In an insect, tracheae are passages that ramify through the body and conduct air from a few openings at the surface, called spiracles. Tracheae are made of epidermis and cuticle. The finest of them are known as tracheoles.

Insect tracheae subdivide over and over again into smaller and smaller tracheoles (1). The terminal tracheoles, which are in contact with the various tissue cells (2), are filled with fluid. When the muscles are relaxed the adjacent tracheoles become fluid-filled. When they contract and need more oxygen the fluid is withdrawn from the tracheole endings, thus allowing more air to enter. This mechanism is assisted by a system of supplementary air sacs (3) that expand (A) when the muscles are relaxed, but are squeezed when they contract (B), the air being forced farther into the tissues.