Part 19

_Circulatory System._--Insects afford an excellent illustration of the remarkable type of blood-system characterizing the Arthropoda. The dorsal vessel is an elongate tube, whose abdominal portion is usually chambered, forming a contractile heart (fig. 10). At the constrictions between the chambers are paired slits, through which the blood passes from the surrounding pericardial sinus. The dorsal vessel is prolonged anteriorly into an aorta, through which the blood is propelled into the great body-cavity or haemocoel. After bathing the various tissues and organs, the blood returns dorsalwards into the pericardial sinus through fine perforations of its floor, and so makes its way into the heart again. Some water-bugs, e.g. of the families _Belostomatidae_, _Nepidae_, _Corixidae_ and _Hydrometridae_ have a pulsating sac at each knee-joint to assist the flow of blood through the legs, while in dragon-flies and locusts (_Acridiidae_) there is a ventral pulsating diaphragm, which forms the roof of a sinus enclosing the nerve-cords.

[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 11.--Ventral Portion of Air-Tubes in Cockroach.]

_Respiratory System._--As mentioned above, respiration by means of air-tubes (tracheae) is a most characteristic feature of the Hexapoda. An air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and a chitinous layer internally, the last-named being continuous with the outer cuticle. The chitinous layer is usually strengthened by thread-like thickenings which, in the region close to the outer opening of the tube, form a network enclosing polygonal areas, but which, through most of the tracheal system, are arranged spirally, the strengthening thread not forming a continuous spiral, but being interrupted after a few turns around the tube. The tracheal system in Hexapods is very complex, forming a series of longitudinal trunks with transverse anastomosing connexions (fig. 11), and extending by the finest sub-division and by repeated branching into all parts of the body. In insects of active flight the tubes swell out into numerous air-sacs, by which the breathing capacity is much increased.

Atmospheric air gains access to the air-tubes through paired _spiracles_ or _stigmata_, which usually occur laterally on most of the body-segments. These spiracles have firm chitinous edges, and can be closed by valves moved by special muscles. When the spiracles are open and the body contracts, air is expired. The subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place. The physiology of respiration has been carefully studied by F. Plateau (1884). In aquatic insects various devices for obtaining or entangling air are found; these modifications are described in the special articles on the various orders of insects (COLEOPTERA, HEMIPTERA, &c.). Many insects have aquatic larvae, some of which take in atmospheric air at intervals, while others breathe dissolved air by means of tracheal gills. These modifications are mentioned below in the section on metamorphosis.

[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 12.--Food Canal of Cockroach.

s, Salivary glands and reservoir. c, Crop (the gizzard below it). coe, Caecal tubes (below them the stomach). k, Kidney tubes. i, Intestine. r, Rectum.]

_Digestive System._--A striking feature in the food-canal of the Hexapoda, as in other Arthropods, is the great extent of the "fore-gut" and "hind-gut," lined with a chitinous cuticle, continuous with the exoskeleton. The fore-gut is composed of a tubular gullet, a large sac-like crop (fig. 12, c) and a proventriculus or "gizzard," whose function is to strain the food-substances before they pass on into the tubular stomach, which has no chitinous lining. This organ, usually regarded as a "mid-gut," gives off a number of secretory caecal tubes (fig. 12, coe). At its hinder end it is continuous with the hind-gut, which is usually differentiated into a tubular coiled intestine (fig. 12, i) and a swollen rectum (fig. 12, r). From the fore-end of the hind-gut arise the slender Malpighian tubes (fig. 12, k), which have a renal function.

On either side of the gullet are from one to ten pairs of salivary glands (fig. 12, s) whose ducts open into the mouth. Some of these glands may be modified for special purposes--as silk-producing glands in caterpillars or as poison-glands in blood-sucking flies and bugs. The food passing into the crop is there acted on by the saliva and also by an acid gastric juice which passes forwards from the stomach through the proventriculus. As the various portions of the food undergo digestion, they are allowed to pass through the proventriculus into the stomach, where the nutrient substances are absorbed.

_Excretory System._--Nitrogenous waste-matter is removed from the body by the Malpighian tubes which open into the food-canal, usually where the hind-gut joins the stomach. These tubes vary in number from four to over a hundred in different orders of insects. The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding body-cavity. This cavity contains an irregular mass of whitish tissue, the fat-body, consisting of fat-cells which undergo degradation and become more or less filled with urates. When the worn-out cells are broken down, the urates are carried dissolved in the blood to the Malpighian tubes for excretion. The fat-body is therefore the seat of important metabolic processes in the hexapod body.

