Stream mayflies (family Heptageniidae), also known as flat-headed or “stone-clinger” mayflies, are a masterclass in aquatic engineering. Unlike typical mayfly nymphs, members of this family feature extremely dorsoventrally flattened bodies and broad heads that allows them to squeeze into tiny crevices and cling tightly to rocks against raging river currents without getting washed away. While most nymphs use specialized mouthparts to scrape algae and detritus from submerged stones, species in the Arthroplea genus have adapted to filter-feeding, perching on vegetation and rapidly sweeping long, hair-like setae through the water to catch passing organic matter. After transforming into adults, they feature delicate, crystal-clear wings and short-lived, mouthless bodies dedicated entirely to reproduction. I crossed paths with this fascinating living fossil—a member of one of the oldest surviving orders of winged insects on Earth—at a quiet riverside campsite in Finland during my cycling tour in June 2026.