Some insects live much of their lives under water, using air bubbles gathered at the surface to survive. Now scientists have discovered just how deep they can go.

Based on a new model of how the air bubbles work, the deepest-diving bugs could as far down as 98 feet (30 meters), researchers said this week. The scientists figured out how the air bubbles work, and why they wouldn't pop at such depths. However, most bugs never go deeper than a few yards (meters), they said.

"Some insects have adapted to life underwater by using this bubble as an external lung," said John Bush, associate professor of applied mathematics at MIT and a co-author of the study, detailed in the Aug. 10 issue of the Journal of Fluid Mechanics.

The air bubble's stability is maintained by hairs on the insects' abdomen, which help repel water from the surface. The hairs, along with a waxy surface coating, prevent water from flooding the spiracles—tiny breathing holes on the abdomen.

The spacing of these hairs is critically important: The closer together the hairs, the greater the mechanical stability and the more pressure the bubble can withstand before collapsing.

However, mechanical stability comes at a cost. If the hairs are too close together, there is not enough surface area through which to breathe.

"Because the bubble acts as an external lung, its surface area must be sufficiently large to facilitate the exchange of gases," said Morris Flynn, a former applied mathematics instructor at MIT who is now at the University of Alberta.

Other researchers have worked on ideas to use similar external lungs to allow humans to dive for long periods. But the surface area required to support human respiration is impractically large — in excess of 100 square meters, Bush and Flynn report.