<P> An insect's respiratory system is the biological system with which it introduces respiratory gases to its interior and performs gas exchange . </P> <P> Air enters the respiratory systems of insects through a series of external openings called spiracles . These external openings, which act as muscular valves in some insects, lead to the internal respiratory system, a densely networked array of tubes called tracheae . This network of transverse and longitudinal tracheae equalizes pressure throughout the system . </P> <P> It is responsible for delivering sufficient oxygen (O) to all cells of the body and for removing carbon dioxide (CO) that is produced as a waste product of cellular respiration . The respiratory system of insects (and many other arthropods) is separate from the circulatory system . </P> <P> Insects have spiracles on their exoskeletons to allow air to enter the trachea . In insects, the tracheal tubes primarily deliver oxygen directly into the insects' tissues . The spiracles can be opened and closed in an efficient manner to reduce water loss . This is done by contracting closer muscles surrounding the spiracle . In order to open, the muscle relaxes . The closer muscle is controlled by the central nervous system but can also react to localized chemical stimuli . Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea . Spiracles may also be surrounded by hairs to minimize bulk air movement around the opening, and thus minimize water loss . </P>

Where does gaseous exchange take place in insects