Part 13

one-fifth of an inch long, slender, with a metallic sheen, almost hairless, black, with dirty yellow legs and wings of glassy clearness. These small flies abound in front of the windows of places where cheese is stored; in summer and autumn the shining white maggots, which are cylindrical in form with tapering ends, and one-third of an inch long, live in large numbers in old cheese, gnawing it through and through, and making it dirty. Now and then they spring forwards by bending their bodies into a circle and suddenly straightening them again. They become pupæ on the walls or in straw, near the cheeses from which they have crept out. _Remedies_: Keeping the cheeses clean; mechanical exclusion (gauze screens outside the windows, enclosure in chests).

The =Green-eyed Flies= (_Chlorops_)

include a number of small flies, under one-sixth of an inch long, with spherical head, rounded greenish eyes, strongly arched thorax, and short egg-shaped abdomen, pointed in the male and blunt in the female. The headless larvæ live in the haulms of grasses and species of grain; the life history of a few forms only is adequately known. A few are harmful, especially as there are two or three generations annually. The late summer generation often appears in large numbers, indeed in actual swarms. Since no species lives exclusively on corn, it is impossible to keep them down for a long period of time. I describe only two species:—

[Illustration:

FIG. 117.—The Ribbon-footed Corn Fly (_Chlorops tæniopus_). ]

The =Ribbon-footed Corn Fly= or =Yellow Haulm Fly= (_Chlorops tæniopus_), nearly one-sixth of an inch long, shining yellow; the antennæ are black, and there are three longitudinal stripes on the dorsal side of the thorax and four transverse bands on the abdomen of the same colour; the latter region is scarcely longer than the thorax. The insect (Fig. 117) is on the wing in cornfields about the middle of May. It lays its eggs separately on the upper leaves of various species of wheat, rye, and barley, choosing the upper side of the blade, not far from the sheath. Only those plants are selected for the purpose in which the ears are still hidden deep down between the leaf-sheaths. Wheat plants are picked out whenever possible. The maggot when hatched works its way between leaf-sheath and haulm, digging into the latter. It is yellowish white, clear and translucent, and about a quarter of an inch long. While still young it penetrates the haulm, and then attacks the developing ear or the upper part of the haulm which immediately adjoins this, and travels gradually up to the first node of the haulm or nearly so, always continuing to slowly suck. Thus a furrow, from 2½ to 3½ inches long (Fig. 118, C and D), is formed along the surface of the upper part of the haulm, and often also along the lower part of the ear. The part of the haulm attacked swells transversely, and the part below often remains short, so that the ear cannot emerge from the leaf-sheath; but, in any case, only small worthless grains are developed. The furrow is always much deeper below than above, and its margins thicken in consequence of the swelling of the tissues of the haulm. At the end of June or in July, the larva becomes a pupa at the lower end of the furrow. The yellowish-brown pupa (Fig. 118, B), one-fifth of an inch long, remains as such in the furrow for three weeks; the fly emerges in August. Very considerable damage may be done by the first generation, of which the habits have just been described. During 1869 in Silesia, from two-thirds to five-sixths of the ears in many fields remained hidden in the leaf-sheaths, and consequently gave no increase. The first generation of the ribbon-footed corn fly can also develop in the way described in the haulms of several grasses, _e.g._ in species of _Poa_ and _Holcus_.

The flies, emerging in late summer, lay their eggs, here, too, separately, on the leaves of grass or corn. Wherever possible, they seek out for the purpose the winter wheat plants then present in the fields, but also content themselves with rye, or even with wild or meadow grasses; they have to be satisfied with grasses if, at the time of egg-laying, the winter corn is not yet up. The maggot, when hatched, works its way to the inner side of the leaf-sheath, and thence to the apex of the still very small haulm; there it remains during the winter. The damage becomes apparent the following spring. The growth in length of the haulm in question is extremely small, while growth in thickness increases to an abnormal extent. Almost all the leaves completely surround the haulm, which swells to an enormous extent (Fig. 119), together with the enclosing leaf-sheaths, which are much broader than usual. The unattacked plants are naturally much larger than the sickly ones, and deprive these of air and light, so that they die down, being overshadowed, not only by the sound haulms, but also by their own secondary shoots. The resulting damage may be tolerably great, especially at the edges of the field. It may happen that both summer and winter generations of the ribbon-footed corn fly are harmful in the same district; but it frequently happens that only one or the other is complained of in a particular spot. It is only natural that the flies, which swarm around in May, and again in August, September, or October, should not always find suitable corn plants upon which to deposit their eggs. In such cases grasses are used. _Remedies_: Sowing the summer corn as early as possible, that it may be developed to a stage which is unsuitable for the purpose of egg-laying, before the flies appear. The winter seed, however, must be sown as late as possible, so that the second generation of flies may find no corn plants fit to lay their eggs upon. Bearded wheat, especially the strongest varieties, should be sown in preference to awnless wheat. Careful tillage and suitable manuring, so that strong plants of rapid growth may be produced.

