Discontinuous gas exchange cycles in Aphodius fossor (Scarabaeidae): A test of hypotheses concerning origins and mechanisms

It has been suggested that discontinuous gas exchange cycles (DGCs) in insects evolved in response to hypoxic and/or hypercapnic environments, and that the flutter phase of the DGC is predominantly diffusive. We tested these hypotheses by examining the rate of CO₂ release at a declining series of oxygen partial pressures in adults of Aphodius fossor, an inhabitant of moist to wet dung pats that can have low oxygen and high carbon dioxide concentrations. This mesic species showed a pronounced DGC, but progressively abandoned it when exposed to declining oxygen concentrations. That is, in response to a reduction in oxygen concentration, the closed phase declined in length and eventually disappeared, the flutter phase declined in length, while the flutter phase rate of CO₂ release increased, the open phase peak rate of CO₂ release declined and DGC frequency increased. Nonetheless, overall CO₂ release rate remained virtually unchanged. Thus, it appears that the flutter phase in A. fossor has a significant convective component. In addition, in response to declining oxygen concentrations, the spiracles are opened for prolonged periods to increase gas exchange. This behaviour is unlikely to incur a significant water loss penalty in the moist habitats in which A. fossor lives. Given the switch to continuous ventilation by A. fossor under hypoxic conditions, we conclude that current adaptive explanations for the DGC are inadequate. Rather, the DGC may be the default state of a system regulated by two interacting feedback loops when demands are absent.