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Wednesday, 9 March 2016

The secret of beetles that waterski so fast they vanish

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This beetle is one extreme waterskiier. It skims across the surface of water so fast that it seems to vanish.
Haripriya Mukundarajan from Stanford University and her colleagues have filmed water lily beetles (Galerucella nymphaeae) in the lab to figure out how they stay on the surface while travelling at speeds of up to 0.5 metres per second –which scaled for size would be equivalent to a human travelling at about 500km/h.

“It’s one of the fastest horizontal speeds ever seen in an insect moving on water but nobody has looked at the physics behind it,” she says.
When a beetle “takes off”, it lifts its middle legs, then angles its body upwards before vigorously flapping its wings to launch itself horizontally, travelling up to a few metres forwards.
The beetles move so fast that they interact with the ripples generated by their own motion, which increases drag and causes a bumpy ride. “It’s as if surface tension acts as a pogo stick that the beetle is jumping on,” says Mukundarajan.
Her team also found that skimming across water expends more energy than flying in air. Water lily beetles are also agile in air, but only fly occasionally, for example when threatened by predators.
They could be moving on water rather than in the air because they feed on water lily leaves floating on ponds, say the researchers.

Elegant solutions

The team also discovered that the beetles are well adapted for gliding on water. Compared with flies and mosquitoesfor example, their wings are stronger and allow them to produce a lot of lift while counteracting drag from the surface.
And their legs are covered with tiny hairs that repel water while a claw at the tip is hydrophilic, allowing them to pin themselves to the surface of the water. “This structure is critical for the beetle to maintain its level exactly on the water surface,” says Mukundarajan.
“I’m surprised that they have something this elegant,” says Jake Socha from Virgina Tech in Blacksburg, who has previously uncovered the aerodynamics of flying snakes. “It suggests that skimming is evolutionarily important.”
Understanding the motion of the beetles could help us develop robots that move across water quickly. Many current designs are based on water striders, which move more slowly.
Mukundarajan also thinks studying the beetle’s wings could give insight into a phenomenon that occurs when an aircraft is flying low. “The beetles flatten their wings when they are close to the water,” she says. “This could create tiny vortices that reflect off the surface to give them a boost.”

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