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 tringuetrella, 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. 
