HENRY J. FRANKLIN. 235 



peculiar to each genus. These will here be described sepa- 

 rately : 



Bombus. — (a). The females (queens and workers) have completely- 

 developed pollen baskets on the outer sides of the hind tibiae while 

 the males, as a rule, have them only partly developed. 



(b). The hind leg of the female has a comb of spines on the inner 

 side of the apical margin of the tibia, guarding a pincer-like cleft, 

 formed between the end of the tibia and a posterior basal extension of 

 the metatarsus. These structures, which make up the " pollen mills," 

 are absent in the male. 



(c). The eyes of the males of some species are greatly swollen and 

 bulge out from the sides of the head as they never do in the females. 



(d.) The malar space of the male is, in some species, much shorter 

 than that of the female. 



(e) . The hind metatarsi of the females are much wider than those of 

 the male and, in some species, have much shorter hair fringing the 

 hind border. 



(f). The queens are always larger than either the workers or males. 

 The size of the males and workers is very variable and the variation 

 has about the same range in each sex. 



Psithyrus. — (a). The labrum of the females is triangular, that of the 

 males rectangular. 



(b). The hypopygium of the female has lateral elevations while that 

 of the male does not have them. 



Constancy of Characters. 



In systematic work on any group of insects, a knowledge of 

 the relative constancy of the various characters presented by 

 the group is always of great assistance. In the family Bom- 

 bidse, all the structural characters, except the "malar space," 

 appear to be very constant indeed within species limits 

 and some of these characters, which are of great value in 

 separating the species into natural groups, show only slight 

 variation even within group limits. While the malar space, 

 as a rule, varies but very little in proportional length between 

 individuals of the same sex of the same species, its variation 

 in some species is very considerable. As examples of spe- 

 cies showing this variation we may cite B. vagans and B. 

 pleuralis. Tables showing this variation have been included 

 with the descriptions of these species. 



The coloration of the pile is very much less constant be- 



TRANS. AM. ENT. SOC, XXXVIII. 



