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.
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.
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).