SAN FRANCISCO, CALIFORNIA—With arms stocky enough to win a bodybuilding competition, it’s little wonder this burly insect is sometimes called a “muscle mayfly.” But those arms aren’t for heavy lifting, researchers say. Instead, they act like wings. At roughly 1 centimeter long, this young Ecdyonurus mayfly lives in fast-flowing river currents, grazing on algae that blankets the rocky riverbed. Rather than sheltering itself from the flow, though, this intrepid insect crawls onto exposed rocks to face the torrent head-on. Why it’s not lifted and swept away puzzled researchers until they looked at its forelegs. When cut down the middle, the leg has a shape similar to an upside-down airplane wing, researchers showed this month at the annual meeting of the Society for Integrative and Comparative Biology here. This inverted shape means water flowing over the leg pushes the nymph toward the ground, much like the rear wing of a racecar, rather than lifting it off the rock. Video clips of nymphs in a water current and wind tunnel experiments with a scaled-up 3D leg model further showed the nymphs only tilt their legs to angles that enhance this downward push so they’re glued to the rock. It’s an evolutionary trick that just goes to show: If you can’t beat the current, you might as well use it.