NOTES ON PSITHYRUS, WITH RECORDS OF TWO 

 NEW AMERICAN HOSTS. 1 



O. E. PLATH, 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS. 



In Franklin's monumental work, " The Bombidas of the New 

 World," we have a taxonomic outline of the American bumble- 

 bees, equal, if not superior, to similar works on the European 

 species. One is rather disappointed, however, on comparing our 

 knowledge, or rather lack of knowledge, concerning the habits of 

 the American species with the interesting accounts of the habits 

 of the European species given by men like Hoffer and Sladen. 

 What we do know about the habits of our North American species 

 we owe chiefly to the efforts of Putnam, Franklin, and Prison, but 

 the field is so large that the surface has hardly been scratched. 

 This is especially true of one of the subdivisions of our Bremidse, 2 

 the genus Psithyrus. Of the 13 or 14 species of Psithyrus de- 

 scribed from the New World, the hosts of only two have thus 

 far been recorded. On July 7, 1914, Sladen ('15), at Agassiz, 

 British Columbia, dug up a nest of Bremus flavifrons Cresson, 

 victimized by Psithyrus insularis Smith, and two years later Prison 

 ('16) recorded that, during the summers of 1910 and 1915, he had 

 repeatedly found the nests of Bremus pennsylvanicus De Geer in- 

 fested by Psithyrus variabilis Cresson. To these two records the 

 writer wishes to add two others, those for Psithyrus laboriosus 

 Fabricius and Psithyrus ashtoni Cresson. 



Before going into detail, however, it seems desirable to give a 

 brief resume of the structure, life history, and habits of these social 

 parasites, as we know them chiefly from the work of Kirby (1802), 



1 Contributions from the Entomological Laboratory of the Bussey Institu- 

 tion, Harvard University, No. 201. 



2 In using Bremus and Bremida: instead of the familiar Bombus and Bom- 

 bida, the writer follows Prison ('19) who in a recent paper pointed out the 

 reasons for making this change (cf. also the extensive paper by Morice and 

 Durrant ('14) on this subject). 



23 



