224 AMERICAN HYMENOPTERA, 



of the claspers, but this may be considered as doubtful. Each clasper 

 is made up of three separate pieces as follows : 



(a) The large basal and lateral piece which in a tapering basal ex- 

 tension reaches the middle line on the ventral side and there fuses, in 

 a narrow connection, with the corresponding extension from the other 

 clasper and which also nearly reaches the middle line in a basal exten- 

 sion on the dorsal side. This piece is smooth and with little or no 

 hair. It is known as the branch or stipes (figs. 71, 73, 119, 120, B). 

 It varies greatly in form, especially as seen on the dorsal side, between 

 the various species. 



(b) A small apical and dorsal piece attached to the distal end of the 

 stipes. This plate is typically two lobed, but either the inner or the 

 outer lobe may be greatly reduced, the former being sometimes 

 entirely absent (fig. 158). This plate is called the squama (figs. 119 

 and 120, T, and figs. 71, 73, 74, 77, 78, T). Its two lobes appear to 

 have different functions. The outer one evidently acts as a brace for 

 the end of the vosella which, in most species of Bombus, would, without 

 it, probably be either bent dorsally or broken by the pressure of the 

 female hypopygium against the middle of the dorsal sides of the 

 claspers, the tips of the volsellae being held fast from slipping on the 

 plates dorsal to the basal arms of the sting by means of their apical 

 projections. The necessity for this bracing is shown by the fact that 

 the branch reaches cauded considerably farther on the dorsal than it 

 does on the ventral side and also by the fact that this outer lobe is, as 

 a rule, longest and best developed in those species which have the ends 

 of the volsellae reach farthest beyond the apices of the branches. The 

 function of the inner lobe is not known. It is, as a rule, best developed 

 in those species which have sagitts with very simple heads. There 

 appears to be a phylogenetic significance connected with this. The 

 squamae are, for the most part, without hair, but the inner lobe often 

 bears considerable. They are of great value in the classification of 

 species. 



(c) A large and usually very hairy piece attached to the inner side 

 of, and partly enclosed by, the stipes. This piece is known as the 

 volsella or lacinia (figs. 71, 73, 93, 119 and 120, V). It varies greatly 

 in its form and in the amount and distribution of its hair between 

 the different species. Its caudad extension, has compared with that 

 of the branch and squama, is also very variable between the various 

 species and is of especial importance, as it seems to have some phylo- 

 genetic significance. The apex of the volsella in Bombus bears a con- 

 spicuous, usually pointed and ventrally recurved projection on its 

 dorsal and inner side. This projection is, in this paper, called the 

 apical projection (figs. 71, PR, and 119, PR). Its function, apparently, 

 is to enable the volsellae to keep a firm hold on the side plates at the base 

 of the sting and so keep that organ in an extruded position during copu- 

 lation, the claspers thus acting not only as pincers, but also functioning 



