SOCIALES PSITHYRUS 59 



species of Bombus has the capacity of variation, and many of the 

 varieties are found in one nest, that is, among the offspring of a 

 single pair of the species, but many of the variations are restricted 

 to certain localities. Some of the forms can be considered as 

 actual (" fertige ") species, intermediate forms not being found, and 

 even the characters by which species are recognised being some- 

 what modified. As examples of this he mentions Bombus silvarum 

 and B. arenicula, B.pratorum and B. scrim sJtiranus. In other cases, 

 however, the varieties are not so discontinuous, intermediate forms 

 being numerous ; this condition is more common than the one we 

 have previously described ; B. terrestris, B. horturum, B. lapidarius 

 and B. pomorum are examples of these variable species. The 

 variation runs to a considerable extent in parallel lines in the 

 different species, there being a dark and a light form of each ; also 

 each species that has a white termination to the body appears in 

 a form with a red termination, and vice versd. In the Caucasus 

 many species that have everywhere else yellow bands possess 

 them white ; and in Corsica there are species that are entirely 

 black, with a red termination to the body, though in continental 

 Europe the same species exhibit yellow bands and a white ter- 

 mination to the body. AVith so much variation it will be readily 

 believed that much remains to be done in the study of this 

 fascinating genus. It is rich in species in the Northern hemi- 

 sphere, but poor in the Southern one, and in both the Ethiopian 

 and Australian regions it is thought to be entirely wanting. 



The species of the genus Psithyrus (Apatkus of many authors) 

 inhabit the nests of Bombus; although less numerous than the 

 species of the latter genus, they also are widely distributed. They 

 are so like Bombus in appearance that they were not distinguished 

 from them by the earlier entomologists ; and what is still more 

 remarkable, each species of Psithyrus resembles the Bombus with 

 which it usually lives. There appear, however, to be occasional 

 exceptions to this rule, Smith having seen one of the yellow- 

 banded Psithyrus in the nest of a red-tailed Bombus. Psitliyrus 

 is chiefly distinguished from Bombus by the absence of certain 

 characters that fit the latter Insects for their industrial life ; the 

 hind tibiae have no smooth space for the conveyance of pollen, 

 and, so far as is known, there are only two sexes, males and per- 

 fect females. The Bombus and Psithyrus live together on the 

 best terms, and it appears probable that the latter do the former 



