Sex as an Adaptation 419 



shells, and are much more resistant to unfavorable condi- 

 tions. They give rise in the following spring to females 

 only, and these are the parthenogenetic individuals that con- 

 tinue to produce during the summer new parthenogenetic 

 eggs. 



It is within the group of insects that some of the most 

 remarkable cases of parthenogenesis that we know are 

 found. In the moth, Psyche helix, only females are present, 

 as a rule, but rarely males have been found. In another 

 moth, Solenobia trinquetrella, the female reproduces by par- 

 thenogenesis, but at times males appear and may then be 

 even more numerous than the females. In the gall-wasps 

 parthenogenetic generations may alternate with a sexual 

 generation, and it is interesting to note that the sexual and 

 the parthenogenetic generations are so different that they 

 were supposed to belong to separate species, until it was 

 found that they were only alternate generations of the same 

 species. 



The aphids or plant-lice reproduce during the summer 

 by parthenogenesis, but in the autumn winged males and 

 females appear, and fertilized winter eggs are produced. 

 From these eggs there develop, in the following spring, the 

 wingless parthenogenetic summer forms, which produce the 

 successive generations of the wingless forms. As many 

 as fourteen summer broods may be produced. By keeping 

 the aphids in a warm temperature and supplying them with 

 plenty of moist food, it has been possible to continue the 

 parthenogenetic reproduction of the wingless forms for 

 years. As many as fifty successive broods have been pro- 

 duced in this way. It has not been entirely determined 

 whether it is the temperature or a change in the amount, 

 or kind, of food that causes the appearance of the winged 

 males and females, although it seems fairly certain that 

 diminution in the food, or in the amount of water contained 

 in it, is the chief cause of the change. 



