How do most insects respire?

Most insects respire by a tracheal system — a network of branching chitin-lined tubes (tracheae) that deliver oxygen directly to tissues and remove carbon dioxide. Air enters and leaves this system through external openings called spiracles located along the thorax and abdomen, and insect respiration is functionally independent of the circulatory system. This direct diffusion-based arrangement allows efficient gas exchange for small bodies and active flight muscles. Lungs are characteristic of many vertebrates, and gills or cutaneous respiration occur only in aquatic or specially adapted invertebrates; skin or gill respiration is not the primary mode for most insects. Therefore the tracheal system best describes how most insects breathe.