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The great variation in insect respiratory cycles can largely be explained by differences in spiracle function, body size and metabolic rate. Gas exchange may occur through a single open spiracle, or the coordination of several spiracles. Spiracle function is controlled almost entirely by the nervous system. In most insects that demonstrate discontinuous gas exchange, spiracle movements and active ventilation are closely coordinated by the nervous system to generate unidirectional air flow within the tracheal system. This coordination leads to the highly regulated bursting pattern of CO2 release. Building CO2 levels during the flutter phase may either directly affect spiracular opening, affect the nervous system while being pumped through the haemolymph, or both. However, the effects of CO2 on both spiracles and the nervous system do not appear to be related to changes in pH.