500 HYMENOPTERA chap, xxii 



words, the act of fertilisation may initiate a different condition 

 of nutrition of the ovaries, and this may determine the sex of 

 the eggs produced. 



Polymorphism, or Castes. — The question of the causes of 

 the modified individuals forming the various castes of the social 

 Hymenoptera has been much discussed. These individuals are 

 many of them very different in size and structure from either of 

 their parents, and are also different in their habits and instincts. 

 This difficult subject is far from being completely elucidated. 

 In the case of the honey-bee it is well established that an egg of 

 the female sex can, after deposition, be made either into a queen 

 or a worker-bee by the mode of nutrition — using that word in 

 the largest sense. On the other hand, Dewitz thought that in 

 the case of the ant Formica rufa, the caste — whether worker or 

 winged female — is already determined in the Insect before leav- 

 ing the egg.i Weismann and others associate the caste with 

 some hypothetic rudiments they consider to exist at the very 

 earliest stage of the embryonic, or oogenetic process. 



Herbert Spencer says : ^ " Among these social Insects the sex 

 is determined by degree of nutrition while the egg is being 

 formed," and " after an egg, predetermined as a female, has been 

 laid, the character of the produced Insect as a perfect female or 

 imperfect female is determined by the nutrition of the larva. 

 That is, one set of differences in structure and instincts is deter- 

 mined hy nutrition iefore the egg is laid, and a further set of 

 differences iji structures and instincts is determined ly nutrition 

 after the egg is laid." 



Spencer's generalisation is not inconsistent with the facts 

 hitherto brought to light, though it is possible that the progress 

 of knowledge may show some variety as to the periods of the 

 development at which the commencements of the modifications 

 occur. 



Fig. 339 represents the chief castes, or adult forms, existing 

 in a community of one of the most highly developed of the 

 species of social Hymenoptera, the leaf-cutting ant, Atta cepha- 

 lotes. We shall, when dealing with Formicidae, enter into some 

 details as to these and other cases of polymorphism. Our object 



^ Zeitschr. wiss. Zool. xxx. Supp. 1878, p. 103. 



^ Rejoinder to Professor "Weismann, p. 11. Reprint from Contemporary Review, 

 December 1893. 



