Tracheal Breathing

Insects, and some other invertebrates, exchange oxygen and carbon dioxide between their tissues and the air by a system of air-filled tubes called tracheae.

Tracheae open to the outside through small holes called spiracles. In the grasshopper, the first and third segments of the thorax have a spiracle on each side. Another 8 pairs of spiracles are arranged in a line on either side of the abdomen.

• valves controlled by muscles that enables the grasshopper to open and close them;
• hairs that filter out dust as the air enters the spiracles.

Spiracles open into large tracheal tubes. These, in turn, lead to ever-finer branches. The branches penetrate to every part of the body. At their extreme ends, called tracheoles, they may be less than 1 µm in diameter. Every cell in the insect's body is adjacent to, or very close to, the end of a tracheole. In some of the flight muscles of Drosophila the tracheoles even penetrate their T-tubules bringing oxygen right next to the mitochondria that power the muscle.

This photomicrograph show how the walls of the tracheal tubes are stiffened with bands of chitin. Even so, there is a limit to the pressure they can withstand without collapsing. This may be one reason why insects are relatively small. The increased weight of the tissues of an animal the size of a rabbit, for example, would crush tracheal tubes filled only with air.

Ventilation of the Tracheal System

However, water vapor as well as carbon dioxide diffuses out of the animal, and this could pose a problem in dry environments. Drosophila avoids the risk by controlling the size of the opening of its spiracles to match the need of its flight muscles for oxygen. When oxygen demand is less, it partially closes its spiracles thus conserving body water. (See Fritz-Olaf Lehmann's report in the 30 November 2001 issue of Science).

Large, active insects like grasshoppers, forcibly ventilate their tracheae. Contraction of muscles in the abdomen compresses the internal organs and forces air out of the tracheae. As the muscles relax, the abdomen springs back to its normal volume and air is drawn in. Large air sacs attached to portions of the main tracheal tubes increase the effectiveness of this bellowslike action.

The experiment illustrated (first performed by the insect physiologist Gottfried Fraenkel) shows that there is a one-way flow of air through the grasshopper. The liquid seals in the tubing move to the right as air enters the spiracles in the thorax and is discharged through the spiracles in the abdomen. The rubber diaphragm seals the thorax from the abdomen.

The one-way flow of air increases the efficiency of gas exchange as CO₂-enriched air can be expelled without mingling with the incoming flow of fresh air.

Gas Exchange in Aquatic Insects

• Some, like mosquito larvae ("wigglers"), get their air by poking a breathing tube — connected to their tracheal system — through the water surface.
• Some insects that can submerge for long periods carry a bubble of air with them from which they breathe.
• Still others have spiracles mounted on the tips of spines. With these they pierce the leaves of underwater plants and obtain oxygen from the bubbles formed (by photosynthesis) within the leaves.
• Even in aquatic insects that have gills, after oxygen diffuses from the water into the gills, it then diffuses through a gas-filled tracheal system for transport through the body.