THE INSECT WORLD 



the queen lays fertilised or unfertilised eggs are probably- 

 some depending on the general condition of the colony which 

 can thus adjust the sex-ratio to current needs. 



In the typical Hymenopterous colony no males are present 

 for most of the year : the workers are sterile females and no 

 corresponding caste of sterile males has been evolved. The 

 termites, in contrast, have both sexes equally represented in 

 the sterile castes. In this group the female seems to be unable 

 to store for long periods the sperm received at her first 

 marriage flight, and she therefore requires frequent fertilisa- 

 tion. Moreover, the method of sex-determination is different : 

 unfertilised eggs do not develop, while fertilised ones give 

 rise to about equal numbers of the two sexes. It may be that 

 the difficulty of evolving two sterile castes is one reason why 

 the bisexual type of society arose only once, in the termites, 

 whereas the female-dominated type evidently arose four or 

 five times in different groups of Hymenoptera. 



39 



