OESTRUS, in natural history, gad-fiy, a genus of insects of the order Diptera. Mouth with a simple aperture, and not exserted : feelers two, of two articula tions orbicular at the tip, and seated each side in a depression of the mouth : anten na of three articulations, the last subglo bular, and furnished with a bristle on the fore-part, placed in two hollows on the front. The face of this singular genus is broad, depressed, vesicular, and glaucous, and has some sort of resemblance to the ape kind. They are extremely trouble some to horses, sheep, and cattle, deposi ting their eggs in different parts of the body, and producing very painful tu mours, and sometimes death. The larva are without feet, short, thick, and annu late, and often furnished with small hooks. There are twelve species, named from the animals which they infest : thus we have 0. bovis, 0. equi, p. ovis, homi nis, he. The principal European species is the 0. bovis, or ox gad fly, which is the size of a common bee, and is of a pale yel lowish colour, with the thorax marked with four longitudinal dusky streaks, and the abdomen by a black bar across the middle ; the lip is covered with tawny orange-coloured hairs ; the wings are pale-brown, and unspotted. The female of this species, when ready to deposit her eggs, fastens on the back of a heifer, or cow, and piercing the skin with the tube situated at the tip of the abdomen, de posits an egg in the puncture, and then proceeds to another spot at some distance from the former, repeating the same ope ration, at intervals, on many parts of the animal's hack. The pain which this ope ration occasions is extreme ; and hence cattle, as if foreseeing their cruel are observed to be seized with the most violent horror when apprehensive of the approaches of the female oestrus, flying instantly to the nearest pond or pool of water ; it having been observed that this insect rarely attacks cattle when standing in water. The eggs are laid in August or September, and the larva remain till the following summer before they under go the change to the pupa state. At this period they force themselves out of their respective cells, and falling to the ground, creep beneath the first convenient shel ter, and lying in an inert state become contracted into an oval form, but without casting the larva skin, which dries and hardens round them. When the included
insect is ready for exclusion, it forces open the top of the pupa coat, and emer ges in its perfect form, having remained within the chrysalis somewhat more than a month.
We shall give an account of the 0. equi, from the Transactions of the Linnxan So ciety, drawn up with great accuracy by Mr. Clarke. " When the female has been impregnated, and the eggs are suffi ciently mature, she seeks among the hor ses a subject for her purpose ; and ap proaching it on the wing, she holds her body nearly upright in the air, and her tail, which is lengthened for the purpose, curved inwards and upwards : in this way she approaches the part where she de signs to deposit her egg ; and, suspend ing herself for a few seconds before it, suddenly darts upon it, and leaves her egg adhering to the hair : she hardly ap pears to settle, hut merely touches the hair with the cggheld out on the project ed point of the abdomen. The egg is made to adhere by means of a glutinous liquid secreted with it. She then leaves the horse at a small distance, and 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 by various flies, till four or five hundred eggs are sometimes placed on one horse. The horses, when they become used to this fly, and find that it does them no in jury, as the Tabani and Conopes, by suck ing their blood, hardly regard it, and do not appear at all aware of its insidious ob ject, The skin of the horse is always thrown into a tremulous motion on the touch of this insect, which merely arises from the very great irritability of the skin and cutaneous muscles at this season of the year, occasioned by the continua] teasing of the flies, till at length these muscles act involuntarily on the slightest touch of any body whatever.