a thrusting its narrow head-end into the depths of its
ofttimes loathsome food-supply, we understand the advantage of access to
the air-tube system being mainly confined to the hinder end of the body.

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
fall