Discontinuous gas exchange

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Discontinuous gas-exchange cycles (DGC), also called discontinuous ventilation or discontinuous ventilatory cycles, follow one of several patterns of

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.[1] 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.[2]

Until recently,

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.[3]

Discontinuous gas exchange cycles have been described in over 50

insects, discontinuous gas exchange cycles are likely adaptive, but the mechanisms and significance of their evolution are currently under debate.[2]

Phases

Discontinuous gas exchange cycles are characterized by a repeating pattern of three phases. These phases are named according to the behaviour of the spiracles and are most commonly identified by their CO2 output, primarily observed using open flow respirometry.[2]

Closed phase

During the closed phase of discontinuous gas exchange cycles, the spiracle

tracheal system drops below a lower limit, activity in the nervous system causes the initiation of the flutter phase.[2]

Flutter phase

During the flutter phase of discontinuous gas exchange cycles, spiracles open slightly and close in rapid succession.

tracheal system has both a direct (acting on the muscle tissue) and indirect (through the nervous system) impact on the spiracle muscles and they are opened widely, initiating the open phase.[2]

Open phase

A rapid release of CO2 to the environment characterizes the open phase of discontinuous gas exchange cycles. During the open phase, spiracular muscles relax and the spiracles open completely.[2] The open phase may initiate a single, rapid release of CO2, or several spikes declining in amplitude with time as a result of the repeated opening and closing of the spiracles. 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]

Variability in discontinuous gas exchange cycles

The great variation in

haemolymph, or both.[1] However, the effects of CO2 on both spiracles and the nervous system do not appear to be related to changes in pH.[1]

Variability in discontinuous gas exchange cycles is also dependent upon external

ectothermic animals, and changes in metabolic rate can create large differences in discontinuous gas exchange cycles.[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. The temperature at which muscular function is lost is known as the chill coma temperature.[4]

Discontinuous gas exchange cycles vary widely among different

insects
.

Evolution of discontinuous gas exchange cycles

Despite being well described, the mechanisms responsible for the

trait must be demonstrated to be a result of natural selection.[2]

Hygric hypothesis

The hygric hypothesis was first proposed in 1953, making it the earliest posed hypothesis for the evolution of discontinuous gas exchange.

insects
.

Chthonic and chthonic-hygric hypotheses

Following work on

hypoxia (low O2 levels) and hypercapnia (high CO2 levels) by spending at least part of their life cycle in enclosed spaces underground.[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.[7] Alternatively, insects could obtain enough O2 by opening their spiracles for extended periods of time. However, unless their environment is very humid, water will be lost from the respiratory system to the environment.[2] Discontinuous gas exchange cycles, therefore, may limit water loss while facilitating O2 consumption and CO2 removal in such environments. Many researchers describe this theory as the chthonic-hygric hypothesis and consider it to support the hygric hypothesis. However, others emphasize the importance of maximizing partial pressure gradients alone and consider the chthonic hypothesis to be distinct from the hygric hypothesis.[2]

Oxidative damage hypothesis

The oxidative damage hypothesis states that discontinuous gas exchange cycles are an

tracheal system reaches levels near that of the external environment. However, over time during the closed phase the partial pressure of O2 drops, limiting the overall exposure of tissues to O2 over time.[2] This would lead to the expectation of prolonged flutter periods in insects that may be particularly sensitive to high levels of O2 within the body. Strangely however, termites that carry a highly oxygen-sensitive symbiotic bacteria demonstrate continuous, diffusive ventilation.[8]

Strolling arthropods hypothesis

The strolling arthropods hypothesis was a very early

metabolic rates in flight muscle necessary for flight, and are grounded.[9]

References