It isn’t a circus trick. An Australian beetle has been seen walking upside down in a pool of water, on the underside of the water’s surface. Researchers think it is a style of locomotion that has never been recorded before in an animal with legs.
at the University of Newcastle in Australia came across the beetle (in the family Hydrophilidae) by chance while searching for tadpoles during fieldwork in the Watagan mountains in Australia. He quickly pulled out his phone to film it. “To see something walk along the underside of the water’s surface as if it was just any regular solid was pretty incredible,” he says.
A beetle’s ability to walk on the underside of water has been mentioned a handful of times in scientific papers, but has never been described or captured on video before.
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The beetle that Gould observed would typically move in a straight line on the water’s undersurface, frequently stopping then continuing. During a stride, a leg would touch the surface of the water, slightly deforming it but not breaking surface tension, then swing through the water to take its next step. A layer of trapped air could also be seen on the beetle’s abdomen.
Aquatic beetles are known to carry a bubble as a temporary oxygen supply when they venture underwater to lay eggs or search for food. But the air bubble can have another role too. Diving insects called backswimmers, for example, use it to achieve buoyancy in a water column, .
, a researcher at the German Centre for Integrative Biodiversity Research in Leipzig, Germany, who studied the upside-down beetle with Gould, thinks that this air bubble on the Australian beetle’s abdomen is used in a similar way and is key to the inverted water-walking. The bubble helps the beetle attach itself to the underside of the water.
It is rare for animals to walk inverted beneath the surface of water, but it isn’t unheard of. Freshwater snails of the kind found in home aquariums can do so. Research suggests that such snails by creating pressure differences in a film of mucus on top of it. However, the beetle is the only known animal with legs to walk this way.
This style of locomotion could help conceal the beetles, allowing them to hide from predators both above and below the water’s surface. “It could also help them avoid detection when hunting prey underwater,” says Valdez.
He would like to further investigate the physical properties that allow this beetle to walk on the undersurface of water. A better understanding of the mechanism involved could be applied to create tiny robots that can move in a similar way to monitor environmental conditions near the water’s surface, for example.
“Such robots could stay underwater indefinitely if powered by the sun and could collect data year-round,” says Valdez.
Ethology
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