76 DIPTERA OF NORTH AMERICA. "[PART II. 



of the old world which have been placed into the genus Gymnop- 

 ternus there certainly is a whole group of closely related species 

 the third and fourth longitudinal veins of which decidedly con- 

 verge. However, as this group must necessarily be separated from 

 the genus Gymnopternus, it could not prevent me from adopting 

 the parallelism of the third and fourth longitudinal veins as cha- 

 racteristic marks of Gymnopternus. Precisely, this character dis- 

 tinguishes in the easiest manner the species of Gymnopternus from 

 those of the following genera, which, like Gymnopternus, have 

 the upper edge of the first joint of the antennae hairy, and are 

 without bristles upon the first joint of the hind tarsi. 



Most of the species of Gymnopternus are small and have shorter 

 antennae, but a more distinctly pubescent arista than the species 

 of Dolickopus. They are much poorer in suitable plastic charac- 

 ters for the distinction of the species than the latter. The feet 

 of the males are very seldom ornamented. 



The color of the cilia of the inferior orbit is also of the highest 

 importance for the determination of the species of the present 

 genus. Unfortunately it cannot be so easily observed as in the 

 species of Dolichopus. Its discrimination in some species, of 

 which I have only single specimens, was totally impossible ; as to 

 others, I often remained uncertain. In the former case I have 

 been silent about their color, and in the latter I did not use any 

 positive expressions. Under these circumstances it was impossible 

 to use the color of the cilia of the lower orbit as a basis for a 

 subdivision. This is, however, but of little moment, as the cilia 

 of the lower orbit seem to be black in almost all North American 

 species of Gymnopternus. Another important mark for the dis- 

 tinction of the species is the hairy or glabrous surface of the scu- 

 tellum, provided there is a sufficient number of well preserved 

 specimens ; otherwise, if the specimens are few or not well pre- 

 served, this mark will be rather uncertain. I did not wish to 

 omit characters of this kind altogether, but have to request the 

 reader not to place too much confidence in them, especially when 

 my expressions seem to imply doubt. The same rule applies to 

 the form of the lamellae of the hypopygium. In many species 

 they have the form of an erect crescent, fastened by its lower 

 point. This form will only then be recognized, when they are not 

 closely applied to the hypopygium with the concave side ; if the 

 latter is the case, then they appear only as small lamella?, with 



