SOCIAL LIFE 233 



quently die of starvation. Such insects, thus taking ad- 

 vantage of the labours of others, are known as inquilines 

 or '* cuckoo-parasites." In some cases the inquiline larva 

 devours the grub of the host, behaving like a beast of prey, 

 having by its mother's action been insinuated into the 

 habitation of its victims. The origin of these types of 

 parasitism among the sting-bearing Hymenoptera (Aculeata) 

 has been suggestively discussed by W. M. Wheeler (191 9), 

 who gives good reason for believing that the parasitic habit 

 arose in all cases among members of the same species, 

 whereof in times of '* scarcity of prey or food . . . individuals 

 . . . might find it as easy as advantageous to steal the pro- 

 visions of other individuals." Wheeler suggests further 

 that the urgency of the egg-laying reflex would reinforce 

 the stimulus due to scarcity in tending to establish the 

 parasitic habit. He insists that many of the inquiline 

 Hymenoptera now distinguishable specifically and often 

 generically from their hosts, are nevertheless closely related 

 to the latter, and the habit as developed among social bees 

 and wasps affords strong evidence in support of this view. 

 In communities of bumble-bees it has been shown by 

 F. W. L. Sladen (1912) that occasional or incipient para- 

 sitism is fairly common. One queen may enter the nest 

 of another of the same kind, kill her and install herself in 

 the vacant place. Among our British species are two 

 nearly related, Bomhus terrestris and B. liicorum, the former 

 of which not infrequently preys on the latter, " killing the 

 lucorum queen and getting the lucorum workers to rear her 

 [own] young." This habit is, of course, abnormal and 

 occasional, but it might easily become the starting-point 

 for a definitely parasitic race. Its further development is 

 illustrated by the peculiar inquiline bumble-bees of the 

 genus Psithyrus, which have no worker-caste and whose 

 queens are destitute of the characteristic pollen-gathering 

 structures on the hind-legs. An over- wintered female of 

 Psithyrus enters a young nest of Bombus in the spring 

 after the first set of the workers have been developed. She 

 seeks " to ingratiate herself with the inhabitants, and in this 



