[Illustration: FIG. 7.--_Hypoderma bovis._ 1, egg; 2, maggot; 4, chrysalis case; 6, fly, magnified (Brauer); 3, 5, chrysalis and fly, natural size (B. Clark).] [Illustration: FIG. 8.--Sac of warble, showing growth of epidermis round aperture.] In countries where cattle are used for draught purposes, goadmarks are a frequent source of injury, and some of the large cattle-ticks do considerable damage to the hides of Spain and South America. From the tanners’ point of view, however, the most injurious insects are the “bot-flies” or “warble-flies” (_Hypoderma bovis_ and allied species, Fig. 7). There is still some controversy as to how the eggs of these insects are deposited. In the horse-bot fly it is known that the eggs, first deposited on the skin, are licked off and swallowed by the animal, and develop in the stomach, where they pass their larval and pupal life hanging on to its interior coats, and only drop off and are passed out with the dung before their final change to the complete fly. Fortified by this, and by some direct observation, some American naturalists are of opinion that the American species at least, hatches in the stomach, and as a minute larva wanders through all the intervening tissues till it reaches the skin, where it undergoes its further development. The late Miss Ormerod, who has made a careful study of the English species,[19] states that the egg hatches on the hair, and that the _larva_ simply eats its way below the skin, leaving a minute red puncture which it subsequently enlarges to obtain air for its spiracles, which are in the tail. As it grows it continues to irritate the lower part of the cavity with hooked mandibles, and lives on the pus and matter so produced. It grows to a length of fully ³⁄₄ inch, and the cavity, Fig. 8, situated between the skin and the subcutaneous tissue is often as large as half a walnut. It remains in the sac not only during its larval, but its pupal stage, which do not differ much in appearance, and falls out on the ground before complete development. In small numbers, the warble seems to do little injury to the general health of the animal, but cases have been known where animals have actually died of the inflammation produced. Some idea of the extent of the plague may be realised from the statement that an Indian kip in the possession of the writer has not less than 680 warble holes, and that almost equal numbers have been counted in English hides. Preventive measures are the sheltering of the cattle during the summer months when the fly is most prevalent; the application of mixtures of oil or grease with tar-oil and sulphur to the hair, to prevent egg-laying; and the destruction of the _larva_ in its early stages, in autumn and winter by smearing the breathing aperture with grease, or better, with mercurial ointment. When this is done sufficiently early, the hole heals up without permanent injury, but when it is allowed to remain open during the