Fanniidae

The Fanniidae are a small (285 species in five genera) group of true flies largely confined to the Holarctic and temperate Neotropical realms; there are 11 Afrotropical species, 29 Oriental, and 14 Australasian. Adults are medium-sized to small and usually have mainly dark body and leg colours. Males congregate in characteristic dancing swarms beneath trees; females are more retiring in habit. Larvae are characterized by their flattened bodies with striking lateral protuberances, and live as scavengers in various kinds of decaying organic matter. Read more at Wikipedia