Very small black flies about 1/8 inch long (3-4 mm). Eyes are reddish, wings are clear. The abdomen is colorless with black banding dorsally. A common trait of members of this family is the presence of an orangey transverse band on the eyes (visible in photo 3). Some of the individuals in Photos 1 and 3 show distended abdomens from eating sap from a weeping serviceberry tree.
These flies are uncommonly seen except at tree wounds or seeps where sap is present. The flies in the photos were photographed on the serviceberry tree in front of the Fontenelle Forest Nature Center on 28 October 2016.
There are three species in the genus Aulacigaster in North America. They are all very similar and difficult to separate from photos. The taxonomy of this family was revised in 1994 by Mathis and Freidberg.
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