OESTRIDAE.--The three common bot-flies are the _Gastrophilus equi_, _Hypoderma lineata_ and _OEstrus ovis_. These flies are important because of the parasitic habits of their larva. They inhabit the stomach and intestines of horses (Fig. 62); the subcutaneous tissue and skin of cattle; and the sinuses of the head and nasal cavities of sheep. _The common bot-fly of the horse_ (_G. equi_) has a heavy, hairy body. Its color is brown, with dark and yellowish spots. The female fly can be seen during the warm weather, hovering around the horse, and darting toward the animal for the purpose of depositing the egg. The color of the egg is yellow, and it adheres firmly to the hair. It hatches in from two to four weeks, and the larva reaches the mouth through the animal licking the part. From the mouth, it passes to the stomach, where it attaches itself to the gastric mucous membrane (Fig. 62). Here it remains until fully developed, when it becomes detached and is passed out with the fasces. The third stage is passed in the ground. This takes place in the spring and early summer and lasts for several weeks, when it finally emerges a mature fly. [Illustration: FIG. 62.--A piece of the wall of the horse's stomach showing the bot-fly larvae attached.] _The bot-fly of the ox_ (_H. lineata_) is dark in color and about the size of a honey-bee. On warm days, the female may be seen depositing eggs on the body of the animal, especially in the region of the heels. This seems to greatly annoy the animal, and it is not uncommon for cattle to become stampeded. The egg reaches the mouth through the animal licking the part. The saliva dissolves the shell of the egg and the larva is freed. It then migrates from the gullet, wanders about in the tissue until finally it may reach a point beneath the skin of the back. Here the larva matures and forms the well-known swelling or warble. In the spring of the year it works out through the skin. The next stage is spent in the ground. The pupa state lasts several weeks, when the mature fly issues forth. _The bot-fly of sheep_ (_O. ovis_) resembles an overgrown house-fly. Its general color is brown, and it is apparently lazy, flying about very little. This bot-fly makes its appearance when the warm weather begins, and deposits live larvae in the nostrils of sheep. This act is greatly feared by the animals, as shown by their crowding together and holding the head down. The larva works up the nasal cavities and reaches the sinuses of the head, where it becomes attached to the lining mucous membrane. In the spring, when fully developed, it passes out through the nasal cavities and nostrils, drops to the ground, buries itself, and in from four to six weeks develops into the mature fly.