Like other members of the Nepidae, they have a pair of nonretractable cerci-like breathing tubes on the terminal abdominal segment, a characteristic which readily distinguishes them from the Belostomatidae.

[6] The following species are included in Nepa:[2][3][4][7] Among these, the type species N. cinerea of Europe, northern Africa and northern Asia and N. apiculata of eastern North American (Canada and United States), are widespread.

[2][5] The remaining have restricted ranges in Corsica, Sardinia, Romania, Morocco and northeastern Asia.

[2][4] One of these, N. anophthalma, is the only cave-adapted species in the family Nepidae, found in Movile Cave.

[4] Linnaeus listed a number of additional species in his description of the genus, most of which either are considered synonyms or have been moved to other genera.