[Illustration: Fig. 6. Apple Aphid (_Aphis pomi_), virgin females, _a_,
wingless; _b_, winged. Magnified 20 times.]

A family of Hemiptera, related to the Aphidae and equally obnoxious to
the gardener, is that of the Coccidae or scale-insects. These furnish an
excellent illustration of features noticeable in certain insect
life-histories. In the first place, the newly-hatched young differs
markedly from the parent in the details of its structure. A young coccid
(fig. 7 _c_) is flattened oval in shape, has well-developed feelers
(fig. 7 _d_) and legs, and runs actively about, usually on the leaves or
bark of trees and shrubs, through which it pierces with its long jaws,
so that it may suck sap from the soft tissues beneath. After a time it
fixes itself by means of these jaws and the characteristic scale or
protective covering, composed partly of a waxy secretion and partly of
dried excrement, begins to grow over its body. 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.]