THE TERMITES 



are produced in colonies containing a royal couple. It is 

 only the substitute sexual forms which are inhibited. The 

 difference is that the substitutes would reproduce in the 

 colony already containing the royal couple, whereas the 

 winged forms would leave it to found new colonies else- 

 where. The inhibition, therefore, is of a peculiar type. 



Eventually Castle and Light tried to test the theory by 

 experiments with Zootermopsis. Royal couples were killed 

 and extracts of them were made in various solvents, such as 

 alcohol or ether, and the extracted material, after removal of 

 the solvent, was added to the food of groups of isolated 

 nymphs. It is broadly true to say that in all the most satis- 

 factory experiments the nymphs nevertheless transformed 

 into sexual adults without much delay. Such experiments are 

 not quite conclusive, since it must always be uncertain 

 whether it is possible to extract an unidentified organic sub- 

 stance unchanged. Yet since there is no objective evidence 

 for the existence of the inhibitory substances, and since the 

 mere absence of inhibition is a very incomplete explanation 

 of the differentiation into several castes of an isolated group 

 of similar nymphs, the theory has now little to commend it. 



The social hormone theory is but one of several " trophic " 

 explanations of caste production in termites. Some authors 

 have supposed that foods of the ordinary sort, together with 

 varying amounts of salivary secretions, might control the 

 process. At present it is very difficult to accept any such 

 theory. The food varies considerably with the age and the 

 condition of the colony. In young colonies, the sexual forms 

 are largely reared on vegetable matter, and the same is true 

 of substitute sexual forms at any time. There is no evidence 

 for a copious supply of glandular food, such as is given to 

 the queen-producing brood of the honey bee. Moreover, the 

 self-differentiating group of nymphs scarcely seems to have 



185 



