This is a somewhat common visitor to carrion traps situated in the lawns around the Field Museum. This will key out to "Undescribed Genus" in the Manual of Neartic Diptera. Dr Steve Marshall IDed this as Bitheca boleta. These are apparently the first images of this genus online.

The pre-apical dorsal bristle on the metatibiae is important when keying this out. As is the long, tapering ovipositor of the female. These specimens are in alcohol, which makes it easier to see the ovipositor shape.