A Picture-winged Fly,
Euxesta species, a little fly (4-5 mm long), that I have see a
few times on rotting material, such as on a slime mold. This one was lapping up fluid from a rotting tomato, and it would pause intermittently to spread and wave its wings. Perhaps this is defensive mimicry, as it resembles a bee or wasp, to some extent. "Euxesta" means "well-polished", and I think that is a good name--these things are quite shiny.This may be
Euxesta notata, but there are a number of species in the genus, and I do not have any information about the species-level identification.