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For Insects, Spiders & Their Kin
For the United States & Canada
Notes on the Dipterous Genus Chyromya R-D
By Malloch JR
Proceedings of the Entomological Society of Washington 16:179-181, 1914
Cite: 2134013 with citation markup [cite:2134013]
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The key includes Chyromya femorella Fallen 1820 and C. flava Linnaeus 1758 as well as 3 species now placed in Gymnochiromyia: Gymnochiromyia concolor Malloch 1914, G. minima Becker 1904, and G. nigrimana Malloch 1914.

An additional Chyromya species listed in Nearctica, C. oppidana Scopoli 1763, isn't included in the key.

C. oppidana
C. oppidana = C. lutea, which Malloch writes is distinguished from the similar C. flava by the coloration of the first flagellomere: yellow in flava, black in oppidana.

 
Also
Also, it appears that minima has been synonymized with Gymnochiromyia flavella, see this checklist. Otherwise, this key covers all described American species in Chyromya and Gymnochiromyia.