How Animals Work. 



in some species a regular alternation of generations 

 exists. Thus, in Vienna, galls on the oak leaves pro- 

 duce about the end of April a Gall-fly called Chilaspis 

 lowii, both male and female insects emerging from 

 the galls. The fertile females later lay eggs on the 

 ribs of the leaves of the same kind of oak, but their 

 work produces a different kind of gall from that in 

 which they were born. These galls, along with the 

 leaves, fall from the trees in the autumn, and in July 

 or August of the following year a Gall-fly emerges from 

 them. It is quite a different creature in appearance 



from the mother, however so 

 different, indeed, that before its 

 life history was known it was 

 thought to be a totally different 

 insect, and had received the name 

 of Chilaspis nitida. Only females 

 of this form are known to de- 

 velop, no male having ever been 

 seen ; and these female Gall-flies, 

 without any intercourse with a 

 male, lay their eggs in the young buds of the oak that 

 are already present in the autumn, and in the follow- 

 ing spring, when the buds begin to open and leaves 

 to develop, those that have had an egg laid in them 

 produce a gall from which true Chilaspis lowii Gall- 

 flies of both sexes emerge in April and May. The 

 complete cycle of the two generations therefore extends 

 over two years, and the generation that occupies the 

 longest time for its production consists only of females. 

 In some species in which this alternation of genera- 

 tion is known to exist, a series of generations com- 



254 



