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, 

 as seen in the young grasshopper 

 m (Fig. 9), the young dragonfly (Fig. 59), 



and the young cicada (Fig. 114), the 

 L 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 in- 

 vaginated into the cavity of the body, 

 in which position they remain through 

 all the active life 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 neces- 

 sary correlation between the exter- 

 nally 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 use- 

 less organs removed from the surface 



[246] 



Fig. 138. Diagram of 



metamorphosis 

 If during the course of 

 their evolution, the 

 adult (/) and the larva 

 (Z,)have independently 

 diverged from a straight 

 line of development 

 («w), the larva must 

 finally attain the adult 

 stage by a transforma- 

 tion (metamorphosis), 

 the degree of which is 

 represented by the 

 length of the line L to / 