[Illustration:

FIG. 118.—The Ribbon-footed Corn Fly (_Chlorops tæniopus_); larva (A) and pupa (B) magnified. To the left a wheat haulm and ear (C) with the furrow (_q_) dug out by the larva; the pupa (_p_) is seen at the bottom of the furrow. To the right a wheat haulm with furrow (D) and the larva (_r_) lying in it. ]

[Illustration:

FIG. 119.—A wheat plant distorted by the winter generation of the Ribbon-footed Corn Fly. ]

The =Frit Fly= (_Chlorops_, or _Oscinis frit_)

(Fig. 120, C) is about one-tenth of an inch long, shining black, with a metallic sheen. Legs short, feet yellow. Maggot (Fig. 120, A) yellowish white, about one-eighth of an inch long, cylindrical, and tapering in front. Usually three generations. 1. The maggots of the first generation are found during May, in the lower part of the haulm of summer corn (especially oats and barley); the plants attacked either die off entirely, or some haulms develop further, remaining small, however, and yielding only a few light grains. The base of the haulm thickens abnormally, but the growth in length is always small; the leaves, too, grow badly, first becoming yellowish at the tip, and then entirely yellow or reddish. The symptoms of disease are exhibited to a less or greater extent, according as few or many (even up to ten) maggots inhabit the base of a plant. The shining brown pupæ (Fig. 120, B) are found in the lower part of the haulm, or between the leaf-sheath and the haulm (Fig. 120, D). The adult insect is on the wing at the end of May and in June. The first generation often appear on wild or meadow grasses, and are chiefly seen on summer corn when this is sown late or develops slowly. 2. The maggots of the second generation are found in average cases during July, on the as yet scarcely ripe grains of late summer corn, principally those of oats and barley, often occurring also in the haulms of grasses. They keep between the awns, and suck the juices of the soft developing grains, which are rendered incapable of growth, and in any case remain light. The maggots of the second generation develop more quickly than those of the first or third; they are mature in three weeks. The pupa rest is very short, and the flies appear in August, September, or October. They lay their eggs on the leaves of winter corn or winter grasses, and from these eggs are developed (3) the maggots of the third generation, which are to be found, during September and October, in the heart of winter corn and grasses, injuring these plants in exactly the same way that the maggots of the first generation injure the summer corn. The insect passes the winter as a pupa in winter corn plants or grasses. It is but very rarely that all _three_ generations infest the corn of any particular region; as a rule, only the first, second, or third generations do this, or the first _and_ third; in such cases the other generations live on grasses. _Remedies_: Extermination of the insect is impossible, since it can always go from corn to grass plants. Oats and barley are almost always attacked in spring, if in the immediate neighbourhood there is winter rye inhabited by the maggots, for the flies, when they emerge the next spring, seek out the summer corn. This may, however, be made impossible, or at least difficult, if a field of peas, clover, lupines, rape, or some other crop not of gramineous nature, is interposed between fields of winter corn and those of oats, barley, or similar late sown crops of summer corn. Sowing the summer corn (especially oats and barley) as early as possible.

[Illustration:

FIG. 120.—The Frit Fly (_Oscinis frit_, L.): A, larva; B, pupa; C, fly; D, a diseased corn plant, as appearing in spring,—the larvæ and pupæ are seen of the natural size in the lower part of the plant. ]

Family: =Syrphidæ= (_Hover_ or _Hawk Flies_).

Chiefly includes brightly coloured flies, marked with yellow or red and black bands or patches (Fig. 67), and flying rapidly with a buzzing noise. They can remain suspended at the same point in the air by moving their wings up and down with great rapidity. A few of them resemble the humblebee in their thick covering of hair; others, with yellow and black abdomen, look like wasps (_Syrphus_). The proboscis is adapted for sucking, but not for piercing; these flies suck their food from flowers. They are fond of hovering in the air in sunny places. The legless larvæ vary, according to species, in their habits, and consequently in their structure. Some (those of the =Drone Flies=, _Eristalis_) live in stagnant water; others (_e.g._ those of _Eumerus lunulatus_) live in onions, which they hollow out; while some, again, develop in rotten wood, etc. The maggots of the =Aphis-eating Flies= (_Syrphus_), however, feed on insects, chiefly aphides, which they suck out completely. They are elongated, tapering in front, thickened behind, move like leeches, and vary much in colour (green, yellow, brown, chequered), according to the species. As they grow quickly, and consequently more than one generation is found each year, and as they are very voracious, we must look upon them as powerful allies for the extermination of aphides.

