POLYMORPHISM IN NEUTER INSECTS 67 



to be of another nature. Recent work such as that of Emerson 

 (1926), John (1925), Heath (1927, 1928), etc., makes it highly- 

 probable that in all primitive termites, the forms which do the 

 work of the colony are not a distinct caste, but are the juvenile 

 forms of the soldiers. There is thus a marked heterogony 

 of the head and jaws between the last worker instar and the 

 first soldier instar (we have already seen that there may be 

 more than one soldier instar : Heath, 1927 : see also Hare, 



I93I-) 



In more specialized forms, however, while some of the 



workers are juvenile soldiers, a true worker caste, distinguish- 

 able by large size and different proportions, also exists 

 (Emerson, 1926, etc.) It would be extremely interesting to 

 measure the head and body of these two types throughout 

 their growth. 



These results, combined with the fact that in some primitive 

 forms, fertile soldiers are met with (Heath, 1928 ; Imms, 1920), 

 and that soldiers with wing-buds are not unknown, whereas 

 no true workers are known ever to be fertile or to possess 

 wing-buds, indicates that workers have been derived from 

 soldiers by a suppression of their final development into the 

 normal big-jawed type — that, in fact, they are neotenic. This 

 neoteny, however, possibly at first facultative, must at least 

 in higher forms have been fixed as a constant caste-character- 

 istic, either by differential feeding or perhaps more probably 

 by some genetic mechanism (see Thompson, 1917). 



This has a bearing on our problem, since the main feature 

 in the evolution of the worker from the soldier would simply 

 be the delay in the onset of the head's heterogony, a delay 

 which, when it exceeded the time to the final instar, would 

 eventually lead to the total absence of soldier characteristics. 

 A less degree of delay would give rise to forms of intermediate 

 soldier-worker type ; these are known to exist in various 

 genera, e.g. Armitermes and related genera. 



Thus, although heterogony appears to play its part in the 

 origin of polymorphism both in ants and termites, its role is 

 different in the two groups. In ants, the control of absolute 

 size through the feeding of the larvae appears to be the main 

 method. In termites true heterogonic polymorphism is rare, 

 and when present seems due to irregularity of moult-number ; 

 but neoteny due to postponement of the onset of heterogony 

 also plays a role in the differentiation of castes. 



