They are oval-bodied aquatic insects with raptorial front legs. Like other members of Nepidae they have a pair of non-retractable cerci-like breathing tubes on the terminal abdominal segment, a character which readily distinguishes them from Belostomatidae. Their primary staple are other insects and small aquatic vertebrates. They can inflict a painful bite when handled.
A partial list of species includes:
Linnaeus lists a number of species in his initial description of the genus, many of which, if not all, have been moved to other genera.