194 Orthorrhapha brachycera. 



with a few bristles above, but none below; middle tibiæ with two 

 rows of bristles above and one ventral bristle ; hind tibiæ likewise 

 with two dorsal rows, and with sonie very small ventral bristles and 

 one stronger towards the apex. Wings almost hyaline; veins blackish 

 brown or black; discai vein with an exceedingly slight bend and only 



Fig. 58. Wing of H. aerof^us d*. 



very slightly converging towards the cubital vein, almost parallel with 

 it ; posterior cross-vein about perpendicular, not oblique as in assimi- 

 lis; axillary lobe moderately developed. Squamulæ yellow, with a 

 black fringe. Halteres pale yellow. 



Female. Epistoma broad, grey. Antennæ shorter than in the 

 male, third joint short triangular. 



Length 3 — 3,5 mm. 



This species is very similar to assimilis^ but in the male it is at 

 once known by the black epistoma; the female is more difficult to 

 distinguish, but it will be known by the quite yellow hind tibiæ, 

 and by the clearer wings with the more parallel cubital and discai 

 vein and the more perpendicular posterior cross-vein: when the front 

 and hind femora are blackish striped, this will also be a distinguishing 

 character. The two species assimilis and aerosus will easily be known 

 from metallicus by the size and the character from the venation given 

 in the table; the feraales of these two species are on the other hånd 

 also similar to the females of celer and brevicornis, they may however 

 be distinguished by the character in the latter species, that the basal 

 part of the costa has a thickening, which is, to be sure, slight, but 

 on close examination distinctly to be seen. 



H. aerosus is rather common in Denmark; Charlottenlund, Or- 

 drup Krat, Dyrehaven, Bøllemosen, Ryget Skov at Farum Sø, Hille- 

 rød and Præstevang at Hillerød, Tisvilde, Faxe Ladeplads; on Funen 

 at Middelfart and Veflinge; in Jutland at Laven and Sminge near 

 Silkeborg, Randers, Ravnkilde near Skørping and at Sæby. My dates 

 are ^/e to the first part of August. It occurs in woods in low herbage 

 at and near water, and on water piants. 



