Never Mind Walking; Some Insects Can Jump on Water



ScienceTake | The Water Strider’s Leap



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Water striders are among the most familiar insects. They skate across puddles and mill around in quiet stream eddies.

But they can also jump straight into the air to escape predators like frogs, fish and bigger insects.

That’s no small trick, even if they are light enough that the surface tension of water supports easy horizontal movement.

The way a water strider takes off is that it draws its legs inward, pushing down on the water surface slowly at first. Even as the leg motion speeds up, the insect never pushes down with enough force to break the surface tension of the water.

That means no splash, and it leaves the water as a stable launch platform.

Researchers at Seoul National University learned how the water strider accomplishes its jump using high-speed video to analyze its motions. Then, to test the validity of their analysis, they joined with Harvard scientists to create a tiny strider-size robot based on the same principles.

The robot was powered by a length of an alloy that can be stretched, but that returns to its original length when heated. The alloy worked like a contracting spring pulling on artificial legs that were highly water-repellent, just like those of the strider. Using a mechanism inspired by the way a flea jumps, the legs were pulled in slowly at first, and then quickly. Videos of the robot show it soaring upward just like a water strider.

It is possible that there may be some kind of application for this kind of robot down the line, but Kyu-Jin Cho, one of the senior authors of a report in Science on the project, and an associate professor at the biorobotics lab at Seoul National University, said, “We did this research to understand one of the most fascinating kinds of locomotion in nature.”

One secret that remains is what muscular or mechanical action the water strider uses to start that inward pull of its legs.

“We don’t actually know how the water strider does that,” Dr. Cho said.