198 Zoology of Colorado 



was discovered that in two Australian genera the tongue is sharply 

 pointed in the male, broad and obtuse in the female. Thus, ac- 

 cording to the old system, the two sexes of these bees would fall 

 in different subfamilies! At the other extreme are those extremely 

 long-tongued bees, which cannot fold up the mouth parts, but 

 carry them under the body like the beak of a plant-bug. Such a 

 bee is Melitoma grisella, gray and of medium size, common about 

 Denver. It visits the flowers of the wild morning-glory, and as 

 that plant is not found at Boulder, the bee is also absent. The 

 most extreme case is that of the South American Glossura, in 

 which the tongue projects beyond the abdomen like a tail. 



Bees may also be divided into working and parasitic groups. 

 The latter, as for instance the wasp-like Nomada, store no food 

 for their young, but lay their eggs in the nests of other bees. 

 The parasitic bees do not constitute a distinct morphological 

 group, but are usually related to and presumably derived from the 

 groups at whose expense they live. Thus Psithyrus, which lives 

 at the expense of the bumble-bee (Bombus), is so like its involuntary 

 host that entomologists have often confused the two genera. 

 Coelioxys, living in the nests of leaf-cutting bees (Megachile), is 

 more distinct, especially by its hairy eyes, but is obviously nearer 

 to Megachile than to various other parasitic groups. Hence it 

 is quite certain that this parasitism has arisen independently a 

 number of times. Sladen has given a graphic account of Psi- 

 thyrus in the nests of bumble-bees. This insect, which is in effect 

 a Bombus which lacks the pollen-collecting apparatus on the hind 

 legs, enters the bumble-bee nest and stings the legitimate queen 

 to death. Then the bumble-bee workers, following their instincts 

 carefully rear the offspring of the usurper. 



"The Psithyrus queen pays close attention to her new-laid 

 eggs for several hours, giving the (Bombus) workers no chance 

 to molest them, but the workers soon get reconciled to them, and 

 henceforth they feed and tend the Psithyrus brood with as much 

 devotion as if it were of their own species." How did such para- 

 sitic forms arise? Sladen observed in England that occasionally 

 a queen of Bombus terrestris enters the nest of the closely related 

 B. lucorum, and kills the lucorum queen. It then gets the lucorum 

 workers to raise its young, precisely as Psithyrus would. Such a 

 habit, once firmly established, may finally result in a permanently 