Family: =Stomoxydæ= (_Stable Flies_).

In many points the stable flies resemble ordinary flies, but their mouths are adapted for piercing. Their painful bites make them known to every one as pests to human beings and cattle. Here belongs the common =Stable Fly= (_Stomoxys calcitrans_), a form often confounded with the house fly, but distinguished from it by a sharp proboscis projecting at right angles, besides which the abdomen is more of a yellowish grey. The maggots usually live in dung. Two generations yearly: the first flies about in March; the second, and much more numerous one, in August and September. _Remedies_: Cp. p. 165.

Family: =Œstridæ= (_Bot Flies_).

Medium-sized or large flies (Fig. 121), with thick hemispherical heads, and mouth-parts not strongly developed. The antennæ can be drawn back into deep pits. The hot flies make a buzzing sound during flight. The headless, twelve-ringed maggots live in the bodies of various mammals. Their skin is provided with numerous wart-like projections, or circlets of spines. When very young the maggots are elongated and cylindrical, and then possess a mouth-hook, which disappears during the later moults. As soon as the maggots are fully developed they leave the body of the animal they inhabit, and let themselves fall to the ground, where they become pupæ within the shrivelled larval skin.

The following genera are distinguished: =Warble Flies= (_Hypoderma_), and =Bot Flies= (_Œstrus_ and _Gastrus_, or _Gastrophilus_). To the first-named genus belongs the

=Ox Warble-fly=, or =Ox Bot Fly= (_Hypoderma bovis_),

two-fifths of an inch long, black. Hair: whitish yellow on the head; reddish yellow on the fore part of the thorax, black on the hinder part; grey on the fore part of the abdomen, black in the middle, and reddish yellow behind. Legs black. Wings brownish, not quite transparent.

On the wing during summer (June to September). As soon as the cattle hear the flies buzzing around (especially on hot days) they become very restless, run about as if mad, and even plunge down steep places. Young cattle are selected for egg-laying; the elongated white eggs are fixed separately to hairs. The maggot, elongated when first hatched, perforates the skin, and gets into the subcutaneous connective tissue, where it does not, as a rule, keep to any one place, but wanders here and there, sometimes penetrating the flesh, or even entering the spinal canal. It always, however, wanders back again later on into the subcutaneous connective tissue, where it gives rise, during the winter or the following spring, to one of the well-known tumours, or “warbles.” After fixing on a definite spot, it moults, becoming broader, and of a yellowish white colour. The maggot first causes an increased flow of blood to the part, and then inflammation. An excavation filled with matter is thus developed, and there is gradually formed a connective tissue sac communicating with the exterior by a minute tube. In spring, or early summer, the warble, which is visible externally, has reached the size of a pigeon’s egg; the maggot meanwhile becomes first greyish yellow, then brown patches appear, and lastly it assumes a dark brown colour, is an inch long or rather more, and somewhat swollen. It is now ready to pass into the resting stage, crawls out, and lets itself fall to the ground, where, within the larval skin, it becomes a black pupa four-fifths of an inch long, from which, about four weeks later, the fly creeps out. _Damage done_: If the warbles occur only in small numbers on an animal, its health is not much affected, though this must undoubtedly be the case if there are many, say fifty, or even up to a hundred, in the same animal. In such cases the yield of milk will be considerably diminished. Holes, too, are present in the skin, which, though they may close again, if the animal remains alive, always leave a thin place. The outer surface of meat from animals infested with warbles is dirty yellow, flaccid, or even soft and jelly-like (“licked beef”); it must be scraped off. _Enemies_: Starlings settle, in spring, on the backs of infested cattle, and seize the parasites with their beaks. Starlings, rooks, and wagtails destroy the maggots ready to become pupæ, as they lie on the ground. _Remedies_: Washing the back, shoulders, and loins with vinegar extract of walnut leaves during the summer, to keep away the bot flies. In spring: squeezing out the maggots from the warbles, having previously opened them, when necessary, with a penknife. If the warble is “ripe,” _i.e._ if it has opened so far that the black hinder end of the maggot can be seen, the opening may be stopped with fat or cart-grease, by which the larva will be killed where it lies.

