If you want to walk on water, it helps to have fancy feet.
A type of water strider known as a ripple bug (Rhagovelia obesa) has frilly fans on its feet. Those fans unfurl when they touch water and close when removed. These ripple bugs use their feet like oars to quickly dart and turn atop choppy river waters.
A new study shows that ripple bugs’ foot fans open and close automatically. No muscles are needed for that, researchers report in the August 21 Science. Their discovery inspired the team to build a water-striding robot with frilly feet of its own.
Rice grain–sized ripple bugs boast some serious agility. With help from their special feet, they can make sharp turns in just 50 thousandths of a second. And they can move up to 120 body lengths per second!
The bugs flit so fast that they appear to fly rather than stride atop water. “If you are blinking, you miss the action,” says Víctor Ortega Jiménez. He studies animal motion at the University of California, Berkeley.
The fans on ripple bugs’ feet are made of tiny, ribbon-shaped barbs. Each barbs is studded with even tinier barbules.
When dipped under water, the stretchy barbs and barbules fan out. But when pulled from the water, the attraction of water molecules to their fibers produces capillary forces. (Those forces result from the water molecules sticking to the fibers and to one another.)
The force also pulls the barbs back together. It’s similar to how a paintbrush spreads when dipped into water and comes to a point when removed.
Ortega Jiménez discovered this through a simple experiment. He attached a severed ripple bug foot to a strand of his wife’s hair. Then, he gently lowered the foot into a water droplet. In just 10 thousandths of a second, it burst open.
Inspired by the ripple bug, his team built a robot with fanlike feet. It performed better than a similar bot without fans. This bug-inspired device also made sharper turns and glided farther after a single push.
In the future, such robots could monitor streams, Ortega Jiménez suggests. Or they could help search flood zones for survivors.