THE SOCIAL INSECTS 



larvae, having no vestiges of wings, and no or only minute 

 eyes, but they have a fixed form and are no longer capable 

 of growth and moulting. 



The substitute sexual forms are individuals capable of 

 reproduction, though not winged and not leaving the nest 

 for a marriage flight. They appear whenever one or both of 

 the royal couple die or is removed from the colony. In 

 bodily form they resemble larvae or nymphs, depending 

 on their mode of production, but the wing pads are often 

 smaller than in the average nymphs. Once a termite has 

 become a soldier, it can no longer become a substitute sexual 

 form, and the same holds for the true workers of the higher 

 termites. 



Grasse and his colleagues in France have obtained the 

 conclusive experimental result in some of the lower termites 

 that an artificial group of, say, twenty or thirty identical 

 third stage larvae can differentiate in isolation into a colony 

 in which all castes are represented. This self-differentiation 

 of a group is the fundamental fact in caste-production. 

 The higher termites, with their more elaborate caste system, 

 have been much less studied, and little can be said about 

 them with certainty. There is, however, no reason at the 

 moment to suppose that their development is essentially 

 different. 



Another element of great importance in the flexible caste 

 system is the influence upon it of the age and size of the 

 colony. In general, the younger and smaller the colony, the 

 fewer moults are necessary to produce the adults of any caste. 

 In Zootermopsis, for instance, a soldier will be produced 

 after five, six, seven or eight moults according as the colony 

 consists of about twenty-five, fifty, one hundred or five 

 hundred individuals, corresponding to ages of one, two, two 

 to three, and four to five years, respectively. In such termites 



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