Discontinuous Gas Exchange - Variability in Discontinuous Gas Exchange Cycles

Variability in Discontinuous Gas Exchange Cycles

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.

Variability in discontinuous gas exchange cycles is also dependent upon external stimuli such as temperature and the partial pressure of O₂ and CO₂ in the external environment. Environmental stimuli may affect one or more aspects of discontinuous cycling, such as cycle frequency and the quantity of CO₂ released at each burst. Temperature can have massive effects on the metabolic rate of ectothermic animals, and changes in metabolic rate can create large differences in discontinuous gas exchange cycles. At a species-specific low temperature discontinuous gas exchange cycles are known to cease entirely, as muscle function is lost and spiracles relax and open. The temperature at which muscular function is lost is known as the chill coma temperature.

Discontinuous gas exchange cycles vary widely among different species of insects, and these differences have been used in the past to support or refute hypotheses of the evolution of respiratory cycling in insects.