THE METAMORPHOSIS OF INSECTS 177 



been evolved forms that exhibit deviations from the usual type of 

 development. The more familiar examples of these are the following : 

 The Saltitorial Orthoptera. In the crickets, locusts, and long- 

 horned grasshoppers, the wings of the nymphs are developed in an 

 inverted position; that surface of the wing which is on the outside in 

 the adult is next to the body in the nymphal instars; and the rudi- 

 mentary hind wings are outside of the fore wings, instead of beneath 

 them, as in the adult. At the last ecdysis the wings assume the normal 

 position. 



The Cicadas. In the Cicadas there exists a greater difference 

 between the nymphal instars and the adult than is usual with insects 

 in which the metamorphosis is gradual. The nymphs live below the 

 surface of the ground, feeding upon the roots of plants ; the adults 

 live in the open air, chiefly among the branches of trees. The forelegs 

 of the nymphs are fossorial (Fig. 200); this is an 

 adaptation for subterranean life, which is not needed 

 and not possessed by the adults. And it is said that 

 the last nymphal instar is quiescent for a period. 



The Coccida. In the Coccidae the mode of develop- 

 ment of the two sexes differ greatly. The female 

 never acquires wings, and in so far as external form is 

 concerned the adult is degenerate. The male, on 

 the other hand, exhibits a striking approach to com- 

 plete metamorphosis, the last nymphal instar being' 

 enclosed in a cocoon, and the legs of the adult are not 

 those of the nymph, being developed from imaginal 

 disks. But the wings are developed externally. 

 - The Aleyrodida.In this family the type of meta- 

 morphosis corresponds quite closely with that described 

 later as complete metamorphosis; consequently the 

 term larva is applied to the immature instars except the last, which is 

 designated the pupa. 



The wings arise as histoblasts in the late embryo, and the growth 

 of the wing-buds during the larval stadia takes place inside the body- 

 wall. The change to the pupal instar, in which the wing-buds are 

 external, takes place beneath the last larval skin, which is known as 

 the pupa case or puparium. The adult emerges through a T-shaped 

 opening on the dorsum of the puparium. Both sexes are winged. 



The Aphidida. In the Aphididae there exists a remarkable type 

 of development known as heterogamy or cyclic reproduction. This is 

 characterized by an alternation of several parthenogenetic generations 



