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Snorkeling in the rain

Ants and most other insects breathe through a complex system of flexible tubes called trachea. Large tracheal tubes branch into successively smaller tubes that extend throughout the body, including the legs and the wings of flying insects. The smallest tubes, tracheoles, are less than a micrometer in diameter (about one-hundredth the size of a human hair), and they exchange gases with the tissues of the body.

Because insects lack lungs, scientists previously thought that gas exchange through the trachea was completely passive—that is, the tendency of gases to move from high to low concentration caused oxygen to flow into the body and carbon dioxide to flow out of the body. Insects use passive gas exchange, but they also breathe through an active mechanism, controlled similarly to how an accordion is played, with its switches and bellows.

As the switches on an accordion open and close banks of reeds, the insect nervous system switches open and closed spiracles—gateways between the tracheal system and the outside air. The insect "bellows" are the muscles that compress the exoskeleton—an insect's tough outer casing—which, in turn, puts pressure on the hemolymph—insect blood that fills the body cavity—and causes it to compress the trachea. The muscles attached to an insect's exoskeleton thus control a cycle of inflation and deflation of the trachea.

The capability to open and close spiracles helps protect insects against drowning. A partially submerged insect could selectively open the spiracles not under water. A fully submerged insect could close all its spiracles. Because insect respiratory systems are very efficient, resting insects sometimes stop breathing for a half-hour at a time, so they could wait out a short rain. For longer rains, they would need to take shelter.

Underground colonies have an intricate architecture that protects ants from the elements, as revealed by research not for the faint of heart. Biologist Walter Tschinkel from Florida State University excavated colonies of Florida harvester ants and made metal and plaster casts of their entire network of shafts and chambers. The largest nests are about 12 feet deep, made by worker ants that had to dig out almost 90 pounds of sand. Young worker ants and the ant brood pack densely into the nest's lower chambers, where air pockets would be found even when it rained.

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