40 Annals Entomological Society of America [Vol. I, 



entiated into distinct castes. This restriction of polymorphism 

 to the female in the social Hymenoptera, with which we are here 

 especially concerned, is easily intelligible if it be traceable, as is 

 usually supposed, to a physiological division of labor, for the 

 colonies of ants, bees, and wasps are essentially more or less per- 

 manent families of females, the male representing merely a fer- 

 tilizing agency temporarily intruding itself on the activities of 

 the community at the moment it becomes necessary to start other 

 colonies. We may say, therefore, that polymorphism among 

 social Hymenoptera is a physical expression of the high degree of 

 social plasticity and efficiency of the female among these in- 

 sects. This is shown more specifically in two characteristics of 

 this sex, namely the extraordinary intricacy and amplitude of 

 her instincts, which are thoroughly representative of the species, 

 and her ability to reproduce parthenogenetically. This, of 

 course, means a considerable degree of autonomy even in the 

 reproductive sphere. But parthenogenesis, while undoubtedly 

 contributing to the social efficiency of the female, must be re- 

 garded and treated as an independent phenomenon, without 

 closer connection with polymorphism, for the ability to develop 

 from unfertilized eggs is an ancient characteristic of the Hymen- 

 optera and many other insects, which made its appearance among 

 the solitary species, like the Tenthredinidee and Cynipidae, long 

 before the development of social life. Moreover, polymorphism 

 may occur in male insects which, of course, are not partheno- 

 genetic. That parthenogenesis is intimately connected with 

 sexual dimorphism, at least among the social Hymenoptera, 

 seems to be evident from the fact that the males usually if not 

 always develop from unfertilized, the females from fertilized eggs. 

 While the bumble-bees and wasps show us incipient stages 

 in the development of polymorphism, the ants as a group, with 

 the exception of a few parasitic genera that have secondarily lost 

 this character, are all completely polymorphic. It is conceivable 

 that the development of different castes in the female may have 

 arisen independently in each of the three groups of the social 

 Hymenoptera, although it is equally probable that they may have 

 inherited a tendency to polymorphism from a common extinct 

 ancestry. On either hypothesis, however, we must admit that 

 the ants have carried the development of the female castes much 

 further than the social bees and wasps, since they have not only 

 produced a wingless form of the worker, in addition to the winged 



