THE SOCIAL INSECTS 



and after several moults the following individuals were 

 produced : 



Large workers Soldiers Substitute Stayed as nymphs 



sexual forms or died 



103 37 82 323 



This result illustrates another characteristic of termite 

 development : that moulting can lead either to progressive 

 or to retrogressive development. Thus the large workers 

 and soldiers have actually lost the wing-pads which were 

 present in the nymphs, whereas the heads and mandibles of 

 the soldiers and the reproductive organs of the sexual forms 

 show a positive change. The possibility, within limits, of 

 developing either forwards or backwards gives the caste 

 system great flexibility. 



THEORIES OF CASTE FORMATION 



It has been mentioned that both the original royal couple 

 and the soldier seem to inhibit the production of forms like 

 themselves, either temporarily or permanently. This obser- 

 vation led certain American students to propose the theory 

 of caste control by " social hormones ". It was suggested, 

 for instance, that the secretions of the royal couple, which 

 are so eagerly licked up by the workers attending them, 

 would be shared all through the colony by the mutual inter- 

 change of food, and might, like the effect of some drug, 

 inhibit the development of sexual maturity in other members 

 of the group. A similar theory might explain the inhibitory 

 effect of the soldier, though in that caste the secretions to be 

 handed round are much less copious. 



Such theories meet two initial difficulties. First, in many 

 termites the variety of castes would require a surprising 

 number of special substances ; secondly, winged sexual forms 



184 



