A few scarabs stroll along the underside of the water's surface

Some small insects can walk along the top of water. This is all thanks to water's unique abilities. Water molecules can attract one another. (A molecule is a group of atoms bonded together.) When these molecules bond together near the water's surface, they make a film on the top of the water. This is what scientists call surface tension. It allows some tiny bugs such as water striders to easily glide on top of the water.

An illustrated graphic showing how water tension works when supporting a water strider or in a water droplet on a leaf.Zoom-in

But new research reveals something even stranger: Some water-dwelling beetles can walk along the underside of the water's surface! It's as if the water were a solid piece of glass. In June 2021, scientists published this finding in a scientific journal. This is the first detailed report of a beetle moving this way.

Upside Down And Below The Water

John Gould is a behavioral biologist. This is someone who studies the behaviors of living things. Gould works at the University of Newcastle in Australia. One night, he was conducting research in Australia's Watagan Mountains. He was looking for tadpoles in pools of water. That's when he noticed a small black object smaller than a pinkie nail.

At first, Gould thought it was a bug that was swimming across the water's surface. Then, he realized that the bug was upside down. And it was below the water's surface!

Gould filmed the beetle, which was later identified as a water scavenger beetle. He saw it walk along the underside of the water's surface. It was as if it was walking on a flat, solid surface. Now and then it rested. Sometimes it changed direction.

Later, Gould mentioned this find to Jose Valdez. He is a wildlife ecologist. This is someone who studies how living things relate to each other and their surroundings. He works at the German Centre for Integrative Biodiversity Research in Germany.

I didn't fully grasp what Gould was describing until he showed me the video, Valdez recalls. He saw the beetle was walking upside down along the underside of the water surface. At that point, he says, "I was floored!"

A Tactic To Avoid Predators

Next, Gould and Valdez teamed up. They searched through scientific reports. But they only found a few passing mentions of beetles moving along the underside of the water. The reports were written decades ago.

Martin Fikáček was surprised by the lack of research on the beetles. He is an entomologist. This is a scientist who studies insects. He works at National Sun Yat-sen University in Taiwan. Many scientists who study water beetles already knew about the beetles' ability to walk along the bottom of the water's surface. However, no one had looked closely at it before, he admits.

It's actually cool that somebody started to think about how the beetles do this, says Fikáček. "We always see it. And we never even thought about it."

Gould and Valdez think these beetles might do it to keep away from predators. A predator is an animal that eats other animals. The predators might lurk along the bottom of the pools of water. So, the beetles might cling to the underside of the water's surface to escape. The scientists don't know how the beetles do this. But they have an idea.

Air Bubble Makes It Happen

Gould's video showed an air bubble trapped along the creature's belly. The bubble floats due to buoyancy. This is an upward force. It pushes on objects submerged in liquids such as water.

Gould thinks that the bubble may flip the beetle. It could pin it to the underside of the water's surface. That might allow the insect to walk on the water-air boundary.

Gould and Valdez think the bubble may flip the beetle. It could pin it to the underside of the water's surface. That might allow the insect to put pressure on the water-air boundary with every step.

Future studies are needed to understand the physics of how the beetles move underwater. They could also inspire advances in robotics. These findings highlight how often we ignore or miss amazing things that small critters do every day, says Gould.