Note 88 — CHAP. V. THE IMPORTANCE OF NATURAL HISTORY; OF INSECTS IN GENERAL, AND OF THEIR CONSTITUENT PARTS. (continued 3)
Reading edition
The obscure and singular habitations of the British oestri are the stomach and intestines of the horse, the frontal and maxillary sinuses of sheep, and beneath the skin of the backs of horned cattle. In other parts of the world they inhabit various other animals.
The larva of the oestrus bovis lives beneath the skin of horned cattle, between it and the cellular membrane, in a proper sack or abcess, which is rather larger than the insect, and by narrowing upwards opens externally to the air by a small aperture. When arrived at its full growth, it effects its escape from the abcess by pressing against the external opening; when the opening has thus obtained the size of a small pea, the larva writhes itself through, and falls from the back of the animal to the ground; and, seeking a convenient retreat, becomes a chrysalis, in which state it continues from about the latter end of June to about the middle of August; the perfect insect, on leaving the chrysalis, forces open a very remarkable marginated triangular lid or operculum. The oestrus in its perfect or fly state is the largest of the European species of this genus, and is very beautiful. Although its effects on the cattle have been so often remarked, yet the fly itself is rarely seen or taken, as the attempt would be attended with considerable danger. The pain it inflicts in depositing its egg is much more severe than in any of the other species: when one of the cattle is attacked by this fly, it is easily known by the extreme terror and agitation of the whole herd; the unfortunate object of the attack runs bellowing from among them to some distant part of the heath, or the nearest water, while the tail, from the severity of the pain, is held with a tremulous motion straight from the body, in the direction of the spine, and the head and neck are also stretched out to the utmost. The rest, from fear, generally follow to the water, and disperse to different parts of the field. The larvæ of this insect are mostly known among the country people by the name of wornuls, wormuls, or warbles, or more properly bots.
The larva of the oestrus equi is very commonly found in the stomach of horses. These larvæ attach themselves to every part of the stomach, but are generally most numerous about the pylorus; and are sometimes found in the intestines. They hang most commonly in clusters, being fixed by the small end to the inner membrane of the stomach, to which they adhere by two small hooks or tentacula. The larvæ having attained their full growth in about a month, on dropping to the ground find some convenient retreat, change to the chrysalis, and in about six or seven weeks the fly appears.
The larva of the oestrus hæmorrhoidalis resembles in almost every respect that of the oestrus equi, and occupies the same situation in the stomach of the horse. When it is ripe, and has passed through the intestines and the sphincter ani it assumes the chrysalis state in about two days, and in about two months the fly appears.
The generally received opinion has been that the female fly enters the anus of the horse to deposit its eggs, and Reaumur relates this circumstance on the authority of Dr. Gaspari; from the account of its getting beneath the tail, it is probable that the fly he saw was the hippobosca equina, which frequently does this: its getting within the rectum appears to have been additional. That a fly might deposit its eggs on the verge of the anus is not impossible, but we know no instance of it: the fact is, that the part chosen by the oestrus hæmorrhoidalis for this purpose is the lips of the horse, which is very distressing to the animal from the excessive titillation it occasions; for he immediately after rubs his mouth against the ground, his fore legs, or sometimes against a tree, or if two are standing together, they often rub themselves against each other. At the sight of this fly, the horse appears much agitated, and moves its head backward and forward in the air to baulk its touch, and prevent its darting on the lips; but the fly, watching for a favourable opportunity, continues to repeat the operation; till at length, the enraged animal endeavours to avoid it by galloping away to a distant part of the field. If still pursued, its last resource is in the water, where the oestrus is never observed to follow him.
The oestrus veterinus is by Linnæus called nasalis, from an idea of its entering the nostrils of the horse to deposit its eggs, which it could not well do without destroying its wings, and is therefore probably as much a fable as the “mire per anum intrans” of the oestrus hæmorrhoidalis.
The oestrus ovis is mostly found in the horns and frontal sinuses of the sheep, though it has been remarked that the membranes lining these cavities were hardly at all inflamed, while those of the maxillary sinuses were highly so; from which it is suspected that they inhabit the maxillary sinuses, and crawl, on the death of the animal, into these situations in the horns and frontal sinuses. When the larvæ are full-grown they fall through the nostrils, and change to the pupa state, lying on the earth, or adhering by the side to a blade of grass. The fly bursts the shell of the pupa in about two months.
The above concise account of the different oestri is extracted from the excellent paper on the subject by Mr. B. Clark, F. L. S. For his more ample description, accompanied with coloured figures of the several British species, see Transactions of the Linnean Society, vol. iii. page 283-329, just published. Edit.