Why do insects shut their spiracles? A meta-analytic evaluation of the adaptive hypotheses of discontinuous gas exchange in insects

The earliest description of the discontinuous gas exchange cycle (DGC) in lepidopterous insects supported the role played by the spiracles and tracheal system in modulating the release of carbon dioxide. Further understanding led to the idea that the adaptive significance of regulated opening and closure of the spiracles is to reduce water loss (hygric hypothesis) and facilitate gaseous exchange in hyperoxia/hypoxia (chthonic hypothesis). With technological advances, other insect orders were investigated and the hygric and chthonic hypotheses were questioned. To allow for broad-scale comparison and conclusion, we conducted a meta-analysis to evaluate the merit of both postulated hypotheses that included 46 insect species in 24 families across nine orders. We also quantified the percent change in metabolic rates per ºC change of temperature during DGC. DGC was shown to reduce water loss (-3.27 ± 0.88; estimate ± 95% confidence limits [95% CI]; P < 0.0001) in insects. However, DGC does not favor gaseous exchange in hyperoxia (0.21 ± 0.25 [estimate ± 95% CI]; P = 0.12) nor hypoxia, but was shown to favor gaseous exchange in normoxia (0.27 ± 0.26 [estimate ± 95% CI]; P = 0.04). After accounting for variation associated with authorship, order phylogeny, family, and species, a phylogenetic model reflected that metabolic rate exhibited a significant, non-zero increase of 8.13% (± 3.48 95% CI; P < 0.0001) per ºC increase in temperature. These data represent the first meta-analytic attempt to resolve the controversies surrounding the merit of adaptive hypotheses in insects.