Vol. 30, pp. 133-144 July 27, 1917 



PROCEEDINGS 



OF THE 



BIOLOGICAL SOCIETY OF WASHINGTON 



ON SOME NORTH AMERICAN SPECIES OF MICRODON. 



[Diptera : Syrphidae.] 



BY FREDERICK KNAB. 



The genus Microdon is full of difficulties for the systematic. 

 There has been considerable uncertainty as to the identity and 

 specific limits of some of our forms, particularly in the group 

 comprising tristis Loew and bombiformis Townsend. The rarity 

 of the flies and their consequent scanty representation in most 

 collections have added to the generally admitted difficulties of 

 the group. Prof. Chas. W. Johnson has recently made the first 

 important step in clarifying the subject by establishing the iden- 

 tity of Loew's Microdon tristis, which has been long misunder- 

 stood.* 



Some time ago the writer worked over the material in the 

 national collection with results that seem worth publishing. 

 He is greatly indebted to Mr. V. A. E. Daecke of Harrisburg, 

 Pa., who has kindly loaned him his entire Microdon material in 

 the tristis group. This additional material has proved most 

 useful in working out the results here presented. The following 

 table should be helpful in separating the forms grouping around 

 tristis ; it is based on material before the writer and species not 

 represented by specimens are excluded. Some additional species 

 of Microdon not especially related are discussed or described at 

 the end of this paper. 



Table of species related to Microdon tristis. 



1. Scntellnm convex, its hind margin rounded, unarmed 2 



Scutellum with distinct, although often small, spines 5 



2. Face clothed with coarse, black and pale hairs intermixed ; abdo- 



men wholly black senilis n. sp. 



Not with this combination of characters 3 



♦Psyche, vol. 23, no. 3 (June, 1916), p. 75. 



33— Proo. Biol. Soc. Wash., Vol. 30, 1917. (133) 



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