_Reproductive System._--All the Hexapoda are of separate sexes. The ovaries (fig. 13) in the female are paired, each ovary consisting of a variable number of tubes (one in the bristle-tail _Campodea_ and fifteen hundred in a queen termite) in which the eggs are developed. From each ovary an oviduct (fig. 13, od) leads, and in some of the more primitive insects (bristle-tails, earwigs, may-flies) the two oviducts open separately direct to the exterior. Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin. The vagina usually opens in front of the eighth abdominal sternite. Behind it is situated a spermatheca (fig. 14, sp) and the ovipositor previously mentioned, with its three pairs of processes (Fig. 14, G, g).

[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 13.--Ovaries of Cockroach, with Oviducts Od and Colleterial Glands CG.]

[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 14.--Hinder Abdominal Segment and Ovipositor of Female Cockroach. Magnified.

T^8 &c. Tergites. S^7, 7th Sternite. S^8, Sclerite between 7th and 8th sterna. S^9, 8th Sclerite. Od, Vagina. sp, Spermatheca. G, Anterior, and g, posterior gonapophyses.]

The paired testes of the male consist of a variable number of seminal tubes, those of each testis opening into a _vas deferens_. In some bristle-tails and may-flies, the two _vasa deferentia_ open separately, but usually they lead into a sperm-reservoir, whence issues a median ejaculatory duet. The male opening is on the ninth abdominal segment, to which belong the processes that form the claspers or genital armature. Accessory glands are commonly present in connexion both with the male and the female reproductive organs. The poison-glands of the sting in wasps and bees are well-known examples of these.

EMBRYOLOGY

_The Egg._--Among the Hexapoda, as in Arthropods generally, the egg is large, containing an accumulation of yolk for the nourishment of the growing embryo. Most insect eggs are of an elongate oval shape; some are globular, others flattened, while others again are flask-shaped, and the outer envelope (_chorion_) is often beautifully sculptured (figs. 20, d; 21, a, b). Various devices are adopted for the protection of the eggs from mechanical injury or from the attacks of enemies, and for fixing them in appropriate situations. For example, the egg may be raised above the surface on which it is laid by an elongate stalk; the eggs may be protected by a secretion, which in some cases forms a hard protective capsule or "purse"; or they may be covered with shed hairs of the mother, while among water-insects a gelatinous envelope, often of rope-like form, is common. In various groups of the Hexapoda--aphids and some flesh-flies (_Sarcophaga_), for example--the egg undergoes development within the body of the mother, and the young insect is born in an active state; such insects are said to be "viviparous."

_Parthenogenesis._--A number of cases are known among the Hexapoda of the development of young from the eggs of virgin females. In insects so widely separated as bristle-tails and moths this occurs occasionally. In certain gall-flies (_Cynipidae_) no males are known to exist at all, and the species seems to be preserved entirely by successive parthenogenetic generations. In other gall-flies and in aphids we find that a sexual generation alternates with one or with many virgin generations. The offspring of the virgin females are in most of these instances females; but among the bees and wasps parthenogenesis occurs normally and always results in the development of males, the "queen" insect laying either a fertilized or unfertilized egg at will.

_Maturation, Fertilization and Segmentation._--Polar bodies were first observed in the eggs of Hexapoda by F. Blochmann in 1887. The two nuclei are successively divided from the egg nucleus in the usual way, but they frequently become absorbed in the peripheral protoplasm instead of being extruded from the egg-cell altogether. It appears that in parthenogenetic eggs two polar nuclei are formed. According to A. Petrunkevich (1901-1903), the second polar nucleus uniting with one daughter-nucleus of the first polar body gives rise to the germ-cells of the parthenogenetically-produced male. There is no reunion of the second polar nucleus with the female pronucleus, but, according to the recent work of L. Doncaster (1906-1907) on the eggs of sawflies, the number of chromosomes is not reduced in parthenogenetic egg-nuclei, while, in eggs capable of fertilization, the usual reduction-divisions occur. Fertilization takes place as the egg is laid, the spermatozoa being ejected from the spermatheca of the female and making their way to the protoplasm of the egg through openings (micropyles) in its firm envelope. The segmentation of the fertilized nucleus results in the formation of a number of nuclei which arrange themselves around the periphery of the egg and, the protoplasm surrounding them becoming constricted, a blastoderm or layer of cells, enclosing the central yolk, is formed. Within the yolk the nuclei of some "yolk cells" can be distinguished.

[Illustration: From Nussbaum in Miall and Denny's, _Cockroach_, Lovell, Reeve & Co.

FIG. 15.--Diagram showing Formation of Germinal Layers. E, ectoderm; M, inner layer. Magnified.]

