214 AMERICAN HYMENOPTERA. 



to a rather sharp apical tubercule and each tubercular area 

 is sometimes represented by a similar rather acute tubercle. 

 The labrum of the Psiihyrus female is, therefore, often 

 noticeably trituberculate, the tubercles being arranged in a 

 triangle. The males of neither Bombus nor Psithyrus have 

 any carina or " tubercular areas " or tubercles on the labrum. 

 In both genera and in all castes the surface of the labrum is 

 more or less strongly punctate. Where " tubercular areas " 

 are present their depressed portions are comparatively 

 smooth. 



Mandibles. — In the females, these organs are well devel- 

 oped and appear moderately powerful. They are broad 

 toward the base and apex, but are narrowed considerably 

 in the middle. In all the New World species of Bo?nbus, the 

 apex of the female mandible is normally either three- or four- 

 toothed. Freaks of species with normally four-toothed man- 

 dibles sometimes have them five-toothed (fig. 37). In the 

 three-toothed mandibles, the two teeth on the front side are 

 small and more or less acute (fig. 41. T. 1 and T. 2) and 

 the third tooth (T. 3) is very broad and forms the greater 

 part of the biting edge. In four-toothed mandibles, the 

 fourth tooth (T. 4) is a small tooth on the hind side. Many 

 species have mandibles showing a partial gradation between 

 the three- and four-toothed condition (fig. 45). In Bombias 

 (fig. 30), the hind side of the biting edge of the female (queen 

 and worker) mandible is more extended distally, in compari- 

 son with the front side, than it is in the subgenus Bombus 

 (fig. 45). In Psithyrus females, this hind side of the biting 

 edge is very greatly extended (figs. 46 and 49) and is much 

 more acutely rounded than in the genus Bombus. The man- 

 dibles of female Psithyri are not distinctly toothed. The 

 biting edge, however, usually has a single notch. The outer 

 faces of the mandibles of both genera are convex and marked 

 by alternating longitudinal ridges and grooves, the grooves 

 being lined to a considerable extent with a very short and 

 fine, usually ferruginous pubescence. The inner faces of their 

 distal halves are strongly concaved and smooth except for a 

 single curved ridge which runs from the front margin over 



