THE SOCIAL INSECTS 



thorax, may vary in a marked fashion. Finally, in some 

 groups, such as the large genus Pheidole, the workers are 

 found in two types with no intermediates, the small workers 

 with normal heads and the larger soldiers with dispropor- 

 tionately large heads. While some variation in size might be 

 expected in any species, especially if there is any likelihood 

 of competition for food amongst the brood, the variation 

 within the leaf-cutters, for instance, seems far more than 

 would be expected from such a cause. Besides, the grubs 

 which give rise to the queens are brought up in very similar 

 way, yet the queens vary little in size. Where the workers 

 are dimorphic, i.e. of two discontinuous types, there must 

 be some special explanation. However, it might be argued 

 that the females of nearly all ants are dimorphic, developing 

 either into queens or workers. The existence of the two sorts 

 of worker in Pheidole might then be regarded as no more 

 than a special case of the more general problem of female 

 polymorphism. If we could explain why it is that some grubs 

 produce queens and others workers, the variation within the 

 worker caste might also be understood. 



The question of what determines the development of a 

 queen or a worker is one about which a debate has gone on 

 for years; it is still not all settled. The answer is not neces- 

 sarily the same for every kind of ant, though it is usually 

 assumed that it is likely to be so. There are two main sorts 

 of theory. It is suggested first, that the queen lays two or 

 more sorts of eggs (genetic theories) ; second, that originally 

 similar grubs are given different quantities or types of foods 

 (trophic theories). In the midst of disagreement, there are 

 at least two certain facts: that the differences between males 

 and females (or workers) are genetic, and are due to the laying 

 of two sorts of eggs ; and that by starving ant-grubs at least 

 some variation in the size of queens or workers, running 



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