a time-lapse photo of a fly tumbling off kilter because its small wing-like halteres have been removed
The common flesh fly possesses specialized sensory organs called halteres that help them detect body rotations during flight. With her halteres removed, this fly can no longer maintain stability.

Biology’s Jessica Fox explains why houseflies can be hard to swat

Why are flies so hard to swat?

NPRJessica Fox, associate professor in the Department of Biology at the College of Arts and Sciences, explained one of the reasons the common housefly is so difficult to swat sometimes: the “halteres”—evolutionary remnants of wings which not only generate lift for the fly, but also function as sensory organs to warn the fly of an imminent swat.