Many insects hold their breath for hours by closing down their air hatches. Now researchers have figured out why. The insects, it seems, are trying to avoid breathing in too much of a poisonous gas--oxygen.
Oxygen may be the breath of life, but too much can be deadly. Exposing body tissues to highly reactive oxygen molecules can damage proteins, lipids, and DNA, hastening aging and cell death. Familiar with research on oxidative damage, insect physiologist Timothy Bradley of the University of California, Irvine, wondered whether breath-holding insects were trying to dodge oxygen overload.
Bradley teamed up with Stefan Hetz of Humboldt University in Berlin, Germany to study the breathing habits of atlas moth pupae (Attacus atlas). The pupae, like all other insects, lack lungs and breathe via tiny valves known as spiracles. Spiracles are connected to a branching network of tubes, or tracheae, which deliver air to tissues. The researchers inserted tiny gas sensors into the tracheae of the pupae and monitored the relative amounts of oxygen and carbon dioxide during normal breathing cycles. The higher the fraction of oxygen, the more readily it diffuses into body tissues.
When the pupae opened their spiracles completely, the researchers found oxygen levels reached the danger zone. The scientists then varied the level of oxygen around the pupae. The more oxygen, the less frequently the insects breathed, the team reports in the 3 February 2005 issue of Nature. Yet the pupae kept interior oxygen and carbon dioxide levels rock steady by fluttering their spiracles.
Breathing in bursts may be the insect's way of decreasing oxygen deliveries to tissues when their metabolic needs are low, says Bradley. But the insects will breathe regularly during periods of high activity, like flight. It's an "incredibly efficient" system, he says.
Oxygen avoidance "is definitely the best hypothesis for rhythmic breathing so far," says insect physiologist Thorsten Burmester of the University of Mainz, Germany. But to settle the argument completely, he says, the experiments should be repeated in more species of insects while they are both resting and active.