Arrived at a state of complete development, the larva of the _OEstrus_ imprisoned in the stomach of the horse leaves the membrane to which it has been fixed, then directing the anterior part of its body towards the pyloric opening of the stomach, allows itself to be carried away with the excrementitious matter. It traverses, mixed with the excrementary bolus, the whole length of the intestinal canal, leaves it by the anal orifice, and on touching the ground at once seeks a suitable place to go through the last but one of its metamorphoses. The skin then gets thick, hardens, and becomes black. All the organs of the animal are composed of a whitish amorphous pulp, which soon assumes its destined form, and the insect becomes perfect. It then lifts a lid at the anterior part of its cocoon, emerges, dries its wings, and flies off. [Illustration: Fig. 45.--Bot-fly (_OEstrus bovis_).] The Bot-fly (_OEstrus bovis_, Fig. 45) has a very hairy body, large head, the face and forehead covered with light yellow hair, the eyes brown, and the antennæ black. The thorax is yellow, barred with black; the abdomen of a greyish white at the base, covered with black hair on the third segment, and the remainder of an orange yellow; the wings are smoky brown. As soon as the cattle are attacked, they may be seen, their heads and necks extended, their tails trembling, and held in a line with the body, to rush to the nearest river or pond, while such as are not attacked disperse (PLATE II.). It is asserted that the buzzing alone of the _OEstrus_ terrifies a bullock to such an extent as to render it unmanageable. As for the insect, it simply obeys its maternal instinct, which commands it to deposit its eggs under the skin of our large ruminants. [Illustration: II. A Herd of Cattle attacked by Bot-flies (_OEstrus bovis_).] Let us now explain how the eggs of the _OEstrus_, deposited in the skin of the bullock, accommodate themselves to this strange abode. The mother insect makes a certain number of little wounds in the skin of the beast, each of which receives an egg, which the heat of the animal serves to bring forth. It is a natural parallel to the artificial way which the ancient Egyptians invented of hatching the eggs of domestic fowls, and which has been imitated badly enough in our day. Directly the larva of the Bot-fly is out of the egg and lodged between the skin and the flesh of its host, the bullock, it finds itself in a place perfectly suitable to its existence. In this happy condition the larva increases in growth, and eventually becomes a fly in its turn. Those parts of the animal's body in which the larvæ are lodged are easily to be recognised, as above each larva may be seen an elevation, a sort of tumour, termed a bot--a bump, as Réaumur calls it, comparing it more or less justly to the bump caused on a man's head by a severe blow. Fig. 46, taken from a drawing in Réaumur's Memoirs, represents the bots of which we speak.