VARIATIONS ON THE TRACHEAL SYSTEM
In smaller, less active insects, tracheal gas exchange is by simple diffusion. Larger, more active insects such as grasshoppers improve upon diffusion by forcibly ventilating their tracheae, analogous to breathing in mammals. Contraction of abdominal muscles compresses their internal organs, forcing air out (like exhaling). Relaxation of abdominal muscles enables air to be drawn back in (like inhaling).

Aquatic insects, whether adult or larva, also rely on tracheal tubes for gas exchange. Some insects, such as mosquito larvae, remain tied to the air and exchange gases at the water's surface. Others may bring a bubble of air under water with them. Even truly aquatic insect larvae—with gills through which O₂ diffuses from the water—still transport the O₂ throughout the body with a gas-filled tracheal system.

Because the tracheal tubes carry oxygen from the air directly to the cells, insects don't need to carry oxygen in their hemolymph, like mammals do in their blood. This is why insect hemolymph isn't red: the oxygen-carrying molecules (hemoglobin) make mammalian blood red.