OESTRUS. 



of the year, occasioned by the continual 

 teasing of the flies, till at length these 

 muscles act involuntarily on the slightest 

 touch of any body whatever. 



" The inside o'f 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 buck 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 

 Elace them promiscuously about the 

 ody, 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 hatched 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 larvx 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 larvae 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 g-o 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 



