500 HYMENOPTERA chap, xxi 



words, the act of fertilisation may initiate a different conditi( 

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

 the eggs prodviced. 



Polymorphism, or Castes. The question of tlie 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 inJ 

 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.^ 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 hj nutrition hefore the egg is laid, and a further set of 

 differences in structures and instincts is determined hy 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 th 

 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 



1 Zeitschr. iviss. Zool. xxx. Supp. 1878, p. 103. 



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

 December 1893. 



