WHEELER— THE PARASITIC ACULEATA. 31 



indicated by the fact, which has not, I believe, been emphasized by 

 other students of the subject, that most of the Aculeate hosts belong 

 to dominant genera. By dominant genera I mean those that are 

 represented by a considerable number of species, some of which are 

 very abundant in individuals and widely distributed as distinguished 

 from genera that are monotypic or represented by few species of 

 rare or sporadic occurrence. Such genera are Andrena, Halictus, 

 Anthophora, Megachilc, Osm'ia and Bomhus among the bees, Vcspa, 

 and PsainnwcJiarcs among the wasps, and among the ants Formica, 

 Lasius, Tapinoma, Pheidole, Crematogastcr, Aphccnogastcr, Tetra- 

 morhim, Monomorium and Leptothorax. It is, nevertheless, sur- 

 prising that no hosts of parasitic ants are known to occur in genera 

 like Camponotiis and Polyrhachis, which comprise hundreds of spe- 

 cies and are widely distributed, the former in all parts of the world, 

 the latter in the old world tropics. Probably the dominant genera, 

 owing to their abundance in individuals and the wide distribution 

 of their species, would act like great nets set to capture any para- 

 sites that have overstepped the bounds of good parasitic manners 

 by bringing their original host species to the verge of extinction. 

 This would account for the close generic affinities which we have 

 seen to be so evident between parasite and host, for a parasite that 

 had endangered or destroyed its original host species would itself 

 more readily escape extinction if the host were already a member 

 of a dominant genus containing many closely allied species, be- 

 cause this would permit a comparatively easy re-adaptation of the 

 parasite to a new host species. Although the parasites would prob- 

 ably differ in their powers of adaptation, the very similar habits of 

 species in the same genus, especially among the bees, would greatly 

 facilitate such a transfer of the parasitic relation. 



Still even if we grant that the Aculeate parasite has arisen from 

 its original host species, we are confronted with two troublesome 

 questions, for it will be asked : What induced certain individuals to 

 become parasites on other individuals of their own species ? And 

 if a parasite originated in this way, what is to prevent it inbreeding 

 with its host and thus losing its peculiar tendencies by swamping, 

 or " panmixia " ? One of the difficulties involved in the first ques- 

 tion lies in deciding on the stimuli that would so afifect some of the 



