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Water walkers surf on the edge

September 28, 2005 By Michael Hopkin This article courtesy of Nature News.

Insects harness surface tension to climb out of ponds.

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Navigating your way out of a puddle might not sound like a challenge, but to a tiny insect it is akin to scaling an extremely steep and slippery mountain. Two mathematicians have outlined exactly how little insects manage this feat: they turn surface tension to their advantage and 'surf' up the edge with scarcely any effort.

The mountainous ridge of water at a puddle's edge arises thanks to capillary attraction, which causes the liquid to shoot up where it touches the surrounding land. This feature, called a meniscus, is also commonly seen around the edge of a glass of water.

Large insects such as water striders (Gerridae), which are often found on pond surfaces, are fast enough and big enough simply to hurdle a puddle's meniscus when they come to it. But smaller insects can't do this.

It's a very striking means of locomotion.
John Bush,
Massachusetts Institute of Technology
Instead, insects that are just a few millimetres long deform the surface of the water with their legs, creating forces that shoot them to the top of the watery hill.

The insects are taking advantage of the same effect that causes pieces of cereal to come together in a bowl of milk, says Bush. Objects that deform a liquid and increase surface tension tend to attract each other once the deformed regions overlap, because this minimizes the overall amount of deformity, and therefore tension.

"It's a very striking means of locomotion," says John Bush of the Massachusetts Institute of Technology in Cambridge, Massachusetts, who studied the phenomenon with his colleague David Hu. "It's rare. It may even be unique."

Wet and wild

Bush and Hu studied three insect species: Mesovelia and Microvelia, which walk on water as adults, and larvae of the beetle Pyrrhalta, which perform a similar trick. They used high-speed video to capture the bugs in action, and then analysed the performance. They report the results in Nature1.

The insects all use their legs to pull the water's surface up at the front and rear, while pressing downwards with their middle segments. Insects that live on ponds and puddles tend to have bodies that are generally water-repellent. But the insects in this study possess retractable 'wetting' claws that attract and hold the water's surface, allowing them to pull it upwards and out of shape.

The uplifted bit of water under the insect's foot then becomes an area of particularly high tension, as is the portion of the puddle's meniscus at the very edge, where it is steepest. Like bubbles on the surface of a glass of champagne, these two areas of high tension attract each other in order to lower the overall tension of the water surface. This attraction pulls the insect to the top of the hill.

"They scamper onto the menisci and are sliding down under gravity, then they lock into position and travel up it," says Bush.

Bush adds that the insect's front legs are mostly responsible for the effect. Their middle legs press down to support their weight and prevent them sinking, while the rear legs pull upwards again for balance, preventing the insect from flipping over in a backwards somersault.

The technique makes for a speedy manoeuvre. The insects surf up the meniscus at around 30 body lengths per second; the fastest human sprinters only manage about five.

References

  1. Hu D.L., Bush J.W.M. . Nature, 437. 733 - 736 (2005).

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