You Will not Imagine This Beetle’s Upside-Down Stroll on Water


After dark, the Watagan Mountains in New South Wales, Australia can seem otherworldly to anyone with a headlamp. But things got stranger than usual in 2015 when John Gould, a behavioral ecologist at the University of Newcastle in Australia, was studying sandpaper frogs in the ephemeral ponds of the woods for his dissertation.

Dr. Gould was crouching in a pond looking for frogs when he saw a pea-sized beetle that he thought had fallen into the water. When he looked closer, Dr. Gould that he was not watching a beetle right side up trying to escape the water, but an upside down beetle that was completely in control of its life and current situation. As if in a parallel world, it scurried along under the surface of the water, the kneeling Dr. Gould under him.

The surface of the pool was immaculately still, hardly a rush of wind in sight, and Dr. Gould took out his cell phone to record the water eater beetle’s casual crawl on the ceiling. Since the footage had nothing to do with his research, Dr. Gould had the beetle video on file and never returned to it for several years after completing his doctorate. Finally, in June, Dr. Gould and Jose Valdez, wildlife ecologists at the German Center for Integrative Biodiversity Research in Leipzig, published the first detailed documentation of this behavior in beetles in the journal Ethology.

Martin Fikáček of National Sun Yat-sen University, Taiwan, was not involved in the initial research, but identified the beetle as a water-eater beetle, likely in the Hydrophilidae family.

Although the paper represents the first time the beetle’s upside down underwater crawl has been recorded in the literature, the behavior was previously reported, according to Manu Prakash, a bio-engineer at Stanford University who was not involved in the research. “It’s a nice observation,” wrote Dr. Prakash in an email.

The movements of the water catcher differ from the traditional way of walking on the water that is on top. Sea and water striders row across the surface of the water with oar-like legs. Surface tension enables some geckos to walk over water by undulating their bodies and hitting the surface of the water with their legs.

Underwater, some creatures live an inverted life. Many are snails. In the ocean, the purple snail clings headfirst to the surface of the sea with a sticky raft of bubbles that holds its shelled body above the water. The freshwater snail Sorbeoconcha physidae is also dependent on slime. “They’re using a different mechanism because they don’t have legs,” noted Dr. Valdez clear about snails.

In fresh water, some insects called back swimmers swim upside down and row their hairy hind legs underwater. The larvae of the water fly Dixidae, also called meniscus mosquitoes, use the hairy structures on their abdomen to attach themselves upside down to the surface of the water.

But these water eater beetles don’t swim like the back swimmer or the larvae. You walk like you would imagine a species of land beetle walking on land or scurrying along your ceiling.

The first question: how?

“It’s a little question mark,” said Dr. Gould, although he and Dr. Valdez have some hypotheses.

One possibility, the researchers say, is an air bubble that clings to the beetle’s belly and keeps the beetle in the air. Although a bubble would inevitably burst this close to the surface, the beetle’s bubble remained resolutely full, suggesting that the beetle is somehow preventing the air from escaping, the researchers say. It is known that beetles that can walk underwater (right side up) trap air bubbles between their feet.

The second question: why?

Although the beetle’s feet seemed to sting with every step into the water, its shallow leap made no waves. The researchers suspect that this mode of locomotion could help the beetle not be eaten by anything lurking nearby. “Any predators above the surface may look down and see a bubble instead of a tasty treat,” wrote Dr. Valdez in an email.

Moving through the drag, buoyancy, and viscosity of water usually requires more energy than moving on land. But the beetle appeared to be able to move around quite easily and even appeared to be lying on its head, suggesting that this behavior is not energetically taxing, the authors say.

But the only way to know for sure is to take the species of beetle to a laboratory for further research.

Now Dr. Gould found another frog on Kooragang Island, not far from the Watagan, that breed in man-made wetlands around the island’s coal ports. The nature there is less untouched, but still holds small wonders; Recently, Dr. Gould was a snail that whirled itself gracefully from the top of a fence to the ground like an aerial artist, using a strand of its own slime like a climbing rope.

And what about that particular beetle? Water catchers have a short lifespan, so this beetle has probably disappeared, its body has returned to the Petrichor-scented soil of the Watagan. But other bugs stay, live and die, and walk on any surface that holds them on.