The =Sheep Bot Fly= (_Œstrus ovis_),

two-fifths to three-quarters of an inch long, yellowish grey, almost hairless; head large, round, reddish; thorax grey, with small black warts; abdomen yellowish white; legs short, bright; wings of a glassy clearness. The flies are found (in September) on the walls of sheepfolds, and in woods near which sheep graze. On sunny days the female flies round the sheep, in order to deposit her brood upon them. The sheep threatened press their nostrils to the ground, though this is not of much use. The maggots are hatched while still in the body of their mother, and are deposited by her on the margins of the nostrils. These little maggots creep about on the mucous membrane lining the internal cavities of the nose, causing intolerable itching. The sheep try to get rid of the intruders by shaking their heads and rubbing their noses on the ground. The maggots, however, creep further into the nasal cavities, and get into the hollows of the frontal bone and upper jaw, perhaps even into the horns. In these places they feed on the fluid which their presence causes the mucous membrane to give out in large quantities. The maggots remain as such for nine months, during which time they alter their shape in various ways. Those just born are white, one-twenty-fifth of an inch long, while those which have reached the length of two-fifths of an inch are yellowish white. Individuals ready to become pupæ are about an inch long, yellowish brown, with dark transverse lines; they wander back to the nasal cavities and thence to the exterior, often being expelled by sneezing. In the soil they become pupæ, first of a brown, then of a black colour, within the larval skin; the flies emerge in six or seven weeks. Different maggots develop at different rates, according to the nature of the cavities into which they penetrate.

The maggots cause the disease known as “false gid,” which appears most obviously from March to May; at this time the parasites are tolerably well developed. They irritate the mucous membrane of the cavities of the head in which they live, causing an unusually large flow of blood to these parts, as a result of which the mucus secreted in the nose increases largely in quantity (sneezing, snuffling), and the brain begins to work abnormally (uneasy movements of the head, high lifting of the feet; in worse cases, rolling of the eyes, gnashing of the teeth, and foaming at the mouth). The sheep have also an intolerable itching of the nose, which they rub in consequence on the ground, against posts, or their own legs; besides this, there may be inflammation of the eyelids and increased secretion of tears. They are also apt to grow very thin. The disease is more frequent, and the symptoms severer, in young than in old sheep. False gid may cause death; the animal is cured, however, after the maggots have crept out. _Preventive Measures_: Keeping the sheep away from the edges of woods, avenues, etc., where the flies live by preference. When sheep are killed, the maggots coming from the head should be collected and destroyed. Smearing the margins of the nostrils with tar, or rubbing them with walnut leaves, before the sheep go to pasture in the morning. For sheep, which in late summer rub their noses up and down tree stems, walls, hedges, or their own legs, substances that cause sneezing may be employed, _e.g._ cheap snuff, which is best introduced into the nose by means of a quill-feather. Later on, when the maggots have passed from the nasal cavities into the frontal sinuses, etc., sneezing does no good. Operations seldom succeed in removing all the maggots.

[Illustration:

FIG. 121.—Horse Bot Fly (_Gastrus equi_): _a_, egg on a hair, strongly magnified; _c_, younger (magnified), and _b_, older larva; _d_, opened pupa case; _e_, fly. ]

The =Bot Flies= (_Gastrus_, or _Gastrophilus_) live in various parts of the stomach (in left side of horse’s stomach) and intestine. I give, first of all, a compressed tabular view of the external characters and mode of life of the four British species. (See next page.)

_Diseases caused by Bot Flies._—The maggots bore into the walls of the stomach and intestines until they reach the layer in which the blood-vessels, lacteals, and lymphatics ramify; they then suck the juices found in these vessels, and also serous fluids. In small numbers they are often almost harmless, but when a great many are present they hinder the secretion of the digestive juices. They may also set up inflammation of the intestinal coats, or may cause death by internal bleeding if they perforate the wall of an artery. In foals they often bore right through the wall of the intestine, and enter the abdominal cavity, where they may set up inflammation of the peritoneum or of the mesentery. There may be as many as a hundred or two hundred maggots in the stomach or intestine, and they then cause, at the very least, digestive disturbances and colic. If the maggots (_G. nasalis_) get into the windpipe (or larynx) they interfere with the breathing; if into the gullet, with swallowing. In either case death may ensue (winter, early spring). _Remedies_: Killing the flies as they lay their eggs. Removing the eggs by brushing, combing, and washing; the last is best done with warm water, to which some caustic potash is added. Rubbing the lips and nostrils, neck, chest, and fore legs with walnut leaves, or a decoction of the same.