NOTES ON PSITHYRUS. 37 



description of the hostile attitude of this rupcstris queen toward the 

 lapidariits workers, it seems probable that he was not dealing with 

 the mother of the Psithyms brood. 



According to Hoffer ('81, '88), Friese ('88), and Sladen ('12), 

 each European species of Psitliynis breeds only in the nests of 

 certain B remits species. Some e.g., Ps. rupcstris and vcstalis 

 seem to be restricted to a single host, while others, like Ps. cam- 

 pcstris and barbutcUus, have two, or even three, hosts. Whether 

 or not our American Psithyri have more than one host still remains 

 to be decided. In determining this question we must understand 

 clearly, however, what we mean by the term " host." While it is 

 true that some of the European Psithyri e.g., Ps. rnpcstris breed 

 only in the nests of a certain Brcmus species, they may nevertheless 

 be found lodging temporarily in the nests of other species. It will 

 therefore be necessary, if we wish to avoid confusion, to restrict 

 the term " host " to those Brcmus species in whose nests a given 

 species of PsitJiynts is known to breed successfully. Using this 

 definition of a Psithyrus host as a criterion, we thus far know of 

 only one host for each of the four American species of PsitJiyrus 

 whose hosts have been discovered. They are as follows : 



TABLE I. 



Psitlnrus. Host. 



Psitliynis ashtoni Crcsson Brcmus affinis Cresson 



Psitliynts insularis Smith Breams flai'ifrons Cresson 



Psitliynis laboriosus Fabricius Brcmus rag<.nis Smith 



Psitliynis z'ariabilis Cresson Breams pennsylvanicus De Geer 



But, as already pointed out by Sladen ('15), the geographical 

 distribution of Ps. insularis and its host, B. flai'ifrons, are not 

 identical, and we must therefore assume that Ps. insularis has more 

 than one host, or that B. flavifrons also occurs in Saskatchewan, 

 Manitoba, Ontario, Quebec, North Dakota, Minnesota, Wisconsin, 

 Michigan, and New York. This second alternative is rather im- 

 probable. 



Similar, though less extensive, discrepancies in the geographical 

 distribution also exist in the cases of Ps. ashtoni and its host, B. 

 affinis, and of Ps. laboriosus and its host, B. vagans. Ps. asJitoni 

 either has more than one host, or B. affinis ought to be present on 



