WHEELER— THE PARASITIC ACULEATA. 29 



lay her eggs. She is at first very prolific, " but she ages and fails 

 more quickly than the Bomhus queen. . . . The Psithyrus kills the 

 Bombiis queen before she has laid the full number of worker eggs,' 

 consequently nests containing Psithyri are not very populous, the 

 number of workers seldom exceeding eighty." Neither queens nor 

 males of Bomhus are reared in such infested nests, but the workers 

 take to ovipositing. Their eggs would, of course, produce males, 

 but the Psithyrus devours them. She " pays close attention to her 

 new-laid eggs for several hours, giving the workers no chance to 

 molest them, but the workers soon get reconciled to them and 

 henceforth they feed and tend the Psithyrus brood with as much 

 devotion as if it were their own species; indeed, they seem some- 

 times to show a greater fondness for it." Sladen's concluding re- 

 marks are very interesting in connection with the case of Psammo- 

 chares rufipes and pectinipcs. He says: 



" The origin of Psithyrus, more especially of its peculiar parasitical in- 

 stincts, is an interesting question. If a specimen of Psithyrus be compared 

 with a specimen of Bombus it is seen that the resemblance is not merely- 

 superficial but extends to nearly all the important details of structure, so 

 that it is impossible to avoid the conclusion that Psithyrus has sprung from 

 Bombus, and this at quite a recent period in the history of life. Moreover, 

 the Bombi — and this is particularly interesting — show parasitical tendencies 

 leading to the parasitism of Psithyrus. We have seen (pages 55-58) how 

 the Bombus queens may enter the nests of their own species and kill one 

 another, and how, in the case of the twin species, B. terrestris and lucorum, 

 tcrrestris has extended this habit so as to prey on lucorum, killing the 

 lucorum queen and getting the lucorum workers to rear her young in practi- 

 cally the same manner as the Psithyri prey on the Bombi. It is a remarkable 

 fact that the sting of the tcrrestris queen differs from that of the lucorum 

 queen and approaches that of Psithyrus in being somewhat stouter and more 

 curved, and having its thickened basal portion more parallel-sided when 

 viewed sideways than in lucorum. There is, however, no evidence to show 

 that any species of Psithyrus has sprung from the particular species of 

 Bombus on which it preys, such resemblances as it may show to it in coat- 

 colour, etc., being pretty clearly attributable to mimicry or exposure to the 

 same conditions of life, and not to ancestry." 



Among the social wasps only two parasitic species are known, 

 Vespa arctica and V. austriaca. The former belongs to our Cana- 

 dian faunal zone and infests the nests of V. diahoUca, as Fletcher 

 (1908) has shown ; the latter has long been known in Europe where 

 it occurs in the nests of V. rufa. Recently Bequaert (1916) has 



