HYMENOPTERA 



987 



The parasitic bumblebees, Psithyrus. — To this genus belong those 

 parasitic bees that infest the nests of bumblebees. They closely re- 

 semble bumblebees in appearance and in structure, except that, as 

 in other parasitic bees, the females do not possess organs for collecting 

 and carr^nng pollen. Although the females of Psithyrus are easily 

 distinguished from those of Bonibus by the absence of the pollen- 

 baskets or corbiculae in the former, the males of the two genera are 

 very similar. In Psithyrus there is no worker caste. 



The conclusions of different observers as to the extent of the 

 parasitism of Psithyrus differ widely. Sladen ('12) from his studies 

 of English species regards them as the deadliest enemies of the 



fioneij J It [ 



buTvdh cf "J 



Cldcoctcn 

 ccnlainirif 

 pollen 



honex^ i, 



Old cccoom 

 filled 

 wi'bh hx>ney 



Fig. 1227. — Nest in mid- summer. (From Sladen.) 



bumblebees whose nests they infest. He found "That it is the 

 practice of the Psithyrus female to enter the nest of the Bombus, to 

 sting the queen to death, and then get the poor workers to rear her 

 young instead of their own brothers and sisters." This conclusion is 

 not in accord, however, with those of other European writers ; and the 

 American species of Psithryus whose habits have been studied, rarely, 

 if ever, kill the host queen. For a detailed account of the relations 

 of these parasites and their hosts and for references to the literature 

 of this subject see Plath ('22). For descriptions of the New World 

 species see Franklin ('i2-'i3). 