_Germinal Layers and Food-Canal._--The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. Along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15). In some cases the inner layer is formed not by invagination but by proliferation or by delamination. The outer of these two layers (fig. 15, E) is the ectoderm. With regard to the inner layer (_endoblast_ of some authors, fig. 15, M) much difference of opinion has prevailed. It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula. No doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it. Further, there is general agreement that the chitin-lined fore-gut and hind-gut, which form the greater part of the digestive tract, arise from ectodermal invaginations (stomodaeum and proctodaeum respectively) at the positions of the future mouth and anus. The origin of the mid-gut (mesenteron), that has no chitinous lining in the developed insect, is the disputed point. According to the classical researches of A. Kowalevsky (1871 and 1887) on the embryology of the water-beetle _Hydrophilus_ and of the muscid flies, an anterior and a posterior endoderm-rudiment both derived from the "endoblast" become apparent at an early stage, in close association with the stomodaeum and the proctodaeum respectively. These two endoderm-rudiments ultimately grow together and give rise to the epithelium of the mid-gut. These results were confirmed by the observations of K. Heider and W. M. Wheeler (1889) on the embryos of two beetles--_Hydrophilus_ and _Doryphora_ respectively. V. Graber, however (1889), stated that in the _Muscidae_, while the anterior endoderm-rudiment arises as Kowalevsky had observed, the posterior part of the "mid-gut" has its origin as a direct outgrowth from the proctodaeum. The recent researches of R. Heymons (1895) on the Orthoptera, and of A. Lecaillon (1898) on various leaf beetles, tend to show that the whole of the "mid-gut" arises from the proliferation of cells at the extremity of the stomodaeum and of the proctodaeum. On this view the entire food-canal in most Hexapoda must be regarded as of ectodermal origin, the "endoblast" represents mesoderm only, and the median furrow whence it arises can be no longer compared with the blastopore. According to Heymons, the yolk-cells must be regarded as the true endoderm in the hexapod embryo, for he states (1897) that in the bristle-tail _Lepisma_ and in dragon-flies they give rise to the mid-gut. These views are not, however, supported by other recent observers. J. Carriere's researches (1897) on the embryology of the mason bee (_Chalicodoma_) agree entirely with the interpretations of Kowalevsky and Heider, and so on the whole do those of F. Schwangart, who has studied (1904) the embryonic development of Lepidoptera. He finds that the endoderm arises from an anterior and a posterior rudiment derived from the "endoblast," that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. K. Escherich (1901), after a new research on the embryology of the muscid Diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. On the whole it seems likely that the endoderm is represented in part by the yolk, and in part by those anterior and posterior rudiments which usually form the mesenteron, but that in some Hexapoda the whole digestive tract may be ectodermal. It must be admitted that some or the later work on insect embryology has justified the growing scepticism in the universal applicability of the "germ-layer theory." Heider has suggested, however, that the apparent origin of the mid-gut from the stomodaeum and proctodaeum may be explained by the presence of a "latent endoderm-group" in those invaginations.

[Illustration: From Nussbaum in Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 16.--Cross section of Embryo of German Cockroach (_Phyllodromia_). S, serosa; A, amnion; E, ectoderm; N, rudiment of nerve-cord; M, mesodermal pouches.]

_Embryonic Membranes._--A remarkable feature in the embryonic development of most Hexapoda is the formation of a protective membrane analogous to the amnion of higher Vertebrates and known by the same term. Usually there arises around the edge of the germ band a double fold in the undifferentiated blastoderm, which grows over the surface of the embryo, so that its inner and outer layers become continuous, forming respectively the _amnion_ and the _serosa_ (fig. 16, A, S). The embryo of a moth, a dragon-fly or a bug is invaginated into the yolk at the head end, the portion of the blastoderm necessarily pushed in with it forming the amnion. The embryo thus becomes transferred to the dorsal face of the egg, but at a later stage it undergoes reversion to its original ventral position. In some parasitic Hymenoptera there is only a single embryonic membrane formed by delamination from the blastoderm, while in a few insects, including the wingless spring-tails, the embryonic membranes are vestigial or entirely wanting. In the bristle-tails _Lepisma_ and _Machilis_, an interesting transitional condition of the embryonic membranes has lately been shown by Heymons. The embryo is invaginated into the yolk, but the surface edges of the blastoderm do not close over, so that a groove or pore puts the insunken space that represents the amniotic cavity into communication with the outside. Heymons believes that the "dorsal organ" in the embryos of the lower Arthropoda corresponds with the region invaginated to form the serosa of the hexapod embryo. Wheeler, however, compares with the "dorsal organ" the peculiar extra embryonic membrane or indusium which he has observed between serosa and amnion in the embryo of the grasshopper _Xiphidium_.

_Metameric Segmentation._--The segments are perceptible at a very early stage of the development as a number of transverse bands arranged in a linear sequence. The first segmentation of the ventral plate is not, however, very definite, and the segmentation does not make its appearance simultaneously throughout the whole length of the plate; the anterior parts are segmented before the posterior. In Orthoptera and Thysanura, as well as some others of the lower insects, twenty-one of these divisions--not, however, all similar--may be readily distinguished, six of which subsequently enter into the formation of the head, three going to the thorax and twelve to the abdomen. In Hemiptera only eleven and in Collembola only six abdominal segments have been detected. The first and last of these twenty-one divisions are so different from the others that they can scarcely be considered true segments.

