136 Proceedings of the Biological Society of Washington. 



which dwells principally on unessential color differences. Characteristic 

 for this species is a pair of rather large depressions on the dorsum of the 

 second abdominal segment, laterally close to the margin ; these leave the 

 lateral margins raised and convex. In tristis and the other species of the 

 group the dorsum of the second abdominal segment is simple, without 

 depressions or raised lateral margins. The abdomen is distinctly narrower 

 and more elongate than in cothurnatus. In addition, the scutellum of 

 ruHcrus is more elongate, less convex, distinctly emarginate apicallyand 

 with a pair of large teeth. The body coloration is dark bluish, like some 

 of the western specimens of cothurnatus, and the pile is pale throughout 

 as in that species. However, the pile appears to be more sparse and 

 shorter ; on the abdominal segments it forms narrow apical bands, nar- 

 rowly interrupted in the middle, rather than large patches. The colora- 

 tion of the legs, as given by Williston, is not diagnostic and varies in the 

 same manner as in cothurnatus. 



The type, a male from Connecticut, and eight other specimens are be- 

 fore me. These latter are: a male taken in the vicinity of Chain Bridge 

 near Washington, June 9, 1905 (D. H. demons); a male, Great Falls, 

 Virginia, June 25, 1915 (C. T. Greene); a male, Bladensburg, Maryland, 

 June 13, 1916 (F. R. Cole) ; a male and female, Hancock, Maryland, Sep- 

 tember 1, 1916 (Cole); a male and female collected by Mr. Daecke at 

 Browns Mills Junction, New Jersey, June 22 and July 5, 1907; a male 

 collected at Franconia, New Hampshire, July 18, 1915, by C. H. T. 

 Townsend. The last mentioned specimen has the pile brassy yellow, but 

 in all other respects agrees with the typical form. The specimens average 

 appreciably smaller than cothurnatus. 



This species, in the modification of the second abdominal segment, 

 shows a distinct approach toward coarctatus Loew and balliopterus Loew, 

 where this segment shows two deep depressions and strong lateral swell- 

 ings. The genus Omegasyrphus Giglio-Tos, based upon this modification, 

 like other genera segregated from Microdon, appears untenable in the 

 light of more complete knowledge. In addition to the intergradient 

 character of M. ruficrus, M. laetus Loew and M. craigheadii Walton show 

 a swollen second abdominal segment, however without the depressions 

 observed in the already mentioned forms. 



Microdon piperi, new species. 



Male. — Dark metallic blue, moderately shining, the mesonotum with 

 slight greenish luster. Frons strongly convex on posterior two-thirds, 

 roughened, posteriorly less than one-third the width of head, strongly 

 narrowed to transverse impression, beyond which point the ocular mar- 

 gins again strongly diverge. Occiput and frons clothed wholly with dense 

 short black pile, this reaching forward to insertion of antenna?. Face 

 clothed wholly with dense recumbent creamy white pile. Antenna? black, 

 long and slender, the first joint about as long as the second and third 

 together, the third moderately thickened and broadly rounded at tip; 

 arista a very stout bristle, shorter than third joint. Mesonotum with a 



