n of galls on various plants, through
the activity of larvae of the hymenopterous family Cynipidae. Many
plant-galls are due, however, to the presence of grubs of tiny dipterous
insects, the Cecidomyidae or Gall-midges. A cecid grub (fig. 19) has an
elongate body with flexible, wrinkled cuticle, tapering somewhat at the
two ends. The head, if rather narrow, is distinct, and beneath the
prothorax is a characteristic sclerite known as the 'anchor process' or
'breast bone.' Along either side of the body is a series of paired
spiracles, each usually situated at the tip of a little tubular
outgrowth of the cuticle; the hindmost spiracles are often larger than
the others. These little grubs live in family communities, their
presence leading to some deformation of the plant that serves to shelter
them. A shrivelled fruit or an arrested and swollen shoot, such as may
be due respectively to the Pear-midge (_Diplosis pyrivora_) or the
Osier-midge (_Rhabdophaga heterobia_), is a frequent result of the
irritation set up by these little grubs. In a larva of the crane-fly
family (Tipulidae, fig. 20) living underground and eating plant-roots,
like the well-known 'leather-jacket' grubs of the large
'Daddy-long-legs' (Tipula) or burrowing into a rotting turnip or swollen
fungus, like the more slender grub of a 'Winter Gnat' (Trichocera), the
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_,
sid