INSECTS
normal procedure of growth by direct development from the embryo to the adult.
It must appear that the nymph, or young of an insect with incomplete metamorphosis, is merely an aberrant development of the normal form of the young as it occurs in an insect without metamorphosis. This is evident from the fact that the nymph has external wings, fully developed compound eyes, and in general the same details of structure in the legs and other parts of the body as has the adult. Most larvae, on the other hand, have
Fig. 140. A bristletail, Thermobia, a member of the order Thysanura, another primitive group of wingless insects (Twice natural size)
few or none of the structural details of the adult that might be expected to occur in a normal postembryonic adolescent form; but they do have many characters that appear to belong to a primitive stage of evolution and that we might expect to find in an embryonic stage of development. The caterpillar, for example, has legs on the abdomen (Fig. 136, AbL), an embryonic feature possessed by none of the higher insects in the adult stage; it has only one claw on its thoracic legs, a character of crustaceans and myriapods, but not of adult winged insects or of nymphs. Likewise, there are certain features of the internal structure of the caterpillar that are more primitive than in any adult insect or nymph; and the same evidence of primitive or embryonic characters might be cited of other larvae. On the other hand, the structural details of some larvae are very much like those of the adults, and such larvae differ from the adults of their species principally in the lack of the compound eyes and of external wings.
Now, if all the insects with complete metamorphosis
[ 248 ]
