NOTES ON PSITHYRUS. 4! 



Group two, which may be called the laboriosus group, and which 

 comprises those Psithyrus species which Franklin ('12) has assem- 

 bled under that name, is restricted to the New World. Here, 

 again, we have a group whose members closely resemble a number 

 of Bremus species, also restricted to the New World, one of which, 

 B. vagans, serves as host to Ps. laboriosus, the chief representative 

 of this group. 



The third and largest group, which may be called the -vest alls 

 group, has representatives in both the Old and the New Worlds. 

 To it belong such European species as Ps. vest alls, distinctus, and 

 quadricolor, and the members of Franklin's ('12) ashtoni and 

 fernaldce groups in America. According to Perez ('83) and 

 Franklin ('12), many of the representatives of this group are also 

 similar in structure. This group likewise closely resembles a num- 

 ber of Bremus species, which in this case are present in both the 

 Old and the New World e.g., Bremus occidentals Greene in 

 America and B. latreillcUiis and jonellns in Europe. The last- 

 named species serves as host of Ps. quadricolor, a widely dis- 

 tributed European species belonging to this group. 



The great difference in coloration (with a parallel difference in 

 structure) between these three groups, the geographical distribution 

 of those Bremus species which resemble them, and the fact that 

 many Psithyrl are parasitic on similarly colored Bremus species, 

 all point to an independent origin of at least three Psithyrus 

 groups. 1 



Just how the parasitic habits of the representatives of the genus 

 Psithyrus may have originated is suggested by certain observations 

 of Sladen ('99, '12). According to this author, many of the later 

 appearing queens of certain Bremus species do not take the trouble 

 to start nests of their own, but enter nests already occupied by 

 their own or other species. In the resulting conflict the foundress 

 of the colony is sometimes killed, and her offspring then assists in 

 rearing the brood of the intruder. He states that this occurs fre- 

 quently between the queens of Bremus lucorum Linnaeus and its 

 larger variety, B. tcrrcstrls, the lucoruin colony serving as a tem- 

 porary host. From this temporary social parasitism, as he points 



i A similar independent origin has been suggested by Wheeler ('10, 

 p. 449) for certain ants which are temporary social parasites on other ants. 



