It is in the larva or caterpillar state that the Bet is most known. The stomach of the horse sometimes contains an almost incredible number of them, the cuticular portion of that organ being in a manner covered with them. In a few instances they have been decidedly injurious. Having mistaken the upper part of the windpipe for their residence, and fastening themselves on the edges of the opening into it, have produced a cough which no medicine could alleviate, and which increased with the growth of the Bot, until a degree of irritation was excited under which the animal sunk. They have also travelled farther than the stomach, and have irritated and choked the first intestine, and thus destroyed the horse ; and, even in their natural habitation, under probably some diseased state of the stomach arising from other causes, they have perforated it and caused death.
These'however are rare occurrences ; they are exceptions to a general rule. The plain matter of fact is, that a horse that has been turned out in July and August, and therefore almost necessarily has bets, enjoys just as good health as another that has been stabled during this period. He ie in as good condition, and as fully capable of work when the cuticular coat is crowded with full-formed bets as be is at any other time ; and his health is unaffected when they are passing through the intestines to seek a new habitation.
A smaller species of Bot, called from its colour the Red-Bot, is occasionally found in the stomach ; but the fly from which it proceeds has never been accurately described. There is no ground for the assertion that the red•bot is more lujurioua than the com mon hot.
A third species, the (Estrets Aersorr1ioidaUa, or Fundement•Bot, is better known. The fly is considerably smaller than the common (Estrus Equi. It is of a brown colour, with the extremity of the body rounded and yellow, and the mouth is furnished with exceedingly sharp pincers. This fly may be seen darting between the thighs of the horse and around its croup, and following the motions of the tail until the animal is preparing to dung. During the evacuation of the dung, and the subsequent protrtudon of the intestine, it darts .upon and tears the gut with its pincers, and deposits an egg in every wound. The horse does not seem to Buffer any pain during this operation, for he stands passive; and the little worm, soon produced from the egg, establishes its abode in the place in which it was deposited. It likewise remains its stated time in the intestine, and escapes at the same time that the common lest does from the stomach. These bota are often seen within the verge of the anus, and occasionally seem to be productive of a slight degree of irritation. They are smaller than the common hot, and distinguished from the red-bot by their colour. An injection of linseed-oil will generally dislodge them.
The (Estrus Cris, or Gad-Fly of the Sheep, is a more formidable insect. It is smaller than the Walrus of the Horse : its body is of a dark-brown colour, spotted with white, the white sometimes so much prevailing as to give a grayish hue to the fly. It may often be seen in copies, and particularly on rails in the neighbourhood of a copse.
Every-shepherd ought to make himself acquainted with it., for it may then be easily crushed and destroyed. It prevails most in June and July, and is sometimes an intolerable nuisance in woody countries.
If only one of them appears the whole flock is struck with terror ; and if there is any place in the field devoid of pasture the sheep crowd to it, turning their heads towards the centre of the group, with their muzzles to the sand, and their feet in continual motion in order to secure themselves from the attack of their foe. The (Estrus endeavours to get at the inner margin of the nostril, and, darting upon it with the quickness of lightning, deposits her egg. The warmth and moisture of the part speedily hatch it, and the little worm escapes. It crawls up the nostril, it threads all the sinuosities of the passage, and finds its way to some of the sinuses connected with the nose. The irritation which it occasions as it travels up the nose seems to be exceedingly great. The poor animal gallops furiously about, snorting violently, and almost maddened by the annoyance.
At length the worm reaches some of the convolutions of the turbi natod bones of-the nose, or the antrum or cavity of the upper jaw, or the frontal sinuses; it fastens itself on the membrane by the two hooks with which, like the others, it is provided, and there it remains until April or May in the succeeding year.
There ar•e seldom more than three or four of these bets in each sheep; and when they have reached their appointed home, like the bets in the stomach of the horse, they are harmless. Some strange but groundless stories have been told of gleet from the nose, giddiness, and inflammation of the brain having been produced by them. The larva or hot remains in the sinus until it has fully grown. It then detaches itself from the membrane, creeps out the same way by which it entered, and again sadly annoys the animal for a little while, the sheep making the most violent efforts to sneeze it out. At length the grub being dropped, burrows in the earth, becomes an oval and motionless chrysalis, and six weeks or two months afterwards it breaks from its prison a perfect fly. The work of propagation being effected the male, like that of the (Estrus Equi, dies ; the female lingers on a little while until sho has safely deposited her ova. She takes no food, for she has no organs to receive or disgest it. She accomplishes her task and expires.