32 WHEELER— THE PARASITIC ACULEATA. 



individuals of a species as to compel them to give up the industrious 

 and nonparasitic habits that have become elaborated and fixed as 

 an integral part of their genetic constitution. I believe, however, 

 that such stimuli exist and that they are frequent and compara- 

 tively simple. If we take such a constantly recurring external 

 stimulus as temporary scarcity of prey or food, we can understand 

 how some individuals of a common species that has outrun its food 

 supply or has emerged in seasons or places of scarcity, might find 

 it as easy as advantageous to steal the provisions of other indi- 

 viduals. This is, in fact, a common practice among normally non- 

 parasitic Aculeata, e. g., in Bcmbix and Psammo chares, in bumble- 

 bees, and honey-bees. And if this external is reinforced by an 

 internal stimulus, such as the urgent need for oviposition, we can 

 see how a parasitic group of individuals might readily arise within 

 the confines of a single species. This urgency of oviposition is 

 very apparent in many parasitic Aculeata, especially among the 

 parasitic bees, which often lay several eggs in a single cell of the 

 host, though only one larva is able to develop. PsitJiyrus and some 

 of the parasitic ants seem to reveal the same urgency. It is even 

 probable that this internal stimulus is more fundamental than the 

 external stimulus above mentioned and that it may incite the insect 

 directly to appropriate the provisions collected by other individuals 

 whose ovarian eggs mature more slowly or in smaller numbers. 

 When the parasitic habit is once started it tends necessarily, owing 

 to the saving of energy which would otherwise be expended in 

 work, to accelerate the maturation of the ova and thus to become 

 more and more confirmed by one of the circular processes so fa- 

 miliar to the physiologist. 



Urgency of oviposition will, I believe, account also for the many 

 cases in which Aculeates have been observed in the act of appro- 

 priating valuable nesting materials or partly constructed nests of 

 other individuals. Fabre (1890) saw wall-bees (Chalicodoma inu- 

 raria) take possession of the partly constructed masonry nests be- 

 longing to other females of the colony and destroy their eggs, and 

 Adlerz (1904) observed other bees (Trachusa scrratnlcc) enter 

 each others' burrows and steal the pine-pitch with which they glue 

 together the pieces of leaves for their nests. Drory (1872) saw 



