. The abdominal ganglia of the ventral chain are concentrated in a single mass, placed in, or close to, the thorax; the thorax has two other approximated ganglia, as well as an anterior one that appears to be the infra-oesophageal.

The metamorphosis is also peculiar; the larva does not differ greatly in appearance from the adult, and has similar mouth-organs and food-habits. The wings are developed outside the body at the sides, and appear first, according to Heeger, after the third moult. The nymph-condition is like that of a pupa inasmuch as no nourishment is taken, and the parts of the body are enclosed in a skin: in some species there is power of movement to a slight degree, but other species are quite motionless. In some cases the body is entirely bright red, though subsequently there is no trace of this colour. Jordan distinguishes two nymphal periods, the first of which he calls the pronymphal; in it the Insect appears to be in a condition intermediate between that of the larva and that of the true nymph; the old cuticle being retained, though the hypodermis is detached from it and forms a fresh cuticle beneath it. This condition, as Jordan remarks, seems parallel to that of the male Coccid, and approaches closely to complete metamorphosis; indeed the only characters by which the two can be distinguished appear to be (1) that the young has not a special form; (2) that the wings are developed outside the body.

Thrips take their food, it is believed, in the same manner as Aphidae, by suction; but the details of the process are not by any means certain, and examination of the stomach is said to have resulted in finding pollen therein. Walsh thought that Thysanoptera pierce and suck Aphidae. An elaborate inquiry by Osborn[459] failed to elicit satisfactory confirmation of Walsh's idea, though Riley and Pergande support it to some extent; Osborn concludes that the ordinary food is not drawn directly from sap, but consists of exudation or pollen, the tissues {530}of the plant being pierced only when a supply of food from the usual sources falls short. Members of this family have been reputed as being very injurious to cultivated plants, especially to cereals, and it is said that as a result the harvests in Europe have been seriously diminished. Several species may take part in the attacks. These appear to be directed chiefly against the inflorescence. Lindeman thought that _Limothrips denticornis_ (= _Thrips secalina_), and _Anthothrips aculeata_ (= _Phloeothrips frumentarius_), were the most destructive species in an attack of Thrips on corn that he investigated in Russia. Uzel suggests that injuries due to other causes are sometimes ascribed to Thrips.[460] In hot-houses these Insects are well known, and sometimes occasion considerable damage to foliage. The German horticulturalists call them black-fly, in distinction from Aphidae or green-fly. Some Thysanoptera live under bark, and even in fungi, and in Australia they form galls on the leaves of trees. This observation is due to Mr Froggatt, and is confirmed by specimens he sent to the writer. Vesicular bodies in the leaves of _Acacia saligna_ were traversed on one side by a longitudinal slit, and on a section being made, nothing but Thrips, in various stages of growth, was found inside them. A second kind of gall, forming masses of considerable size on the twigs of _Callistemon_, is said by Mr Froggatt to be also due to Thrips, as is a third kind on _Bursaria spinosa_. It is curious that Thrips' galls have not been observed in other parts of the world.

Thysanoptera are devoured by small bugs of the genus _Triphleps_, as well as by beetles; a small Acarid attacks them by fixing itself to the body of the Thrips. Nematode worms and their eggs were found by Uzel in the body-cavity. He found no less than 200 Nematodes in one Thrips, and noticed that they had entirely destroyed the ovaries. Woodpeckers, according to him, tear off the bark of trees and eat the Thysanoptera that are concealed thereunder, though one would have surmised that these minute Insects are too small to be game for such birds. They have, it appears, no special protection, except that one species (a larva of _Phloeothrips_ sp.) is said to emit a protective fluid.

Parthenogenesis seems to be frequent amongst Thysanoptera, {531}and is found in concurrence with diversity as to winged and wingless females of the same species, so as to have given rise to the idea that the phenomena in this respect are parallel with those that are more widely known as occurring in Aphidae. Under certain circumstances few or no males are produced (one of the circumstances, according to Jordan, being season of the year), and the females continue the species parthenogenetically. In other cases, though males are produced they are in very small numbers. Some species of Thysanoptera are never winged; in others the individuals are winged or wingless according to sex. But there are other cases in which the female is usually wingless, and is exceptionally winged. The winged specimens in this case are, it is thought, of special use in disseminating the species. Jordan has suggested that these phenomena may be of a regular nature, but Uzel does not take this view. Another condition may be mentioned, in which the species is usually wingless, but winged individuals of the male as well as of the female sex occasionally appear. _Thrips lini_ apparently makes regular migrations, feeding at one time underground on the roots of flax, and then changing to a life in the open air on other plants.

Numerous forms of Thysanoptera, belonging to both of the great divisions of the Order, have been found fossil in Europe and North America, but all are confined to deposits of the Tertiary epoch.

Of the 135 species known to Uzel, 117 are European; they are divided into two Sub-Orders. 1, Terebrantia, in which the females are provided with an external toothed ovipositor, of two valves; 2, Tubulifera, in which there is no ovipositor, and the extremity of the body is tubular in both sexes. The British species are about 50 in number, and were described by Haliday about 60 years ago;[461] of late they have been very little studied.

The name Physopoda or Physapoda is used for this Order, instead of Thysanoptera, by several naturalists.

{532}