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Stephen,

This is a sp. of Atomosia (tribe Atomosiini, usually placed in Laphriinae but sometimes treated as a separate subfamily). It probably is A. puella (Wied.) - the commonest eastern U.S. Atomosia sp.; but it could also be A. glabrata (Say) - another widespread (though somewhat less common) e. U.S. fly. These two cannot be distinguished based on the photo.

Regards, Eric.