[2] Independent of cycles of insect ventilation which may be discontinuous, cellular respiration on a whole animal level continues at a constant rate.
[2] During the flutter phase of discontinuous gas exchange cycles, spiracles open slightly and close in rapid succession.
The flutter phase may continue even after tracheal pressure is equal to that of the environment, and the acquisition of O2 may be assisted in some insects by active ventilatory movements such as contraction of the abdomen.
During the open phase, a complete exchange of gases with the environment occurs entirely by diffusion in some species, but may be assisted by active ventilatory movements in others.
[1] The great variation in insect respiratory cycles can largely be explained by differences in spiracle function, body size and metabolic rate.
[1] 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.
Discontinuous gas exchange cycles have long been thought to be an adaptation to conserve water when living in a terrestrial environment (the hygric hypothesis).
For discontinuous gas exchange cycles to be considered adaptive, the origin and subsequent persistence of the trait must be demonstrated to be a result of natural selection.
[1] The hygric hypothesis proposes that the discontinuous release of CO2 is an adaptation that allows terrestrial insects to limit respiratory water loss to the environment.
[5] This hypothesis is supported by studies that have demonstrated that respiratory water loss is substantially higher in insects forced to keep their spiracles open, than those of the same species who exhibit discontinuous gas exchange.
Following work on harvester ants in 1995, doctors John Lighton and David Berrigan proposed the chthonic hypothesis.
[1] Lighton and Berrigan hypothesized that discontinuous gas exchange cycles may be an adaptation to maximize partial pressure gradients between an insect’s respiratory system and the environment in which it lives.
[2] The oxidative damage hypothesis states that discontinuous gas exchange cycles are an adaptation to reduce the amount of O2 delivered to tissues under periods of low metabolic rate.
[2] The strolling arthropods hypothesis is supported by evidence that tracheal parasites can substantially limit O2 delivery to the flight muscles of active honeybees.