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Sundew, Susan, thanks very much for your help with this one! Your ID sent me to the site www.nearctica.com, and it lists six Melanostoma spp. for the Nearctic, and they are:

Melanostoma ambigua Fallen 1817 (Scaeva)
Melanostoma angustatum Williston 1887 (Melanostoma)
Melanostoma bicruciata Bigot 1884 (Melanostoma)
Melanostoma dubia Zetterstedt 1838 (Scaeva)
Melanostoma mellina Linnaeus 1758 (Musca)
Melanostoma parva Williston 1882 (Cheilosia)

At www.bugguide.net I found a similar gravid female photos identified by Gerard as Melanostoma mellina (acid green halteres too):

http://bugguide.n...760/bgpage

I don't know if mine is the same species as in the BugGuide photo. At any rate I am very glad to know the genus of this one. Thanks again, Sundew and Susan!