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- 



FlG. 32. — .'Vn oak Rail, 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. 



