any member of a family of insects in the fly order, Diptera, that are tiny and metallic blue, green, or copper in colour. These flies prey on smaller insects and are found around damp, marshy places.

The male has conspicuous genitalia at the end of the abdomen and cuplike structures on his legs, which cover the female's eyes during mating. Many species perform highly stylized and complicated mating dances. The elongated larvae have almost no external head structure and are found in mud, decaying vegetation, and water and under bark.