Onychohydrus is a dytiscid “diving beetle” with very large larvae (up to 50mm) and adults that are active swimmers. The larvae have a distinctive head with large sickle-like jaws, and teeth along the front of the head. The adults are oval, black or green, with 11-segmented antennae, and a visible scutellum (triangular area between the two wing covers and the thorax).
Onychohydrus are rarely recorded, but they are most likely to be found in North Island ponds or wetlands.
The dytiscid beetles are predators, feeding on other pond invertebrates.
The presence of Onychohydrus is more likely to reflect pond-like habitat conditions rather than any particular water quality conditions. They have a tolerance value of 5 for hard bottom sites, but there is no soft bottom value (though the family soft bottom value is 8).