448 AMERICAN HYMENOPTERA. 



or " Bumble-bees," for only entomologists recognize the dif- 

 ferences between the two genera. In entomological litera- 

 ture, the species of Psithyrush.2Lve been called "Guest Bees," 

 " Inquiline Bees" and "Guest Bumble-bees." They have 

 been given these names because of the fact that the pub- 

 lished observations, concerning their life history and habits, 

 indicate that they are habitually inquilinous in the nests of 

 species of Bombus. Many of the European records concern- 

 ing this are evidently valid, but there is not yet a single true 

 New World account of a Psithyriis having been found in a 

 Bombus nest. As there are other bees which are inquilinous 

 in the nests of solitary bees, the terms "Guest Bees" and 

 " Inquiline Bees " cannot be distinctive names for Psithyrus 

 species. The name " Guest Bumble-bees " is distinctive 

 and appropriate as these insects have undoubtedly descended 

 with Bombus from a common ancestral stock. 



Degeneracy. — Most of the structural characters, in which 

 Psithyrus differs from Bombus, appear to show a more or 

 less marked degeneration. Among these marks of degen- 

 eracy, the following may be particularly mentioned : 



1. The absence of corbiculae, which is correlated with their having 

 given up the habit of gathering pollen. 



2. The absence of the " pollen mills," which is also apparently cor- 

 related with their loss of the pollen collecting habit. These structures 

 and the corbiculae are present in Apis as well as in Bombus, and are 

 very much alike in those two genera. 



3. The comparatively small eyes, apparently correlated with their 

 habit of spending so much of their time in loafing in the darkness of 

 the nests of their hosts instead of flying actively abroad. 



4. The general form and the untoothed condition of the mandibles 

 of the females are probably primitive and they may be degenerate also. 

 They are much like those of the honey-bee, but the honey-bee does 

 not build a nest of grass or similar coarse materal, and so does not 

 need mandibles so well developed for tearing and cutting. 



5. The narrower hind metatarsi. They are very broad in Apis as 

 well as in Bombus. 



6. The membraneous volsellse of the male genitalia, with apical 

 projections absent, apparently correlated with a somewhat different 

 method of copulation. The lateral elevations on the female hypopy- 

 gium perhaps have some significance in this connection. 



