Mayfly nymph (Genus Oniscigaster) Swimming Mayfly nymph
Species: O. distans
Binominal name: Oniscigaster distans
Common name: Swimming Mayfly nymph
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 usually inhabit in pools in stony or gravely, bush-covered streams.
They 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.
Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/