Skip to content

Swimming mayfly (Oniscigaster)

Taxonomic group

Oniscigastridae: Oniscigaster

Diagnostic features

Oniscigaster mayfly nymphs have a torpedo-like body form, fringed caudal filaments (“tails”), large spines along the sides and top of the abdomen, and rounded gills lying on the top of the abdomen.

Typical habitats

Oniscigaster larvae are most likely to be found in pools in stony or gravelly, bush covered streams.


Oniscigaster larvae are thought to be detritivores, feeding on streambed algae and decomposing plant matter.

Indicator value

  • Hard bottom: 10
  • Soft bottom: 5.1

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 Oniscigaster suggests good habitat and water quality conditions, especially if other mayfly or stonefly groups are also present. They have tolerance values of 10 (hard bottom sites) and 5.1 (soft bottom sites).