Discontinuous Gas Exchange

Discontinuous Gas Exchange

Discontinuous gas-exchange cycles (DGC), also called discontinuous ventilation or discontinuous ventilatory cycles, follow one of several patterns of arthropod gas exchange that have been documented primarily in insects; they occur when the insect is at rest. During DGC, oxygen (O₂) uptake and carbon dioxide (CO₂) release from the whole insect follow a cyclical pattern characterized by periods of little to no release of CO₂ to the external environment. Discontinuous gas exchange is traditionally defined in three phases, whose names reflect the behaviour of the spiracles: the closed phase, the flutter phase, and the open phase.

Until recently, insect respiration was believed to occur entirely by simple diffusion. It was believed that air entered the tracheae through the spiracles, and diffused through the tracheal system to the tracheoles, whereupon O₂ was delivered to the cells. However, even at rest, insects show a wide variety of gas exchange patterns, ranging from largely diffusive continuous ventilation, to cyclic respiration, of which discontinuous gas exchange cycles are the most striking.

Discontinuous gas exchange cycles have been described in over 50 insect species, most of which are large beetles (order Coleoptera) or butterflies or moths (order Lepidoptera). As the cycles have evolved more than once within the insects, discontinuous gas exchange cycles are likely adaptive, but the mechanisms and significance of their evolution and are currently under debate.