a, Eggs of the breeze-fly of the horse. b, the same magnified. c, larva or bot. d, chrysalis. e, perfect insect. f, position of the female in the act of oviposition.

CHAPTER III.

HISTORY OF BREEZE-FLIES.*

THE name of gad-fly has been repeatedly given to the insects whose history is detailed in this chapter, but that term more properly belongs to those blood-sucking flies which cause such pain and misery to our horses, in places abounding in wood, especially in warm cloudy weather and before thunder showers. The present insects when in the larva state are termed "bots" in horses, "maggots" in sheep, and "wornils" or "warbles" in cows and oxen; and these three represent three divisions of the family, differing essentially in their history. The perfect insect produced from each kind of larva is properly termed a breeze-fly.

* Authority:-An Essay on the Bots of Horses and other animals, by Bracy Clark, F.L.S.

The opinions respecting the breeze-fly of the horse, or bot as it is usually termed, as to the benefit or injury derived from it, are very opposite; some observers go so far as to assert that the larvæ occasionally completely perforate the stomach of the horse, causing disease, pain, and even death; others regard them as perfectly innocuous; and one author, whose careful and laborious investigations entitle his opinions to the greatest respect, believes the effect of bots to be salutiferous rather than otherwise: this author is Mr. Bracy Clark, and from his masterly essay the following particulars are extracted.

The female fly in approaching the horse for the purpose of oviposition, carries her body nearly upright in the air, the protruded ovipositor being curved inwards and upwards. Suspending herself for a few seconds before the part of the horse on which she intends to deposit the egg, she suddenly darts upon it, and leaves the egg adhering to the hair: she hardly appears to settle, but merely touches the hair with the egg held out on the extreme point of the ovipositor (see fig. f), the egg adhering by means of a glutinous liquor with which it is covered. She then leaves the horse at a small distance, prepares a second egg, and, poising herself before the part, deposits it in the same way the liquor dries, and the egg becomes firmly glued to the hair. This is repeated till four or five hundred eggs are sometimes placed on one horse. The skin of the horse is usually thrown into a tremulous motion on the touch of the insect, which merely arises from the very great irritability of the skin and cutaneous muscles at this season of the year, occasioned by the heat and continual teazing of the flies, till at length these muscles appear to act involuntarily on the slightest touch of any body whatever.

The fly does not deposit her eggs at random on the horse's body, but selects those parts which are most likely

to be nibbled by the horse: the inside of the knee is frequently chosen, but all naturalists must have remarked how commonly the eggs of the bot are deposited on that part of a horse's shoulder which he can never reach with his mouth, and thus, to a casual observer, it would seem that they must perish, and fail in the object for which their parent designed them. Now there is a provision of nature which exactly counteracts this difficulty. When horses are together in a pasture, and one of them feels an irritation on any part of the neck or shoulder which he cannot reach with his mouth, he will nibble another horse in the corresponding part of his neck or shoul

der, and the horse so nibbled will immediately perform the kind office required, and begin nibbling away in the part indicated. The horses, when they become used to this fly, and find it does them no injury by sucking their blood, hardly regard it, and do not appear at all aware of its object.

When the eggs have remained on the hairs four or five days they become mature, after which time the slightest application of warmth and moisture is sufficient to bring forth in an instant the latent larva. At this time, if the lips or tongue of the horse touch the egg, its operculum is thrown open and the young larva liberated: this readily adheres to the moist surface of the tongue, and is from thence conveyed with the food to the stomach. It is worthy of remark, that it is probable the greater part of the eggs deposited by this fly are taken up in consequence of the irritation of other flies, as the Tabani and Stomoxides, which, by perpetually settling on the skin, occasion a horse to nibble himself in those parts, and thus receive the larvæ on the tongue and lips whence they are introduced into the stomach. The egg is glued on the hair with the broad end

downward (page 25, figs. a, b), and is thus well disposed for the operation of the tongue in removing the operculum, which is of an oval figure, and surrounded with a prominent margin. The microscope shows the case of the egg to be shagreened in squares, or impressed longitudinally and transversely with delicate striæ. When the larva or grub is hatched from the egg it is a small active worm, long in proportion to its thickness; but as its growth advances it becomes proportionably thicker and broader, and beset with bristles. These larvæ are very frequent in horses that have been at grass, and are in general found adhering to the white insensible tissue or coat which comes from the lining of the oesophagus, and extends over the upper part of the stomach. They make small, deep, round holes wherever they adhere to this white tissue, and sometimes so deep as to pass through it, but not through the other layers or coats of the stomach.

The larvæ usually hang in dense clusters from the lining of the stomach, and maintain their hold by means of two dark brown hooks; between these a longitudinal slit or fissure is seen, which is the mouth of the larva. When the larvæ are removed from the stomach with a sudden jerk so as not to injure them, they will, if fresh and healthy, attach themselves to any flaccid membrane, and even to the skin of the hand. For this purpose they sheath or draw back the hooks almost entirely within the skin, till the two points come close to each other; they then present them to the membrane, and keeping them parallel till it is pierced through, they expand them in a lateral direction, and afterwards, by bringing the points downwards towards themselves, they include a sufficient piece of the membrane to enable them to remain firmly fixed for any length of time as at anchor, without requiring any further exertion. The body of the larva is of a whitish red colour, and ap

pears to be composed of eleven segments, surrounded with a double row of horny bristles, a longer and a shorter series, and placed alternately: the two last segments appear to be naked or destitute of them. These spines are of a reddish colour, except the points, which are black, and are directed towards the tail or large end of the larva.

The larva, when matured, quits the stomach of the animal and falls to the ground, and finding a convenient place of retreat, undergoes its change to a chrysalis, the skin then losing its organization, and changing in colour to a reddish brown. After remaining torpid in the chrysalis state a few weeks, the superfluous moisture being removed and the parts of the future insect hardened by drying, it bursts from its confinement, and the fly makes its exit at the small end of the case. A few hours after quitting their shell they become dry, take wing, and seek their mates.

A second species of breeze-fly has a still more wonderful history its eggs are laid in the nostrils of sheep, from one to seven or eight in each individual, and these on becoming larvæ, enter the frontal and maxillary sinuses, and even the horns, and feed on their secretions: when the larvæ are young they are perfectly white and transparent, except two small, black, horny plates: as they increase in size the upper surface becomes marked with two transverse brown lines on each segment, the anterior being shorter and narrower than the posterior; and some spots are also observable on the sides. The body consists of twelve segments besides the head. These larvæ move with considerable activity, holding with their tentacula to a fixed point and drawing up the body. When full grown the larvæ fall through the nostrils of the sheep, and change to the pupa state lying on the earth or adhering to the side of a blade of grass in about two months the case of the chrysalis opens, and the fly makes its appearance.