GROWTH 93 



(iii) By the action of internal parasites - notably the effect of Stylops 

 upon certain Hymenoptera. This effect was formerly called 'para- 

 sitic castration'; but it is more than the mere assumption of a neuter 

 condition, it often involves a partial reversal of sex : females acquire 

 male characteristics, and vice versa. In other words, the latent sex 

 factor is again manifesting itself. This result seems to be due to star- 

 vation, which may lead to lack of some specific nutrient factor, for 

 it occurs only in those Hymenoptera which receive a fixed ration to 

 last the whole of larval life, and not in those social forms which are 

 fed according to their individual needs ; and the effects are exagger- 

 ated if more than one parasite is present. 



Determination of other characters 



There are other types of dimorphism or polymorphism which have 

 much in common with the phenomenon of sex reversal. Certain para- 

 sitic insects exist in several forms with strikingly different mor- 

 phology : the type which appears depends upon the size of the host 

 or the species of host in which they are reared. Social insects, 

 notably termites, exist in a variety of different forms or castes which 

 are likewise determined by the environmental conditions and, in- 

 directly, by the needs of their societies. The reproductive forms of 

 termites prevent the appearance of other reproductive forms by a 

 chemical factor, or 'pheromone', given off in their excreta; if the 

 young workers do not receive this secretion certain of them develop 

 into 'supplementary reproductives', or neotenic adults. The appear- 

 ance of the soldier caste is probably regulated in the same way. The 

 normal female, or 'queen', of the honey-bee is produced by special 

 feeding of female larvae, which receive a liberal supply of 'royal 

 jelly' from the salivary glands of the nurses throughout their growth. 

 'Solitary' and 'gregarious' phases of locusts are induced by the sparse 

 or crowded conditions of life in the growing larvae (p. 81). Likewise 

 winged parthenogenetic Aphids are produced by overcrowding (the 

 so-called 'group effect') and the wingless parthenogenetic forms by 

 solitary development. In all these cases the form of the body is in- 

 fluenced by factors, presumably of a chemical nature, acting upon 

 latent morphogenetic systems. To that extent they are comparable 

 with the phenomenon of metamorphosis. 



