144 HYMENOPTEEA OF AMERICA. [PART I. 



but never as much petiolated as in Eumenes, the base only of the 

 first segment being slender, if at all, while its extremity always 

 envelops the base of the 2d segment. The form of the different 

 parts of the body is quite variable. The thorax is short or elon- 

 gate, angulate or rounded, the abdomen may be thick or slender, 

 quite conical and sessile or sessile and not conical, but ovate and 

 cylindrical ; it becomes also subpetiolate, the first segment being 

 more cup-shaped or campanulate. The abdomen is also often 

 attenuated anteriorly and posteriorly, which makes it spindle- 

 shaped, etc. Several of these forms make transitions to other 

 genera. In Odynerus, as in all the genera with more or less 

 sessile abdomen, the styliform appendages at the end of the poste- 

 rior tibiae are not as much developed as in the petiolate genera. 

 The exterior spine of the hinder femora is always small. The 

 Odynerus are distinguished : from Alastor by their wings, which 

 have the 2d cubital cell not petiolate upon the radial cell ; from 

 Pterochilus by their labial palpi, comparatively small and not 

 pectinate or feathery; from Montezumia by a different facies and 

 by the composition of their palpi ; from Monobia only by the 

 formation of the palpi. 



It is to Nortonia that these insects have the most similarity. 

 They differ by their less petiolate abdomen (really not petiolate) 

 and by the emarginate or variable metathorax, which in Nortonia 

 remains as in Eumenes, convex and stubbed. 



Nevertheless it would be difficult to set a distinct limit between 

 Odynerus and the most closely allied genera. = Ex.: — 



Transitions to Alastor. — In most species of Odynerus the 2d 

 cubital cell has a little radial edge which makes it very distinct 

 from Alastor, but in some intermediate types the 2d cubital cell 

 becomes quite triangular, only touching the radial cell with its 

 anterior angles. This is really quite a transition to Alastor, but 

 we keep the intermediate types in the genus Odynerus, reserving 

 for the genus Alastor only the species in which the 2d cubital 

 cell is distinctly petiolate upon the radial cell. 



Transitions to Pterochilus. — In the subgenus Epipona, the 

 thorax becomes short and rounded, the abdomen oval and de- 

 pressed. The whole insect assumes quite the appearance of a 

 Pterochilus and can only be distinguished by its normal labial 

 palpi. 



Transitions to Monobia. — As said above, the two genera can 



