Evolutionary responses of discontinuous gas exchange in insects.

The discontinuous gas-exchange cycles (DGCs) observed in many quiescent insects have been a cause of debate for decades, but no consensus on their evolutionary origin or adaptive significance has been achieved. Nevertheless, three main adaptive hypotheses have emerged: (i) the hygric hypothesis suggests that DGCs reduce respiratory water loss; (ii) the chthonic hypothesis suggests that DGCs facilitate gas exchange during environmental hypoxia, hypercapnia, or both; and (iii) the oxidative-damage hypothesis suggests that DGCs minimize oxidative tissue damage. However, most work conducted to date has been based on single-species investigations or nonphylogenetic comparative analyses of few species, despite calls for a strong-inference, phylogenetic approach. Here, we adopt such an approach by using 76 measurements of 40 wild-caught species to examine macrophysiological variation in DGC duration in insects. Potential patterns of trait variation are first identified on the basis of the explicit a priori predictions of each hypothesis, and the best phylogenetic generalized least-squares fit of the candidate models to the data is selected on the basis of Akaike's information criterion. We find a significant positive relationship between DGC duration and habitat temperature and an important interaction between habitat temperature and precipitation. This result supports the hygric hypothesis. We conclude that the DGCs of insects reduce respiratory water loss while ensuring adequate gas exchange.