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.

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.