_Head Segments._--In the adult insect the head is insignificant in size compared with the thorax or abdomen, but in the embryo it forms a much larger portion of the body than it does in the adult. Its composition has been the subject of prolonged difference of opinion. Formerly it was said that the head consisted of four divisions, viz. three segments and the procephalic or prae-oral lobes. It is now ascertained that the procephalic lobes consist of three divisions, so that the head must certainly be formed from at least six segments. The first of these, according to the nomenclature of Heymons (see fig. 17), is the mouth or oral piece; the second, the antennal segment; the third, the intercalary or prae-mandibular segment; while the fourth, fifth, and sixth are respectively the segments of the mandibles and of the first and second maxillae. These six divisions of the head are diverse in kind, and subsequently undergo so much change that the part each of them takes in the formation of the head-capsule is not finally determined. The labrum and clypeus are developed as a single prolongation of the oral piece, not as a pair of appendages. The antennal segment apparently entirely disappears, with the exception of a pair of appendages it bears; these become the antennae; it is possible that the original segment, or some part of it, may even become a portion of the actual antennae. The intercalary segment has no appendages, nor rudiments thereof, except, according to H. Uzel (1897), in the thysanuran _Campodea_, and probably entirely disappears, though J. H. Comstock and C. Kochi believe that the labrum belongs to it. The appendages of the posterior three or trophal segments become the parts of the mouth. The appendages of the two maxillary segments arise as treble instead of single projections, thus differing from other appendages. From these facts it appears that the anterior three divisions of the head differ strongly from the posterior three, which greatly resemble thoracic segments; hence it has been thought possible that the anterior divisions may represent a primitive head, to which three segments and their leg-like appendages were subsequently added to form the head as it now exists. This is, however, very doubtful, and an entirely different inference is possible. Besides the five limb-bearing somites just enumerated, two others must now be recognized in the head. One of these is the ocular segment, in front of the antennal, and behind the primitive pre-oral segment. The other is the segment of the maxillulae (see above, under _Jaws_), behind the mandibular somite; the presence of this in the embryo of the collembolan _Anurida_ has been lately shown (1900) by J. W. Folsom (fig. 18, v. 5), who terms the maxillulae "superlinguae" on account of their close association with the hypopharynx or lingua. In reference to the structure of the head-capsule in the imago, it appears that the clypeus and labrum represent, as already said, an unpaired median outgrowth of the oral piece. According to W. A. Riley (1904) the epicranium or "vertex," the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging to the ocular segment), while the mandibular and maxillary segments form the hinder parts of the genae and the hypopharynx.

[Illustration: After Heymons.

FIG. 17.--Morphology of an Insect: the embryo of _Gryllotalpa_, somewhat diagrammatic. The longitudinal segmented band along the middle line represents the early segmentation of the nervous system and the subsequent median field of each sternite; the lateral transverse unshaded bands are the lateral fields of each segment; the shaded areas indicate the more internally placed mesoderm layer. The segments are numbered 1-21; 1-6 will form the head, 7-9 the thorax, 10-21 the abdomen. A, anus; Abx1 Abx11, appendage of 1st and of 11th abdominal segments; Ans, anal piece = telson or 12th abdominal segment; Ant, antenna; De, deuterencephalon; Md, mandible; Mx1, first maxilla; Mx2, second maxilla or labium; O, mouth; Obcl, rudimentary labrum and clypeus; Pre, protencephalon; St1 St10, stigmata 1 and 10; Terg, tergite; Thx1, appendage of first thoracic segment; Tre, tritencephalon; Ul, a thickening at hinder margin of the mouth.]

Great difference of opinion exists as to the hypopharynx, which has even been thought to represent a distinct segment, or the pair of appendages of a distinct segment. Heymons considers that it represents the sternites of the three trophal segments, and that the gula is merely a secondary development. Folsom looks on the hypopharynx as a secondary development. Riley holds that the hypopharynx belongs to the mandibular and maxillary segments, while the cervical sclerites or gula represent the sternum of the labial segment. The ganglia of the nervous system offer some important evidence as to the morphology of the head, and are alluded to below.

_Thoracic Segments._--These are always three in number. The three pairs of legs appear very early as rudiments. Though the thoracic segments bear the wings, no trace of these appendages exists till the close of the embryonic life, nor even, in many cases, till much later. The thoracic segments, as seen in an early stage of the ventral plate, display in a well-marked manner the essential elements of the insect segment. These elements are a central piece or sternite, and a lateral field on each side bearing the leg-rudiment. The external part of the lateral field subsequently grows up, and by coalescence with its fellow forms the tergite or dorsal part of the segment.