1908] The Polymorphism of Ants 65 



In the lives of the social insects the threptic, or philoprogeni- 

 ti\-e instincts are of such transcendent importance that all the 

 other instincts of the species, including, of course, those of ali- 

 mentation and nest-building, become merely tributary or ancillary. 

 In ants, especially, the instincts relating to the nurture of the 

 young bear the aspect of a dominating obsession. The very 

 strength and scope of these instincts, however, renders the insects 

 more susceptible to the inroads of a host of guests, commensals 

 and parasites. Besides the parasitic larvae of Chalcidids, Lome- 

 chusini and Phorids there are many adult beetles and other insects 

 on which the ants lavish as much or even more attention than 

 they do on their own brood. And when the ants themselves 

 become parasitic on other ants, it is always either for the sake 

 of having their own brood nurtured, as in the temporarily and 

 permanently parasitic forms, or for the purpose of securing the 

 brood of another species, as in the slave-making, or dulotic 

 species. 



The philoprogenitive instincts arose and were highly developed 

 among the solitary ancestral insects long before social life 

 made its appearance. In fact, social life is itself merely an exten- 

 sion of these instincts to the adult offspring, and there can be no 

 doubt that once developed it reacted rapidly and powerfully in 

 perfecting these same instincts. It is not so much the fact that 

 all the activities of the social insects converge towards and center 

 in the reproduction of the species, for this is the case with all 

 organisms, as the elaborate living environment developed for the 

 nurture of the young, that gives these insects their unique posi- 

 tion among the lower animals. A full analysis of the threptic 

 activities would involve a study of the entire ethology of the 

 social insects and cannot be undertaken at the present time. 

 Nevertheless the bearing of these instincts on the subject of poly- 

 morphism can hardly be overestimated and deserves to be em- 

 phasized in this connection. 



All writers agree in ascribing polymorphism to a physiological 

 division of labor among originally similar organisms. This is 

 tantamount to the assumption that the phylogenetic differentia- 

 tion of the castes arose in the sphere of function before it mani- 

 fested itself in structural peculiarities. Although this view im- 

 plies that the female, or queen, was the source from which both 

 the instincts and structures of the worker were derived, it has been 

 obscured by an improper emphasis on the instincts of the honey- 



