Bad breath: insects zip air holes to cut oxygen risks.

A need to avoid overdosing on that dangerous gas--oxygen--may be what drives some insects to shut down their breathing holes periodically.

That's two researchers' proposal to explain why many ants, grasshoppers, moths, and some other insects on occasion close--for hours at a time--the air holes, or spiracles, that line their bodies. The animals' breathing systems work so efficiently that resting insects have to take care not to overdose on oxygen, contends Timothy Bradley of the University of California, Irvine. Studies of the pupal stage of a moth show that oxygen concentrations stay constant inside internal respiratory tubes despite external changes in gas concentration. Bradley and Stefan Hetz of the Humboldt University in Berlin report in the Feb. 3 Nature.

"It's a new idea, and this is the first evidence for it," says Bradley.

Another researcher who has studied the problem, Steven Chown of the University of Stellenbosch in South Africa, calls the new paper "a remarkable piece of work." He points out that lungfish and some amphibians also do stop-and-start breathing. The new paper, he predicts, will inspire a rethinking of the phenomenon.

An insect's spiracles lead to branching trees of internal airways that let in oxygen for fueling metabolism and get rid of carbon dioxide. These airways deliver oxygen some 200,000 times as fast as a mammal's blood vessels do, and they whisk away carbon dioxide some 10,000 times as fast, according to Thorsten Burmester of the University of Mainz in Germany.

The cumulative wear and tear of oxygen exposure has been implicated in tissue deterioration during animals' aging, so Bradley proposed that insects' respiratory shutdowns might minimize such damage. To test that idea, Bradley and Hetz inserted their probes through two of the spiracles into airways of Atlas moth pupae.

As the researchers varied external oxygen concentrations, they found that concentrations inside the airways stayed steady. Internal oxygen concentrations were about one-quarter normal atmospheric concentration, even when external oxygen was more than double the normal amount.

The traditional explanation for the opening and closing of air holes was that the cycle saves water. A more recent explanation linked on-off spiracles with efficient gas exchange during life underground or in other confined spaces.

Researchers have challenged both explanations as failing to explain the pattern of breathing observed among species and the timing of the cycling.

Burmester says, "The major point is that most people think that oxygen is good, but this isn't always the case."