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.