Mayfly nymphs of the scrapers group evolved and adapted for living in fast water environments. Their anatomical structure is perfect for strong current conditions. The flat body is hydrodynamic and built to take advantage of the flowing water pressure to hold the grip on the rock substrate.
They can crawl fast, but are no great swimmers. They are the typical dwellers of the high and mid part of the rivers. The family of the Heptageniidae is a typical representative of this group.
On the left, family Heptageniidae genus Ecdyonurus.