THEIR RELATION TO PLANTS 79 



and females, without variation; always making the 

 same kind of gall. Others appear in spring from over- 

 wintered galls, normal males and females: the latter 

 lay eggs, galls appear, but from them only females 

 make their appearance; these in turn lay eggs and from 

 their galls males and females appear again the year 

 following. This is termed an "alternation of genera- 

 tions" and so long as the galls are similar and the fe- 



FIG. 32. An oak gall, made by Cynips q-spongifica; a, showing the 

 larva in its cell; b, exit hole of adult. 



males are similar, no confusion is caused. But it some- 

 times happens that the summer generation is very 

 different in appearance from the hibernating form, 

 while the gall itself is different and on a different kind 

 of tree, so that there appears to be no sort of connec- 

 tion between them until the life history has been com- 

 pletely followed out no light task in species of this 

 character. For some species no males have ever been 

 found and, so far as we know, these are maintained 

 by parthenogenetic females alone. 



