SYNOPSIS OF NORTH AMERICAN SYRPHIDiE. XVII 



d. Face evenly rounded and pilose Pipiza Fallen. 



dd. Face not evenly rounded and pilose, tuberculate. 



e. Abdomen with only four visible segments ; arista short, subapical. 



Nausigasteu Will. 

 ee. Abdomen with at least five visible segments. 

 /. Face partly or wholly yellow ; abdomen puuctulate, antennae elongate; small 



species Paragus Latr 



ff. Face wholly black, or at least only with luteous spots; antennae short. 



g. Hind femora slender Chilosia Meig. 



gg. Hind femora thickened ; epistoma projecting. 

 7(. Scutellum unusually large, nearly square in outline ; males dichoptic. 



Chalcomyia Will. 



7i/i. Scutellum not unusually large, considerably broader at the base; males holop- 



tic , Myiolepta Newm. 



There is considerable diversitj" in this group, but there can be no 

 natural division, so closely do the characters blend from one to the 

 other, and because the relationships do not permit a linear ari'ang'ement 

 of the genera. Chilosia may be taken as its most predominant type, from 

 which Chalcomyia and N'nusigaster depart the most. The genus itself 

 is a large one, with a considerable diversity of forms, but which, so far, 

 has eluded attempts at rational subdivision. The presence of a swollen 

 frontal triangle, or frontal groove, is characteristic in not a few species, 

 but they are evanescent, and not of sufBcient uniformity to warrant their 

 use exclusively as a generic distinction. The neuration varies from a 

 Pipizoidtoa Syrphoidtype ; none except artificial distinctions can be made 

 use of here. The antenni© show only slight variations; generic charac- 

 ters based chiefly upon these organs are not generally reliable through 

 out the family. The plumosity of the arista has been proposed as a 

 means of distinction ', but the plumosity is never very strong, and the 

 ultimate generic boundary would often depend upon the power of the 

 lens employed ; nor can this character be used in connection with the 

 pilosity of the eyes and the hairiness of the face, unless we are prepared 

 to dub every ijossible permutation of the three characters with an inde- 

 pendent title. The pilosity of the eyes is also unsatisfactory ; its use 

 will separate allied forms and bring together irrelevant ones ; moreover, 

 here, likewise, the microscope and possibly the imagination would 

 sometimes be the ultimate generic distinctions. The hairiness of the 

 face is in itself wholly insufficient, and bears no constant relation to the 

 other more important characters. The thoracic bristles imperceptibly 

 change to hair and then to pile. The limits of the genus are also doubt- 

 ful. From Chrysogaster on the one hand, and Melanostoma on the other, 

 nothing more than specific separation is yet possible. The relation to 

 Pipiza., through Chrysogaster and Psilofa, is equally obvious. Yet again 

 we cannot overlook the natural affinity with species of Myiolepta. 



Pipiza and Psilota, as here employed, are unquestionably nearly re- 

 lated forms ; indeed further discoveries may require their union. The 

 former genus has been very much subdivided. A knowledge of nu- 

 merous species, however, constantly tends to break down the distinc- 



