Cockroaches breathe discontinuously to reduce respiratory water loss

Abstract

SUMMARYThe reasons why many insects breathe discontinuously at rest are poorly understood and hotly debated. Three adaptive hypotheses attempt to explain the significance of these discontinuous gas exchange cycles (DGCs), whether it be to save water, to facilitate gas exchange in underground environments or to limit oxidative damage. Comparative studies favour the water saving hypothesis and mechanistic studies are equivocal but no study has examined the acclimation responses of adult insects chronically exposed to a range of respiratory environments. The present research is the first manipulative study of such chronic exposure to take a strong-inference approach to evaluating the competing hypotheses according to the explicit predictions stemming from them. Adult cockroaches (Nauphoeta cinerea) were chronically exposed to various treatments of different respiratory gas compositions (O 2 , CO 2 and humidity) and the DGC responses were interpreted in light of the a priori predictions stemming from the competing hypotheses. Rates of mass loss during respirometry were also measured for animals acclimated to a range of humidity conditions. The results refute the hypotheses of oxidative damage and underground gas exchange, and provide evidence supporting the hypothesis that DGCs serve to reduce respiratory water loss: cockroaches exposed to low humidity conditions exchange respiratory gases for shorter durations during each DGC and showed lower rates of body mass loss during respirometry than cockroaches exposed to high humidity conditions.

A hierarchy of factors influence discontinuous gas exchange in the grasshopper Paracinema tricolor (Orthoptera: Acrididae)

“…Much recent support has been found for both the hygric and oxidative damage hypotheses (e.g. Hetz and Bradley, 2005;White et al, 2007;Terblanche et al, 2008;Schimpf et al, 2009;Williams et al, 2010;Schimpf et al, 2012; but see Boardman et al, 2012), suggesting that both may provide mechanistic explanations for the evolution of DGE. If this is indeed the case, the next logical step is to understand the prioritization of control within the organism, so further elucidating the regulation of gas exchange, and the integration of inputs and homeostatic setpoints in the system.…”

Metabolism and water loss rate of the haematophagous insect,Rhodnius prolixus: effect of starvation and temperature

“…This unexpected result could not be explained as a consequence of dehydration because assays were short-term recordings (about 30-35 min). It could also not be explained by better control of the spiracles (Schimpf et al, 2009;Wigglesworth, 1972) because we distinguished between RWL and CWL, and the lower CP at 35°C is observed both on GCP and corrected CP, i.e. calculated only from CWL.…”

The effect of discontinuous gas exchange on respiratory water loss in grasshoppers (Orthoptera: Acrididae) varies across an aridity gradient

“…For example, a shorter burst phase and cycle length have been reported in cockroaches in response to low humidity (Schimpf et al, 2009); xeric species exhibited a shorter burst phase, longer interburst phase or longer DGC, with or without an associated reduction in metabolic rate, compared with their mesic counterparts (Duncan et al, 2002;Chown and Davis, 2003;White et al, 2007).…”

“…The hygric hypothesis has received considerable experimental support. For example, chronic exposure of cockroaches to low humidity results in shorter bursts of gas exchange, correlated with reduced rates of body mass loss (Schimpf et al, 2009). Employing DGC is also associated with increased desiccation and starvation resistance (Schimpf et al, 2012).…”