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.

In a previous chapter reference was made to the exopterygote insects,
stone-flies, dragon-flies, and may-flies, whose preparatory stages live
in the water. Among the endopterygote orders many Neuroptera and
Coleoptera, all Trichoptera, a very few Lepidoptera and many Diptera,
have aquatic larvae. One or two examples of the adaptations of dipteran