The antennae are very reduced. Species in the genus "Nymphomyia" have atrophied mouthparts. Nymphomyiidae are unusual in that the adults are ventrally holoptic, meaning they possess two eyes that meet on the underside of the head. Adults form large swarms above water. One or two generations may breed in a single year depending on the region and climate. Currently all species are treated as members of a single genus: