Discontinuous gas-exchange cycles in Scarabaeus dung beetles (Coleoptera :Scarabaeidae): Mass-scaling and temperature dependence

Although discontinuous gas exchange cycles (DGC) are known from many insect s, the effects of body size and temperature on DGC have not been widely exa mined. Here, these effects are investigated in five Scarabaeus dung beetle species from mesic and xeric habitats. The investigation tests two hypothes es: that previous estimates of the scaling exponents for the DGC and its ch aracteristics are more broadly applicable to insects, and that, in response to temperature, both DGC frequency and the quantity of CO2 emitted during the open (O) phase (O-phase emission volume) are modulated. Like previous w orkers, we find that (V) over dot (CO2) scaled as mass(0.968) and that O-ph ase emission volume scaled as mass(0.968) . However, temperature-associated increases in (V) over dot (CO2) (Q(10)'s of 2.19-2.65) were modulated most ly by increases in DGC frequency since O-phase volumes remained constant ac ross temperature. Flutter (F)-phase and O-phase durations were closely coup led to DGC duration, although the relationship between closed (C)-phase dur ation and DGC duration was less pronounced. We show that ventilation phase coefficients, previously considered a measure of the proportional duration of each phase of the DGC, calculated from the slopes of these relationships are a measure of change in phase duration with change in DGC duration and not a measure of the way in which total DGC duration is apportioned among p hases. We suggest that proportions be used to estimate the contribution of each of the phases to the total duration of the DGC.