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.