Length: Fly in photo 3.5 mm to wing tip
Scatella stagnalis is a cosmopolitan species, found essentially worldwide.
Dahl (1959) recorded details of courtship within this species. The male approaches the female from the front and stops at a distance of about 3 mm, "spreading his wings and scissoring them three times. This ceremony is repeated several times," but if the female gives no reaction the male shifts his position and tries again.
Photo location: north of Dawson, Yukon, Canada. What initially appeared to be a red stripe on the fly was actually a pair of mites under the fly's wing.
American Insects site