Nematocera are generally primitive flies, typically recognized by filamentous, multi-segmented antennae which may be plumose in some males. Nematocera is a paraphyletic suborder, because one of its constituent families (Anisopodidae) is apparently the sister taxon to the entire suborder Brachycera; an alternative classification has been proposed in which the family Nymphomyiidae (traditionally classified within the Blephariceromorpha) is removed to its own suborder, the Archidiptera, and all the remaining Nematoceran families are placed in a suborder called Eudiptera - however, Eudiptera is also paraphyletic, and this classification has not been widely accepted. Largely due to its long history, the name Nematocera continues to be used.
Examples of the Nematocera include the mosquitoes (Culicidae), crane flies (Tipulidae) and black flies (Simuliidae) - many of the remaining families (esp. Mycetophilidae, Anisopodidae and Sciaridae), are called gnats, while others (esp. Chironomidae, Cecidomyiidae and Ceratopogonidae) are called midges.
The larvae are mostly aquatic and have distinct head with mouthparts that may be modified for filter feeding. The pupae are orthorrhaphous (meaning adults emerge from the pupa through a straight seam in the pupal cuticle). The body and legs of the adults are usually elongate and these flies often have relatively long abdomens.
Many species form mating swarms of males, and in some of these competition for females is extreme. Although many species (as larvae) have a strong association with water, even within a single family there may be a trend toward semiaquatic and terrestrial habitats.