The habit of parasitism by maggots in back-boned animals has led to some remarkable modifications of the larva and to curious adventures in the course of the life-story. The Bot-fly of the Horse (_Gastrophilus equi_) and the Warble-fly of the Ox (_Hypoderma bovis_, fig. 22) lay eggs attached to the hairs of grazing animals, which, at least in the case of Gastrophilus, lick the newly-hatched larvae into their mouths. The 'bot,' or maggot of Gastrophilus, comes to rest in the horse's stomach; often a whole family attach themselves by their mouth-hooks to a small patch of the mucous coat of that organ. The maggot is relatively short and stout, with rows of strong spicules surrounding the segments, and with spiracles capable of withdrawal through a cup-like inpushing of the tail-region of the body, so that the parasite is preserved from drowning when the host drinks water. The young maggot of Hypoderma (fig. 22 _e_) is elongate and slender, spends its first two stages burrowing in the gullet wall and then wandering through the dorsal tissues of its host; ultimately it arrives beneath the skin of the back and assumes for its third and fourth instars a broad barrel-like form (fig. 22 _b_). The supply of free oxygen within the ox's tissues being now insufficient, the warble-maggot bores a circular hole through the skin and rests with the tail spiracles directed upwards towards the outer air. When fully grown the maggot works its way through the hole in the host's skin, and falling to the ground pupates in some sheltered spot, the life cycle occupying about a year. Similarly the Horse-bot escapes from the host's intestine with the excrement, and pupates on the ground. A curious modification of the maggot is noticeable in the larva of the Hover-flies (Syrphus). These, unlike most of their allies, live exposed on the foliage of plants, where they feed by preying on aphids. [Illustration: Fig. 22. Ox Warble-fly (_Hypoderma bovis_), _a_, female; _b_, full-grown maggot from back of ox, dorsal view; _c_, egg; _d_, empty puparium, ventral view; _e_, young maggot from gullet, ventral view. Magnified (lines show natural size). _a-d_, after Theobald, _2nd Report Econ. Zool._ (_Brit. Mus._).] In agreement with this manner of life, the cuticle is roughly granulated, often greenish or reddish in hue, and the maggot, despite its want of definite head and sense organs, moves actively and purposefully about, often rearing up on its broad tail-end with an aphid victim impaled on its mouth-hooks.