" The inside of the knee is the part on which these flies are most fond of de positing their eggs, and the next to this, on the side and back part of the shoulder, and, less frequently, on the extreme ends of the mane. But it is a fact wor thy of attention, that the fly does not place them promiscuously about the body, but constantly on those parts which are most liable to be licked with the tongue ; and the ova, therefore, are al ways scrupulously placed within its reach.
" The eggs thus deposited I at first supposed were loosened from the hairs by the moisture of the tongue, aided by its roughness, and were conveyed to the stomach, where they were hatched : but on more minute search I do not find this to be the case, or at least only by acci dent ; for, when they have remained on the hairs four or five days they become ripe, after which time the slightest appli cation of warmth and moisture is sufficient to bring forth, in an instant, the latent lar va. At this time, if the tongue of the horse touches the egg, its operculum is thrown open, and a small active worm is produced, which readily adheres to the moist surface of the tongue, and is from thence conveyed with the food to the stomach. If the egg itself be taken up by accident, it may pass on to the intes tinal canal before it hatches ; in which case its existence to the full growth is more precarious, and certainly not so agreeable, as it is exposed to the bitter ness of the bile.
" I have often, with a pair of scissars, Clipped off some hairs with eggs on them from the horse, and on placing them in the hand, moistened with saliva, they have hatched in a few seconds. At other times, when not perfectly ripe, the larva would not appear, though held in the hand under the same circumstances for several hours ; a sufficient proof that the eggs themselves are not conveyed to the stomach. It is fortunate for the animal infested by these insects, that their num bers are limited by the hazards they are exposed to. I should suspect near a hun dred are lost for one that arrives at the perfect state of a fly. The eggs, in the first place, when ripe, often hatch of themselves, and the larva, without a ni dus, crawls about till it dies ; others are washed off by water, or are batched by the sun and moisture thus supplied toge ther. When in the mouth of the animal they have the dreadful ordeal of the teeth and mastication to pass through. On their arrival at the stomach, they may pass mixed with the mass of food into the intestines ; and when full grown, in drop ping from the animal to the ground, a dirty road or water may receive them. If on the commons, they are in danger of being crushed to death, or of being pick ed up by the birds who so constantly at tend the footsteps of the cattle for food. Such are the contingencies by which na ture has wisely prevented the too great increase of their numbers, and the total destruction of the animals they feed on.
" I have once seen the larva of this oestrus in the stomach of an ass ; indeed there is little reason to doubt their exis tence in the stomachs of all this tribe of animals. These larva attach themselves
to every part of the stomach, but are gen erally most numerous about the pylorus, and are sometimes, though much less fre• quently, found in the intestines. Their numbers in the stomach are very various, often not more than half a dozen, at oth er times more than a hundred ; and, if some accounts might be relied on, even a much greater number than this. They hang most commonly in clusters, being fixed by the small end to the inner mem brane of the stomach, which they adhere to by means of two small hooks, or ten tacula. When they are removed from the stomach they will attach themselves to any loose membrane, and even to the skin of the hand. The body of the larva is composed of eleven segments, all of which, except the two last, are surround ed with a double row of horny bristles, directed towards the truncated end, and are of a reddish colour, except the points, which are black. The larvm evidently re ceive their food at the small end, by a longitudinal aperture, which is situated between two hooks, or tentacula. Their food is probably the chyle, which, being nearly pure aliment, may go wholly to the composition of their bodies, without any excrementitious residue, though on dissection the intestine is found to con tain a yellow or greenish matter, which is derived from the colour of the food, and shows that the chyle, as they receive it, is not perfectly pure. They attain their full growth about the latter end of May, and are coming fron the horse from this time to the latter end of June, or some times later. On dropping to the ground they find out some convenient retreat, and change to the chrysalis ; and in about six or seven weeks the fly appears.
" The perfect fly but ill sustains the changes of weather ; and cold and mois ture, in any considerable degree, would probably be fatal to it. These flies never pursue the horse into the water_ This aversion I imagine arises from the chill ness of that element, which is probably felt more exquisitely by them, from the high temperature they had been exposed to during their larva state. The heat of the stomach of the horse is much greater than that of the warmest climate, being about 102 degrees of Fahrenheit, and in their fly state they are only exposed to 60, and from that to about 80 degrees. This change, if suddenly applied, would in all probability be fatal to them ; but they are prepared for it, by suffering its first effects in the quiescent and less sensible state of a chrysalis. I have often seen this fly, during the night time and in cold weather, fold itself up with the head and tail nearly in contact, and lying apparent ly in a torpid state through the middle of the summer." 0. ovis : wings pellucid, punctured at the base; abdomen variegated with white and black. It deposits its eggs on the inner margin of the nostrils of sheep, oc casioning them to shake their heads vio lently, and hide their noses in the dust or gravel. The larva crawl up into the fron tal sinuses, and when full fed are again discharged through the nostrils. See PI. III. Entomology, fig. 7 and 8.