Diving beetle (Onychohydrus)

Taxonomic group

Dytiscidae: Onychohydrus

Diagnostic features

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] adult with scutellum visible. Image: Stephen Moore Image — Onychohydrus adult with scutellum visible. Image: Stephen Moore

[Onychohydrus] larva with teeth along the front of the head. Image: Stephen Moore Image — Onychohydrus larva with teeth along the front of the head. Image: Stephen Moore

Typical habitats

Onychohydrus are rarely recorded, but they are most likely to be found in North Island ponds or wetlands.

Feeding

The dytiscid beetles are predators, feeding on other pond invertebrates.

Indicator value

Hard bottom: 5
Soft bottom: unassigned; general dytiscid value 8

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

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).