HYMENOPTERA 993 



wasps and the ants, there is in addition to the males and the egg- 

 laying females a worker caste ; with all other bees there are only two 

 forms, the males and the females. 



The parasitic bees do not constitute a natural division of the 

 group of bees, as was formerly supposed, instead of that it isevident 

 that members of several of the families of bees have acquired the 

 parasitic habit. The bees of the genus Psithyrus, which are parasitic 

 in the nests of btmiblebees, are closely allied to the bumblebees and 

 should be placed with them in the family Bombida?; the parasitic 

 genera Stelis and Ccelioxys are evidently members of the leaf-cutter- 

 bee family, the Megachilida?; and there are many parasitic genera 

 belonging to the family Andrenidas. 



The nests of solitary bees, like those of the digger-wasps, are of 

 many forms. The mining-bees dig tunnels in the ground ; the mason- 

 bees build nests of mortar-like material; the carpenter-bees make 

 tunnels in the stems of pithy plants or bore in solid wood ; and some 

 bees make nests of comminuted vegetable matter. The distinctive 

 characteristic of the nests of bees is the fact that they are always 

 provisioned with honey and pollen. In many cases closely allied spe- 

 cies of bees differ in their nesting habits; for example, different spe- 

 cies of the genus Osmia build very different kinds of nests. 



Although many entomologists have studied the bees intensively, 

 no classification of them has been proposed that is generally ac- 

 cepted. Some writers regard them as constituting a single family, 

 the Apidffi; other writers recognize several families and restrict the 

 term Apidag to the honey-bee family ; but these writers differ among 

 themselves as to the number of families that should be recognized. 



When several families of bees are recognized thev are commonly 

 grouped together as the superfamily Apoidea; but the writers whose 

 classifications I have adopted believe that the bees are a group of 

 sphecoid wasps that have acquired the habit of provisioning their 

 nests with honey and pollen, and should not, merely for this reason, 

 be placed in a separate superfamily. An analogous case is that of 

 the subfamily Masarinse some members of which differ from other 

 Vespidas in nest-provisioning habits in the same way that bees differ 

 from other sphecoid wasps. 



Family PROSOPID^ 

 The Bi fid-ton gued Bees 



The members of this family differ from all other bees in having 

 the tip of the labiimi either shallowly emarginate at the apex or deeply 

 bifid. Inallof them the labium is comparatively short. This family 

 has been commonly known as the wasp-like bees. It includes two 

 quite distinct subfamilies. 



Subfamily PROSOPIN^ 



This subfamily is represented in our fauna by a single genus, 

 Prosopis. The members of this genus, of which there are many 



