Beetle Bubble Serves as an Underwater Lung

Biological Strategy

Aquatic beetle

Mary Hoff

Image: Martin Str / Pixabay / Free non-commercial use

Capture, Absorb, or Filter Gases

Oxygen, carbon dioxide, and nitrogen are all gases of particular importance to living systems. Living systems obtain these gases from water and air by trapping them, absorbing them across surfaces, or filtering them. Gases are low in density and viscosity, and when dissolved in liquids, are very low in concentration–so extracting them requires specialized strategies. As a result, living systems use the properties of gases, liquids, and solids to assist in capture, absorption, and filtering. For example, fish gills have many filaments to create a large surface area to maximize how many blood vessels come in close contact with oxygen dissolved in the water.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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A bubble attached to an African beetle allows it to breathe underwater by absorbing air generated by a fast-moving stream.

Introduction

Can an air-breathing beetle be completely submerged and still get enough oxygen to stay alive? It can if it carries its own refillable oxygen tank. That’s exactly what Potamodytes tuberosus does—with a little help from fast-flowing water and the unique shape of its body.

The Strategy

An inch-long beetle, P. tuberosus is found in fast-flowing streams in western Africa. It lives underwater, clinging to rocks near the surface with its body oriented so its head is pointed upstream. Tiny hairs and other protuberances on its body cause air bubbles to collect as the water flows by. The beetle’s streamlined shape and the position of its legs cause the bubbles to coalesce into a single big bubble that starts at the “elbow” of its forelegs and extends beyond its back end. Because it, like other insects, breathes through tiny holes in its skin, the beetle can take in oxygen from the bubble rather than having to pop to the surface to breathe, much as scuba divers draw air from the tanks on their back while underwater. At the same time, the shape of the insect’s legs and the bumps direct water flow in a way that prevents the bubble from detaching.

But what happens when the beetle uses up all the air in the bubble? Thanks to the fast-flowing water, it doesn’t. A tenet of physics known as Bernoulli’s principle holds that as the speed of flow of a fluid goes up, the pressure it produces goes down. (This is the same principle used to create lift for airplanes by lowering the pressure around the upper surface of the wings.) As the water flows past the bubble, it has a longer distance to travel than the unimpeded water next to it, so it speeds up, creating a low-pressure zone in the water. That causes the air dissolved in the water to move out of solution and into the bubble.

The Potential

The ability to use the movement of water to cause the water to release dissolved air could come in handy in numerous circumstances. For example, it could provide inspiration for those in search of a source of air for submerged combustion devices. And it potentially could open the door to innovative and energy-efficient ways of providing needed air to occupants of underwater settlements in the future.

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References

“On several occasions, however, the beetles were found in smoothly-flowing shallow water which was unbroken, and which apparently contained no obvious air bubbles. Such conditions were found, for instance, when the water from a large still pool flowed away over a smooth rock sill. Under such circumstances the beetles behaved quite normally, moving but little, and showing no signs of respiratory stress. Further, all the beetles in the same situation possessed bubbles of approximately the same size, an unlikely occurrence if the bubbles were of the well-known “shrinking air store” type (Ege, 1915). It seemed likely, in fact, that under these conditions the respiratory bubbles were acting as permanent physical gills.” (Stride 1955: 345)

Journal article

On the respiration of an aquatic African beetle, Potamodytes tuberosus Hinton

Annals of the Entomological Society of America | September 1, 1955 | George O. Stride

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Reference

“Stride demonstrated that Potamodytes tuberosus orients its body so as to create a stationary, cavitation bubble in its lee. As noted by Stride and examined in greater detail below, the cavitation bubble results from Bernoulli pressure lows resulting from large flow speeds. Indeed, Stride’s field observations suggest that Potamodytes tuberosus relocates to more briskly flowing streams once the flow velocity in its local habitat drops below a well-defined critical value.” (Flynn & Bush, 208: 277–78)

Journal article

Underwater breathing: the mechanics of plastron respiration

Fluid Mechanics | 2008 | M. R. Flynn and John W.M. Bush

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