https://www.theguardian.com/environment/2021/jul/09/beetle-that-can-walk-upside-down-under-water-surface-filmed-in-australia-in-world-first

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Insects

Beetle that can walk upside down under water surface filmed in
Australia in world first

Researcher accidentally spots tiny insect walking on the underside of
the water surface as if it were a pane of glass

00:34
Tiny Australian aquatic beetle captured walking upside down on water
surface - video

Donna Lu
@donnadlu

Thu 8 Jul 2021 23.14 EDT[ ]

Last modified on Fri 9 Jul 2021 12.06 EDT

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An Australian beetle has been observed walking upside down along the
surface of water - the first instance that such behaviour has been
visually documented.

The tiny aquatic beetle, about 6mm to 8mm in length, has been
recorded scuttling along the undersurface of a pool of water in New
South Wales.

John Gould, a PhD student at the University of Newcastle, discovered
the beetle by accident while researching a frog species in the
Watagan Mountains.

 
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Gould had been crouching down next to a body of water, searching for
tadpoles, when he noticed what initially appeared to be a bug
swimming.

In a rare documentation of the behaviour, Gould said he then realised
the beetle seemed to be walking on the underside of the water's
surface as if it were a pane of glass.

"I think I was just lucky," Gould said. "The most interesting thing
is that it's able to rest on the underside of the water's surface as
well as walk."

While the specific species of beetle hasn't been identified, the
researchers believe it belongs to the Hydrophilidae family of
beetles, commonly known as water scavengers.

The beetle Gould observed appeared to have trapped a layer of air
along its abdomen, giving it the buoyancy to push it up to the
surface of the water.

"We also propose that the beetle has attachment organs on its legs
that are trapping more air bubbles," he said, which allows it to move
without breaking the surface tension of the water.

In this way, the beetle is similar to water striders, which can
travel across the top surface of water. "They also have hairlike
projections along their legs that stop them from breaking surface
tension," Gould said.

The researchers concluded: "The water's surface seems to be acting as
a support for the beetle, with the force needed for forward movement
generated by the beetle's legs as each transition between the stance
and swing phases of walking."

They hypothesise the behaviour might have evolved as a means to avoid
predators at the bottom of bodies of water.

Some water scavenger species do not have to ever leave their aquatic
environments to breathe. "They're actually able to remain under the
water and they're able to collect additional air [trapped underwater]
using their antennae," said Gould.

The documented sighting was published in the journal Ethology.

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