Trachea systems Insects

The life of an Insect Most insects are small terrestrial animals (live on land). They have a large surface area to volume ratio. Their exoskeletons are hard and waxy to minimise water loss. One advantage terrestrial animals have over aquatic animals is that oxygen is more abundant in air than in water

Gas exchange system - Trachea Insects exchange oxygen and carbon dioxide between their tissues and the air by a system of air-filled tubes called trachea. Trachea open to the outside through small holes called 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, and are filled with liquid. Every cell in the insect's body is adjacent, or very close to, the end of a tracheole.

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. Trachea

As water vapor and carbon dioxide diffuse out of the animal this could pose a problem in dry environments. Drosophila avoids the risk of dehydration by controlling the size of the opening of its spiracles to match the need of its flight muscles for oxygen. Drosophila When oxygen demand is less, it partially closes its spiracles thus conserving body water. 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.

Tracheae and spiracle of an insect. At top center is a spiracle (brown), from which numerous branching tubes – the tracheae – can be seen running to many parts of the insect’s body. The spiracles are usually located on the sides of the body segments, the number varying in different kinds of insects. (See previous figure for a drawing of terminal branches, or tracheoles.

Spiracles of two insects. (A) Scanning electron micrograph of a fully open ant spiracle. The pointed projections are sensory hairs that monitor external conditions and can trigger spiracle closing when necessary. (B) A nearly closed grasshopper spiracle; the black areas are the valves. Note the resemblance to the stoma of a leaf.

Aquatic insects Biozone p 194