Sexual and Partheiiogenetic Forms 333 



Wingless parthenogenetic individuals are found during the 

 winter on the roots, where they cause sweUings, or gall-like 

 thickenings. Immense damage has been done to the vineyards 

 of Europe by these root-inhibiting forms. At the end of June, 

 winged parthenogenetic individuals develop, and wander up- 

 ward to the leaves. They deposit eggs of two kinds, — large 

 ones that produce the sexual females, and smaller ones that 

 produce the males. The males and females pair, and the fe- 

 male deposits on the bark a single large winter egg. In the 

 spring this egg produces a wingless female that goes to the 

 leaves and produces there gall-Hke growths. Offspring are 

 produced, some or all of which find their way in time to the 

 roots, where they remain over winter. 



In the phylloxerans of the hickories the hf e history is somewhat 

 the same. Following Pergande the cycle for most of the Ameri- 

 can species is as follows : From the winter egg a female — the 

 stem-mother — is produced. She wanders to the young leaves 

 and fixes her proboscis in the under surface. A gall develops 

 around her, within which she lays eggs that give rise to winged 

 migrants. These leave the gall by the opening on its under 

 surface, and fly away. Those that alight on the bark of the 

 hickory deposit their eggs in its crevices. These eggs are large 

 or small, and produce respectively female or male individuals. 

 These pair, and the female lays a single large egg that over- 

 winters and gives rise in the following spring to the stem-mother. 

 In a few species the stem-mother produces forms that are, as 

 a rule, without wings. These deposit the large and small eggs 

 within the gall. Presumably the sexual forms that hatch from 

 these eggs leave the galls and deposit the fertilized ^gg on the 

 stem. 



The hfe histories of these different forms offer many points 

 for consideration, and what is more important would seem to 

 give unusual opportunities to test the factors that lead to the 

 transformations of one form into another. What would happen, 

 for example, if the life-cycle were to be artificially altered at dif- 

 ferent points in its course by transferring the individuals prema- 



