INSECTS
been preserved; and many European entomologists use the word "nymph" for the stage we call a pupa.
A larva is distinguished from a nymph by the lack of wing rudiments visible externally, and by the absence of the compound eyes. Many larvae are blind, but some of them have a group of simple eyes on each side of the head substituting for the compound eyes. Nymphs in general have the compound eyes of the adult insect, and,
Fig. 138. Diagram of metamorphosis
If during the course of their evolution, the adult (I) and the larva (L) have independently diverged from a straight line of development (nm), the larva must finally attain the adult stage by a transformation (metamorphosis), the degree of which is represented by the length of the line L to I
as seen an the young grasshopper (Fig. 59), the young dragonfly (Fig. 59), and the young cicada (Fig. 114), the nymphal wings are small pads that grow from the thoracic segments after the first or second molt. The larva, however, is not actually wingless any more than is the nymph; its wings are simply developed internally instead of externally. When the groups of cells that are destined to form the wings begin to multiply, the wing rudiments push inward instead of outward, and become small sacs invaginated into the cavity of the body, in which position they remain through all the active lire of the larva. Then, at the time of the transformation, the wing sacs are everted, and appear on the outside of the pupa when the last larval skin is cast off.
It is difficult to discover any necessary correlation between the externally wingless condition of the larva and the existence of a pupal stage in the life of the insect; but the two for some reason go together. Perhaps it is only a coincidence. To have useless organs removed from the surface
[ 246 ]
