The Black Onion Fly, aka the Onion bulb maggot, Tritoxa flexa, has been reported in North American entomological publications dating to the 1865. The black onion fly is mentioned as an occasional pest of onions that can cause some damage to the bulbs, but has never been elevated to “serious” pest status.
The flies are known as “picture wing flies” of the family Ulidiidae (formerly placed in Otitidae and Ortalidae). The wings have interesting patterns which possibly have a role in mating and mate recognition. These flies are not uncommon in gardens of the Midwest where onions are grown. They will land on flat surfaces holding their wings like oars to display the charming patterns.