Maggots, differing from that of the Bluebottle only in minor details, are the larval forms of a vast multitude of allied species and display great variation in the nature of their food.  Most, however, hide their soft defenceless bodies in some substance which affords shelter as well as food.  The Bluebottle maggot burrows into flesh, that of the House-fly into horse-dung or vegetable refuse.  The maggot of the Cabbage-fly eats its way into the roots of cruciferous plants, that of the Mangel-fly works out a broad blister between the two skins of a leaf, into which the newly-hatched larva crawls directly from the egg.  A large number of species, forming an entire subfamily (the Tachininae) have larvae that feed as parasites within the bodies of other insects.

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.).]