ART. 9 AMERICAN XYLOTINE SYRPHID-FLIES SHANNON 9 



widely spaced; bases of all the femora with spinose areas; anal cell 

 broadened, the lower vein bowed downward; scutellum 3-5 times as 

 broad as long. 



An unusual character may be noted here for the female of Poly- 

 donto7nyia curvipes. The first and third abdominal sternites are 

 greatly swollen, whereas the second is reduced and deeply sunken 

 between the first and third, thus forming a broad deep channel. 



Pterallastes differs from the Helophilini by : Males holoptic ; front 

 and ocelli of female normal; face of female concave from antennae 

 to oral margin, small tubercle in the male; slopes of face without 

 pile; spinose area at base of femora not so pronounced; hind femur 

 but little thickened, hind tibia very slightly bowed; scutellum about 

 tAvice as broad as long; third vein not deeply curved; anal cell not 

 unusually broadened, its lower vein nearly straight. 



PTERALLASTES THORACICUS Loew 



A fairly common species in the eastern part of the United States. 

 Not recorded from Canada or south of North Carolina. 

 Type. — In Museum of Comparative Zoology. 



Genus TROPIDIA Meigen 



KEY TO THE SPECIES OF THE GENUS TROPIDIA MEIGEN 



A\ First tergite entirely blacli ; scutellum not edged with yellow ; small species 

 4.5-9 mm. 



B\ Abdomen largely red incana Townsend. 



B^ Abdomen largely black or with yellow markings. 



C\ Second and third tergites with small yellowish or reddish marking 



on anterior corners; tarsi broad montana Hunter. 



C^ Second and third tergites with large yellow markings. 



jy. Hind margins of tergites 2, 3, and 4 bright yellow; species of 



average size coloradensis (Bigot). 



Dl Hind margins not strikingly yellow ; very small species. 



pygmaea, new sjjecies. 

 A^ First tergite with yellow lateral margins ; scutellum edged with yellow ; 

 usually larger species, 10-12 mm. 

 B'. Males. 



C\ A large basal spur on ventral surface of hind femur, 10-12 mm. 



mamillata Loew. 

 C^ Without such spur. 



D\ Hind trochanter produced spur-like calcarata Williston. 



D^ Hind trochanter not produced. 



E\ Hind femora entirely black quadrata (Say). 



E^ Hind femora reddish at base albistylum Macquart. 



Bl Females. 



C\ Frontal aspect of face dark. 



D\ Fourth tergite grayish pollinose; wings infuscated; third joint 

 broader than front across ocelli ; 10-11 mm. 



calcarata Williston. 



3042—26 2 



