Insects 199 



parasitic bee, which has even lost the means of feeding its own 

 young, and has in fact no worker cast. 



No observations on the biology of Psithyrus have been made 

 in Colorado, but we have three species in the mountains. All 

 three have been named twice, one name for the male, another for 

 the female, but the sexes are now correctly associated. In the 

 queen, P. suckjcyi is known by the black hairs of the head, with 

 at most a little yellow intermixed above. In P. insularis the 

 face between the bases of the antennae shows yellow hair, and 

 there is much yellow on the upper part of the head behind. In 

 P. fernaldae the face has entirely dark hair, or there may be a 

 very little yellow, but the upper part of the head behind has 

 yellow. There are, of course, other characters, but these are 

 readily seen. In all these females the hind tibiae are convex and 

 hairy on the outer side, instead of being concave and smooth, 

 with long fringes on the margins, as in Bombus. In other respects 

 they look just like bumble-bees. 



In the study of bees, it is necessary to note particularly the 

 structures for collecting pollen. These of course exist only in 

 the females, and are absent in both sexes of the parasitic genera. 

 In the large group Megachilidae, except for the parasitic members, 

 there is an extensive ventral scopa or pollen brush on the under 

 side of the abdomen. The hairs of the scopa may be white, 

 yellow, red or black, according to the species, but very often they 

 appear magnificently yellow, because full of pollen gathered from 

 plants such as the sunflower and goldenrod. Many other bees 

 possess no distinct ventral scopa, but instead a well-developed 

 one on the hind legs. Other important features are derived from 

 the wings, and these have the advantage of being easily seen with 

 a lens. On examining the upper wings, a thickening will be 

 observed on the upper or anterior margin. This is the stigma; it 

 may be large or small, slender or short and thick. The surface 

 of the wing is divided by the so-called veins into spaces termed the 

 cells. A large one beginning at the stigma and extending toward 

 the apex of the wing is the marginal cell. It is exceedingly long 

 in the honey bee, very short and cut off squarely or obliquely in 

 Perdita. Below the marginal cell are the cubital cells, primitively 

 three, but in many bees (as the Megachilidae) reduced to two by 

 the obliteration of one of the cross-veins. So distinctive is the 



