The female loses legs and
feelers, and never acquires wings, becoming little more than a sluggish
egg-bag (fig. 7 _e_). The male on the other hand passes into a second
larval stage in which there are no functional legs, but rudiments of
legs and of wings are present on the epidermis beneath the cuticle, as
shown by B.O. Schmidt for Aspidiotus (1885). The penultimate instar of
this sex in which the wing-rudiments are visible externally lies
passively beneath the scale, its behaviour resembling that of a
butterfly pupa. The adult winged male (fig. 7 _a_) leads a short, but
active life.

[Illustration: Fig. 7. Mussel Scale-insect (_Mytilaspis pomorum_). _a_,
male; _b_, foot of male; _c_, larva, ventral view; _d_, feeler of larva;
_e_, female, ventral view. After Howard, _Yearbook U.S. Dept. Agric._
1904. Magnified, _a, c, e_ x 20; _b, d_ x 120.]

Another family allied to the Aphidae is that of the Cicads, hardly
represented in our fauna but abundant in many of the warmer regions of
the earth. Here also the young insect differs widely from its parent in
form, living underground and being provided with strong fore-legs for
digging in the soil. After a long subterranean existence, usually
extending over several years, the insect attains the penultimate stage
of its life-story, during which it rests passively within an earthen
cell, awaiting the final moult, which will usher in its winged and
perfect state.

In the life-histories of cicads and coccids, then, there are some
features which recall those of the caterpillar's transformation into the
butterfly. The newly-hatched insect is externally so unlike its parent
that it may be styled a larva. The penultimate instar is quiescent and
does not feed. But while the caterpillar shows throughout its life no
outward trace of wings, external wing-rudiments are evident in the young
stages of the cicad. In the male coccid we find a late larval stage with
hidden wing-rudiments, the importance of which, for comparison with the
caterpillar, will be appreciated later.

CHAPTER IV

FROM WATER TO AIR

Insects as a whole are preeminently creatures of the land and the air.
This is shown not only by the possession of wings by a vast majority of
the class, but by the mode of breathing to which reference has already
been made (p. 2), a system of branching air-tubes carrying atmospheric
air with its combustion-supporting oxygen to all the insect's tissues.
The air gains access to