the Bees of the Family NomadidfB. 41 



that a valid taxonoraic group must consist of forms not less 

 related to one another than to forms outside of the group, or, 

 at least, that the group must have a common ancestor ex- 

 hibiting the group-characters, then the family Nomadida3, by 

 the admission of its autlior, is not valid, or else must be 

 restricted to a small portion of tlie geneva now included in it. 

 In other words, convergent evolution must be distinguished 

 from blood-relationship *. 



In our studies of these bees we have met with facts whicli 

 convince us that they are descended from two or three entirely 

 diverse groups, Nomada itself coming from the Panurginaa. 



NOMADA, Scopoli, 1770. 



Mr. E. Saunders (Journ. Linn. Soc, Zool. xxiii. p. 423) 

 says : — " This genus forms a curious transition between the 

 Andrenidae and Apida3 ; it has many of the characteristics of 

 the latter division, such as the long tongue, the acute para- 

 glossas, and the long basal joint of the labial palpi ; on the 

 other hand, however, it has the joints of the labial palpi sub- 

 cylindrical, like the Andrenida^, without any tendency to the 

 flat or concave sheath-liiie form of the Apidte, and its species 

 are also parasitic on the Andrenidae (with the exception of 

 N. seafasciata, which frequents the burrows of Eucera). . . . 

 1 should feel inclined to treat it as a very abnormal genus of 

 the Andrenidse." 



When we come to compare tlie mouth-parts of Nomada 

 with those of the Panurginaa (which are themselves obviously 

 derived from the Andrenidaj) all the difficulties mentioned by 

 h'aunders disappear. In the Panurgine mouth we find tlie 

 long tongue, the long basal joint of the labial palpus, and 

 ]jaraglossa3 of the same type as Nomada. Moreover, certain 

 species of Nomada (e. g. N. similis) areparasitic on Panurgus. 

 Of all the Panurginas Fanurgus is most like Nomada, 

 having the same slender tapering galea, the same long 

 maxillary palpi, &c. The greatest difference is in the para- 

 glossge, which in Nomada are much more slender ; but 

 another Panurgine bee, Dufourea vulgaris, has slender para- 

 glossa3, as in Nomada, though it differs from Nomada in the 

 palpi. Nomada, to be sure, has three submarginal cells in 



* In the famous paper by De Vries (cf. ' Science,' Maj' 9, 1902, p. 72G) 

 new " species " are said to appear repeatedly anioiifr the offsprinj,'- uf 

 G^notheia or Omigra Lamarckiana (apparently it sliould be Oruifjra 

 ijrandijlora) ; thus the •' species " CEnothera ul}hmja originated several 

 hundred times independently. Uf course the fact is that Cfc,'. Lamarckiana 

 is polymorphic, and the various mutations have not yet reached the status 

 of species. — T. D. A. C. 



