THE OPEN TYPE OF WING-GROWTH 73 



the autumn sexual insects may be all winged or all wingless, 

 or the females may be wingless and the males winged 

 (Fig. 40). The striking feature in the aphid life-cycle from our 

 present standpoint is that a wingless mother gives birth to 

 young that develop wings, and that the offspring of 

 these may, in their turn, be wingless. A marked and sudden 

 change of form is thus seen in the course of a single 



P. Detmers. del. 



FIG. 40. SEXUAL FORMS OF CABBAGE APHID (Aphis . 



a, male, X 15 ; 6, head and feeler, x 30. ; c, female, x iz ; d, head and feeler, x 

 From Weed, " Insect Life ", III. 



generation. But occasionally intermediate forms with use- 

 less wings half the normal size or still more rudimentary, 

 are produced. 



The aphids may also serve to introduce another subject of 

 much importance, one moreover that lends itself to extended 

 treatment ; the varying amount of divergence between adult 

 and young displayed by various insects that practise the open 

 type of wing-growth. In the two examples described in the 



