158 HYMENOPTERA OF AMERICA. [PART I. 



lengthened and rather flattened above. The clypens is pyriform 

 among the females, but is usually wide or wider than long; its ex- 

 tremity terminates in a little border, channelled by a small trench 

 which often makes it appear bidentate; among the males it is 

 polygonal and bidentate. The disk of the mesothorax offers two 

 or four grooves on its hinder part. Above all the metathorax is 

 very characteristic. It is a little oblique. Its posterior plate 

 always presents an excavation, which is divided by a vertical 

 carina. The post-scutellum is truncate, at times subcrenulate, 

 but its trench never coincides with that of the metathoraeic 

 fossette. The abdomen, always depressed and sessile, has an 

 ovalo-conic, never flatly conic form. It is at times velvety, never 

 coarsely punctured, except perhaps on borders of the segments. 

 The first segment is wide, cut squarely and shortly above. 



In fine, even when the form is much lengthened (0. tigris), the 

 general appearance is preserved, the body being rather flattened 

 than cylindrical. In this group the males are quite variable. 

 Among certain species they nearly assume the size of the females. 

 Among others they are very much smaller and vary endlessly in 

 form, so that one can scarcely refer them to their females, but 

 by a" series of groupings. 



Tulle to assist in the determination of the species of Ancistrocerus 

 (principally based upon the females'). 



1. Wings fusco-violaceous. 



2. Margin of second segment smooth; post-scutel black. 



11. Spinolos. 



2, 2. Margin of second segment rougli ; post-scutel yellow. 



24. unifasciatus. 

 1, 1. Wings subhyaline or smoky. 



2. Margin of the second segment reflexed 



3. Color rufous, vertex tuberculate. 26. tuber culiceps. 



3,3. Color black, argenteoua. 29. Arista. 



2, 2. Margin of the second segment not reflexed. 



3. Abdomen and antennae mostly yellow. 27. Sutterianus. 

 3,3. Abdomen and antennae mostly black. 



4. Metathorax with two long spines; angle of prothorax dentate. 



19. Clarazianus. 



4, 4. Metathorax with short spines or not dentate. 



5. Thorax black, body velvety. 



6. The first abd. segment not margined with yellow. 



23. Parredesi. 



