English: Most small insects breathe passively, relying on diffusion and their physical movement to move air through the system. Air diffuses through a series of tubes (trachea) that branch and subdivide through the body. Gases enter and leave the body through holes along the sides of the body called "spiracles". Inside the body, the tracheal trunk divides and subdivides again and again into smaller and smaller tracheal branches. The branches reach bits of tissue and terminate directly on the cell membranes as fluid-filled tips. This contact at the terminal tracheole permits the exchange of gases. This passive system of diffusion is considered largely responsible for limiting the size of insects. In the carboniferous era, when the oxygen content of the atmosphere was higher, insects could achieve a larger size.

The trachea are reinforced by rings of chitin (taenidia) that spiral through the tracheal walls. Chitin is the same protein that protects the insects by forming their exoskeleton. Some insects have regions that lack chitin. These areas form collapsible reservoirs for storing air.

Some larger insects have the ability to open and close some of their spiracles. By moving their abdominal muscles, they may be able to move air more effectively through the body though interconnecting tracheal trunks. Even here, diffusion is the means by which gases are exchanged at the terminal connections.