THE SOCIAL INSECTS 



their number by its parasitic action. The life of the species 

 is therefore rather precarious. 



SUMMING UP PARASITISM 



The story of the parasitic wasps, bees and ants which 

 avoid some of the difficulties of colony foundation by their 

 dependence on other species is suggestive in several ways. 

 There is of course no need to draw the moral that — even in 

 organic evolution — crime does not pay. Many parasites, 

 both amongst the insects and in other animal groups, are 

 highly successful and show no signs of dying out. Some 

 dependence on other organisms is universal amongst animals, 

 for all depend ultimately on the green plants. A successful 

 parasite, however, is one which does least damage to its 

 host, which allows if not the individual at least the species 

 to flourish. Apart from Formicoxenus, all the types dealt 

 with in this chapter prevent their host from producing sexual 

 forms. This by itself might not be a fatal objection to their 

 mode of life, from the evolutionary point of view. Yet once 

 the parasitic habit has begun, there seems to be a general 

 tendency for the worker caste to be lost. 



This loss can be looked at in two ways. First, in any 

 species, varieties lacking structures or types of behaviour 

 which contribute towards successful reproduction and sur- 

 vival will tend to be eliminated. In most social insects, the 

 presence of many workers is essential to successful repro- 

 duction in the colony ; any tendency to reduce the number 

 of workers would be suppressed. But in a parasitic species 

 whose brood may be reared by the host workers, the workers 

 of the parasite are no longer essential to its survival. A 

 situation therefore exists in which the suppression of the 

 worker caste will not be an immediate disadvantage. 



Secondly, while we are uncertain what determines whether 



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