GRUBS, WARBLES, BOTS.
Fig. 5.—Screw-worm fly (Chrysomyia macellaria). Enlarged. (From Bureau of Entomology.)
Ox warbles are whitish or, when full grown, dark-colored grubs or maggots that develop from the eggs deposited on the hairs of cattle by certain flies known as warble flies. In the United States there are two species of ox-warble flies, technically known as Hypoderma lineatum and Hypoderma bovis. These flies somewhat resemble bees in their general appearance, but like all flies have only two wings.
The first named, H. lineatum, is commonly called the heel fly and is more generally distributed over the United States than the other species. The tail has a distinctive reddish-orange color and the legs are rough and hairy. This fly commonly deposits its eggs about the coronet, whence the name of heel fly, and on the fetlocks, knees, and hocks. When cattle are resting, eggs are deposited along the line of contact of the body with the soil. Cattle are frequently indifferent to the activity of this fly in depositing its eggs. Commonly 8 to 10 eggs, sometimes as many as 14, are attached to a single hair.
In the United States the other warble fly, H. bovis, has been found only in the North (New England, New York, Pennsylvania, Maryland, Michigan, Iowa, Missouri, and Washington) and has not yet been found in the Southern States. The tail is orange-yellow, lighter in color than that of the other species, and the legs have but few hairs. This fly commonly deposits its eggs on the outside of the hind quarters and above the fetlocks when the animals are moving, or lower down if they are quiet. Cattle are usually much disturbed by the activity of this fly and not infrequently appear terror stricken. The eggs are attached singly, one egg to a hair near its base.
The eggs of the warble flies hatch as a rule in about a week, the time varying with local conditions. The young warble is about 1 mm. (1/25 inch) long. It crawls to the base of the hair and burrows into the hair follicle. The entrance of the larvæ frequently causes sudden appearance of swellings. The larvæ of H. bovis in entering the skin rarely cause a flow of serum
