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. The spiracles are guarded by
- valves controlled by muscles that enables the grasshopper to open and close them
- hairs that filter out dust as the air enters the spiracles
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
Even aquatic insects use a tracheal system for gas exchange.
- 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.