SOCIAL PARASITES 



a particular egg produces a queen or a worker, it seems that 

 the balance between the two possible types of development 

 may be quite easily tipped one way or the other. It may well 

 be that the original balance of the colony is upset by the 

 presence of the parasite in such a way as to suppress queen- 

 production in the host. It has been suggested, for instance, 

 that in Strongylognathus it requires less effort to rear the 

 small sexual forms of the parasite than the much larger ones 

 of the host, Tetramorium. This might lead both to the 

 suppression of the sexual forms of the host and to the con- 

 version of the majority of the parasite brood into sexual forms 

 rather than workers. This argument, of course, raises again 

 the question whether castes are determined by feeding or 

 genetically. 



Probably in the more degenerate parasites such as Aner- 

 gates the power to produce a worker caste has been com- 

 pletely lost as the result of a long process of selection. Once 

 the workers have lost almost all their functions it is more 

 advantageous to have all the females fully fertile. It is possible 

 that Strongylognathus provides an example of what has been 

 called social regulation, a process (as we shall see) much 

 better known in the termites. This is the method by which 

 different types of individuals and of developmental stages are 

 produced in the proportions best suited to the needs of the 

 colony. It arises from the natural interplay of the normal 

 behaviour of the species with its environment. In Anergates, 

 social regulation has been replaced by an innate physiological 

 process. 



Social parasites are not very numerous, perhaps because 

 species which have evolved along these lines are rarely 

 successful for long. Amongst the eighty-eight social species 

 in Great Britain, eleven are parasitic, if one counts the slave- 

 making ant. According to our present knowledge, the 



167 



