138 THE AMERICAN NATURALIST [Vol. LV 



Correlated with such habits, structural modifications of 

 the body appear, such as the loss of the pollen-collecting 

 apparatus in parasitic bees. 



In certain ants there occurs a third type, social para- 

 sitism (Wheeler, '04) whereby the young females of 

 some ants that do not establish their own colonies insin- 

 uate themselves into the nests of other species of ants, 

 do away with the queen, and take on themselves the func- 

 tion of egg-laying. As the larvae from these eggs are 

 raised to maturity, they produce worker individuals of 

 the parasitic species which gradually supplant the orig- 

 inal population. Finally, the colony becomes pure and 

 maintains itself through its own efforts, giving no evi- 

 dence of the temporary social parasitism by which it 

 has originated. In a very few cases social parasitism 

 may become permanent with the complete elimination of 

 the worker caste. 



The term entomophagous parasite may be applied with 

 some appropriateness to all of the three types described, 

 but is most suitable for the first one, since there the par- 

 asite not only consumes its host, but feeds upon nothing 

 else during its developmental stages. By far the largest 

 number of species in the order exhibit this type and it is 

 the only one which I shall consider in any detail. 



There are several ways in which such parasitism may 

 have originated, but the question of origin is best de- 

 ferred until its several phases have been discussed at 

 greater length. 



Defining parasitism in its several forms as enumerated 

 on a previous page, we find that there are parasitic 

 genera included in nearly all of the larger groups of Hy- 

 menoptera. Thus, the Ichneumonoidse, Serphoidea and 

 Chalcidoidea, each represented by a number of families, 

 are almost exclusively entomophagous parasites, while 

 in the Aculeata, numerous parasitic genera appear, scat- 

 tered through a series of families with generally non- 

 parasitic habits. In addition to these there is the prim- 

 itive family Oryssidae, now known definitely (Eohwer, 



