THE SOCIAL INSECTS 



Of the 20,200 species of insects in the British fauna, only 

 thirty-seven ants, about forty-four bees, and seven wasps 

 are social. The proportions in other parts of the world are 

 probably much the same. 



The insect society, with its store of food for the young, 

 is bound to provide a tempting site for a parasite to lay its 

 eggs. Moreover, as explained in Chapter 7, other females of 

 the same or allied species will often attempt to usurp the 

 incipient colony. Thus a certain " jealousy " of oviposition 

 by other females is nearly always shown by the female 

 founder of the colony. In humble bees and in most kinds of 

 social wasps the queen attacks any other individual attempting 

 to lay eggs, and this behaviour is aroused much more by the 

 act of oviposition than by the mere presence of another fertile 

 female. It is also true that if many females all lived together 

 and laid eggs indiscriminately, the number of young would 

 soon be too great for the available food-supply and for the 

 number of nurses. In all social forms a distinction, not always 

 very sharp, has thus developed between egg-laying queens 

 and sterile workers. Even if the workers do, in some circum- 

 stances, lay eggs, these are not fertilised and normally produce 

 males only. 



It appears that the special form of sex-determination which 

 is found in most Hymenoptera made the attainment of social 

 life more easy in that group. The female can store up inter- 

 nally for long periods the sperm transferred to her by the 

 male ; and she is able to expose some of her eggs to sperm 

 just before they are laid, whereas others can be laid un- 

 fertilised. In nearly all Hymenoptera the fertilised eggs 

 produce females and the unfertilised ones males. Thus for 

 the maintenance of the colony males are needed only at 

 long intervals, when a new brood of young queens requires 

 fertilisation. Among the stimuli which determine whether 



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