HENRY J, FRANKLIN. 231 



what longer. The hind coxae are very close together and are 

 inserted between the lower plate of the metapleuron and the 

 front plate of the metasternum in front and the hind plate of 

 the metasternum behind. They are triangular, flattened 

 pieces and they taper from base to apex. The trochanters 

 and femora do not differ, in any very important respect, from 

 those of the fore and middle legs. In Bombus, the hind 

 tibiae (fig. B) of the queens and workers are modified for 

 pollen collecting. In this modification, they have become 

 considerably widened apically and their outer faces have be- 

 come flattened, destitute of clothing, except for a fringe of 

 long hairs on the front and hind margins, and smooth, except 

 for a faint reticulation, which is plainly visible with a good 

 lens. These flattened outer surfaces of the tibiae, together 

 with their bordering fringes of long hairs, make up what are 

 known as the corbiculce or pollen baskets and the bordering 

 fringes are, in this paper, called the corbicular fringes. The 

 color of these fringes is usually quite constant within species 

 limits and is, therefore, of much taxonomic value. In Psi- 

 thyrus (fig. A), the outer faces of the hind tibiae of both 

 sexes are convex and hairy all over. In the females, how- 

 ever, the hair on the front and hind margins of these tibiae 

 is somewhat longer than that between so that there is often 

 a slight suggestion of a corbicula. In the males of Bombus, 

 there is usually a considerable corbicular development, though 

 it is always more or less weak in comparison with that of the 

 females. There is considerable variation in this regard be- 

 tween the males of the different species. Those of the Ter- 

 restris group have very well developed corbiculae. In the 

 Borealis group, the outer faces of the hind tibia of the males 

 are longitudinally concaved. 



Each hind tibia, in all groups and castes, bears two long 

 ' and prominent spines on the front side of its distal end, the 

 inner being the longer (fig. 48, SS). Each of these spines 

 has two lines of denticles or spinules on one side running 

 from base to apex. The surface of these spines is very finely 

 reticulated. 



In Bombus., the hind tibiae of the queens and workers have 



TRANS. AM. ENT. SOC, XXXVIII. 



