Insects take in oxygen through spiracles, small openings in the exoskeleton that lead to their internal tracheal system. Unlike fish gills, spiracles do not extract oxygen from water; they admit atmospheric air directly into the body.
After air enters a spiracle, it travels through larger tubes called tracheae and then through finer branches called tracheoles. Oxygen diffuses from these tubes directly into nearby tissues, while carbon dioxide moves out along the reverse path. This arrangement means an insect's circulatory system generally does not carry oxygen the way human blood does.
Spiracles are often positioned along the sides of the thorax and abdomen. Many insects can close or partially close them with valves, helping reduce water loss. The number and arrangement vary by species, and some aquatic insect larvae use specialized structures such as tracheal gills instead. A common mix-up is to answer “tracheae”: tracheae are the internal tubes, whereas spiracles are the external openings through which air enters.