Hairy Insects Use Bubble "Lungs" to Stay Underwater

Sara Goudarzi
for National Geographic News

August 13, 2008

Of the hundreds of insect species that rely on air bubbles to dive underwater, some can use the bubbles like external lungs to stay submerged for long periods, according to new research that describes how insects manage the feat.

Scientists have known since the 19th century that when aquatic insects dive underwater, they trap an air layer similar to a thin bubble around their bodies and use this air supply to breathe.

The new study is the first to describe exactly how the insects' air layers work, said Steven Vogel, a biology professor at Duke University who was not involved with the paper.

"You might say, with only mild hyperbole, that our understanding [of the bubbles] is now complete," Vogel said.

Hairy Problem

When aquatic insects dive, the layer of air forms between their waxy body surfaces and their rough covering of hairs.

The bubble is held to their hairs by surface tension, the same force that allows a leaf to float on a pool of water.

(Related: "Hairy Legs Help Bugs Walk on Water" [November 3, 2004].)

Air pressure inside the bubble is less than that of the surrounding water, allowing oxygen to flow into the air layer and then into the insect's body via spiracles—small openings that ultimately connect to its bloodstream.

For some insects, gases are exchanged across the bubble's surface, with oxygen being absorbed from the surrounding water while carbon dioxide is expelled.

Such a system allows the bugs to submerge indefinitely, said study co-author Morris Flynn of the University of Alberta.

"For example," Flynn said, "the pygmy backswimmer Neoplea striola, which is found in lakes in New England, actually hibernates underwater and arises from its winter slumber only when the water temperature reaches 10 to 12 degrees Celsius [50 to 54 degrees Fahrenheit]."

Flynn and study co-author John Bush of the Massachusetts Institute of Technology found that a tightly packed covering of hair could secure the air bubble in place much better than a sparse one, allowing insects to dive deeper before the bubble collapses.

Some insects, the researchers found, can dive down to 98 feet (30 meters) while maintaining their air supplies.

But there's a tradeoff: Denser hairs reduce the surface area of the bubble, leaving less space for gases to move through.

In such a case, "the insect cannot extract sufficient oxygen from the water column to meet its metabolic demands," Flynn said.

Breathing is more effortless with a thinner hair network, but it's easier for the bubble to dislodge, forcing the insect to return to the surface in a hurry.

The study is detailed in the August 10 issue of the Journal of Fluid Mechanics.

Better Scuba?

In addition to the external-lung model, some insects use bubbles primarily as diminishing air stores, similar to how scuba divers carry a set supply of air in their tanks.

Flynn and Bush were initially hoping to examine bugs' breathing methods to improve scuba technology.

"The unfortunate fact is that humans are big, warm-blooded animals," Flynn said. Air bubbles that could stay strong at depth wouldn't have enough surface area to supply the amount of oxygen people would need.

"Humans' high metabolic rates therefore make it impractical to try to directly exploit the tricks used by insects to breathe underwater."

Duke's Vogel also expressed doubts that this research could benefit scuba technology as we now recognize it.

"But [the work] well may open the door to a range of underwater surface tension-based gas extractors for purposes that are as yet largely unimagined."