Some Bugs Walk on Water, but This Talented Beetle Scurries Underneath Its Surface

The aquatic critter may use bubbles to stay inverted

A photo of a water scavenger beetle on top of some algae suspended in a body of water
In total, there are 3,200 species of water scavenger beetles belonging to the family Hydrophilidae. These beetles have previously been known to use tiny hairs lining their abdomens and legs to trap air bubbles. Andrew C via Wikicommons under CC BY 2.0

Various aquatic insects can use the water's surface tension to skate around along the top lakes and ponds. However, one particularly acrobatic water beetle can scoot along upside down beneath the surface as if it were walking on the underside of a glass-top table, according to a new study published in the journal Ethology.

In the first detailed documentation of an insect showing this behavior, researchers observed a water scavenger beetle walk, rest and change directions—all while remaining under the surface of water, reports Jake Buehler for Science News.

Study author John Gould, a behavioral ecologist from the University of Newcastle in Australia, discovered the beetle on accident while searching for tadpoles in shallow pools of water in the Watagan Mountains of Australia, reports Sandrine Ceurstemont for New Scientist. When he first spotted the insect, he thought it fell into the water and was only swimming on the surface. But upon closer look, he noticed the beetle's inverted walking method. Gould quickly took a video of its movements with his phone and shared his findings with Jose Valdez, a wildlife ecologist at German Centre for Integrative Biodiversity Research. Though the behavior has been briefly described in previous studies, their paper is the first to analyze it in depth with video evidence, reports New Scientist.

"The moment I realized the beetle was moving across the underside of the water's surface, I knew I had found something truly bizarre," Gould recounted to Live Science's Ben Turner. "The beetle was casually walking along the underside of the water's surface with ease while upside down. Every now and then, it would come to a stop, and then kept plodding along across the surface as if it was walking across any regular solid surface on land."

Water scavenger beetles differ from other aquatic insects because of their ability to hang suspended from the water's surface using its head instead of its abdomen. The beetles swim by moving their middle and hind legs together to propel forward. In total, there are 3,200 species of water scavenger beetles belonging to the family Hydrophilidae. These beetles have previously been known to use tiny hairs lining their abdomens and legs to trap air bubbles. The bubbles keep the beetles oxygenated while they stay underwater for long periods, reports Rachael Funnell for IFLScience.

In Gould's video, the beetle is seen holding an air bubble near its upturned belly. The researchers suspect that this air bubble may be key to how the beetle can walk on the underside of the water's surface. The bubble could provide the beetle enough buoyancy to flip and pin the beetle on the underside of water and allow it to put pressure on the water without breaking the surface tension, reports Science News. This inverted water walking technique may also help the beetles conceal themselves from predators, reports New Scientist.

"That means it can remain at the water's surface without expending any energy," Gould tells IFLScience. "This is in contrast to large animals that move across the water's surface, such as lizards, which need to keep running in order not to sink through the surface. This ability could also mean that it could avert predation by minimizing the amount of noise it makes while remaining still."

Snails are also known to surf along the underside of the water's surface as well. They use their mucus as a barrier between the water and themselves to create differences in pressure. Then, they slide along the underside of water, New Scientist reports.

The finding may help researchers further develop robotic "insects" that can monitor environmental conditions at the water's surface, Live Science reports.

"Such robots could stay underwater indefinitely if powered by the sun and could collect data year-round," Valdez tells New Scientist.