discontinuous, intermediate forms being numerous; this condition is more common than the one we have previously described; _B. terrestris_, _B. hortorum_, _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 versa_. 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 termination to the body. With 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 hemisphere, 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_ (_Apathus_ 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_. _Psithyrus_ 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 perfect females. {60}[Illustration: Fig. 23—_Psithyrus vestalis_, Britain. A, Female, x 3/2; B, outer side of hind leg.]