Males use that flat face to bump heads in defence of their territories. Larvae of this family typically feed on bacteria in decaying plants, fungus or dung, although some feed inside the root nodules of legumes.
Duomyia

Small and with the characteristically-marked wings of this genus … but this behaviour is novel. We commonly see Rivellia in mating swarms around fresh wallaby and wombat dung. But these two females were alone and quite intent on laying eggs into this dry grass seed head. I guess there are bacteria in all kinds of places.
Rivellia