Chappell MA, Rogowitz GL (2000) Mass, temperature, and metabolic effects on discontinuous gas exchange cycles in eucalyptus-boring beetles (Coleoptera: Cerambycidae). IJournal of Experimental Biology 203: 3809-3820
SUMMARY -- Ventilatory accommodation of changing metabolic rates is a relatively little-studied aspect of the discontinuous gas exchange cycles (DGC) that occur in a wide variety of terrestrial arthropods. We used correlation analysis of resting metabolic rates (RMR, measured as rates of CO2 emission; VCO2) and several components of the DGC to examine accommodation to both temperature-induced changes and individual variation in RMR in two wood-boring beetles (Phorocantha recurva and P. semipunctata; Coleoptera: Cerambycidae).
At low to moderate ambient temperatures (Ta; 10-20 °C), Phorocantha displayed a characteristic DGC with relatively brief but pronounced open (O) phase bursts of CO2 emission, separated by longer periods of low VCO2 the flutter (F) phase. However, the VCO2 never fell to zero and we could not reliably differentiate a typical closed (C) phase from the F phase. Accordingly, we pooled the C and F phases for analysis as the C+F phase. At higher Ta (30 °C), the duration of the combined C+F phases was greatly reduced. There were no differences between the two species or between males and females in either RMR or characteristics of the DGC. We found large variation in the major DGC components (cycle frequency, durations and emission volumes of the O and C+F phases); much of this variation was significantly repeatable. Accommodation of temperature-induced RMR changes was almost entirely due to changes in frequency (primarily in the C+F phase); that is similar to results from several other discontinuously ventilating arthropods. Frequency changes also contributed to accommodation at constant Ta, but modulation of emission volumes (during both O and C+F phases) played a larger role.
The DGC is often viewed as a water conservation mechanism, based on the belief that respiratory evaporation is minimal during the C and F phases. That hypothesis assumes that the F phase is primarily convective (due to a reduction in tracheal pO2 and total intratracheal pressure during the C phase). To test this, we measured the DGC in beetles subjected to varying degrees of hypoxia in addition to normoxia. As predicted for a largely diffusive F phase, we found an increase in the volume of CO2 emitted during the C+F phase in hypoxic conditions (10.4% oxygen). That finding, along with a reduced tendency to utilize a DGC at high Ta (when water stress is greatest) and a natural history in which water availability is probably not limiting for any life stage, suggest that reduction of respiratory evaporation may not have been critical in the evolution of the Phorocantha DGC. Instead, selection may have favored discontinuous ventilation because it facilitates gas exchange in the hypercapnic and hypoxic environments commonly encountered by animals (like Phorocantha) that live in confined spaces.