NOTES ON PSITHYRUS. 39 



found breeding in the only nest of B. flavifrons on record. These 

 figures indicate that in some places, at least in certain years, about 

 half of the colonies of those Brcnius species which serve as hosts 

 to certain species of Psithyrus are parasitized by the latter. 



Some of the European Psithyri e.g., Ps. quadricolor and rupes- 

 tris closely resemble their respective hosts in coloration, while 

 others, like Ps. cauipcstris, have no resemblance to their host what- 

 soever. In the case of the four American species of Psithyrus 

 whose hosts are known we have a similar state of affairs. Between 

 Ps. laboriosus and its host, B. vagans, there is considerable simi- 

 larity, especially between Psithyrus male and host. On the other 

 hand, there is little or no similarity between Ps. ashtoni and insu- 

 laris and their respective hosts, B. affinis and flavifrons, and be- 

 tween Ps. variabilis and its host, B. pennsylvauicns, there is practi- 

 cally none. 



Hoffer ('88, pp. 115, 116) states that those of the European 

 Psithyri which are parasitic on Bremus species, which are equally 

 large (queen against queen) or larger than they themselves, resem- 

 ble their hosts more closely than those Psithyrus species which prey 

 on smaller Bremus species. This correlation in color and size be- 

 tween parasite and host may be true of the European Psithyri, but 

 it does not hold for our American species. According to Franklin 

 ('12), the queens of Ps. ashtoni and variabilis are both consider- 

 ably smaller than their hosts, B. affinis and pennsylvanicus, and 

 still, in this case, there is little or no similarity in coloration between 

 parasite and host. On the other hand, the Ps. laboriosus queen is 

 somewhat larger than that of its host, B. vagans, and here, espe- 

 cially between Psithyrus male and host, we have a considerable 

 degree of similarity. 



We now come to the very interesting question as to the origin of 

 the members of the genus Psithyrus. M tiller ('/O* Perez ('83), 

 Hoffer ('88), Sladen ('12), Lutz ('16), and Wheeler ('19), all of 

 whom have paid more or less attention to this question, are agreed 

 that the members of the genus Psithyrus are a degenerate offshoot 

 from the genus Brcuius.' 1 One of these authors (Perez, '83, pp. 



1 All observations on Bremus-Psithyrus colonies indicate that in no case 

 is there a genetic relationship like that suggested by Patterson and Pack- 

 Beresford ('03) for Vespa austriaca and Vespa rufa. According to this 



