student notices a somewhat tough cuticle, a relatively small but
distinct head, and frequently prominent finger-like processes on the
tail-segment. Further examination shows a striking modification in the
arrangement of the spiracles. Instead of a paired series on most of the
body-segments, as in caterpillars and the vast majority of insects
whether larval or adult, there are two large spiracles surrounded by the
prominent tail-processes, and a pair of very small ones on the
prothorax, the latter possibly closed up and useless. This restriction
of the breathing-holes to a front and hind pair (amphipneustic
condition) or to a hind pair only (metapneustic type) is highly
characteristic of the larvae of Two-winged flies.

[8] Known as the Orthorrhapha and the Cyclorrhapha; these terms are
derived from the manner in which the larval or pupal cuticle splits, as
will be explained in the next chapter (p. 88).

[Illustration: Fig. 20. Crane-fly (_Tipula oleracea_), _a_, female; _b_,
larva ('leather-jacket' grub). Magnified twice.]

[Illustration: Fig. 21. Maggot of House-fly (_Musca domestica_), _a_,
side-view, magnified 5 times; _b_, prothoracic spiracle; _c_, feeler;
_d_, hind-region with posterior spiracles; _e_, _f_, head-region with
mouth-hooks; _g_, head-region of young maggot; _h_, eggs. All magnified.
After Howard, _Entom. Bull._ 4, _U.S. Dept. Agric._]

Turning now to the _maggot_, characteristic of the House-fly section
(fig. 21) of the Diptera, we see the greatest contrast between the larva
and the imago that can be found throughout the whole class of the
insects. The Bluebottle's eggs, the well-known 'fly blow' laid in summer
time on exposed meat, not unnaturally arouse feelings of disgust, yet