Dixidae

The Dixidae (meniscus midges) are a family of aquatic nematoceran flies (Diptera). The larvae live in unpolluted, standing fresh waters, just beneath the surface film, usually amongst marginal aquatic vegetation. They are found in all continents except Antarctica.

Description
For terms see Morphology of Diptera

Dixidae are small (body length not more than 5.0 mm) slender gnats with thin legs. Adults are black to yellowish-brown. The head is relatively broad. The antennae are thin and the flagellum has 14 segments. The proboscis is short and thick and the palpi are five-segmented. The thorax is slightly convex. The wing veins are without scales (with scales in the closely related family Culicidae. The subcosta is fused with the costa at the level of the base of Rs or slightly proximal to this. The wing venation exhibits radial, medial, and cubital forks (R 4 branched, M 2 branched, Cu 2 branched). R 2+ 3 is strongly arched, the r–m crossvein is distinct, and the discal cell is absent. The anal vein of the wing is long. The genitalia of the male is inverted at 180° by torsion of segments 5–8.

Evolutionary history
The oldest known fossils of the group come from the Jurassic of Asia, assigned to the extinct genera Syndixa and Eucorethrina, members of modern genera are not known until the Eocene.