LIGHTON, J.R.B.; TURNER, R.J.; Univ of Nevada Las Vegas & Sable Systems; SpanLabs, Inc, Las Vegas NV: Discontinuous gas exchange and respiratory water loss during hypoxia challenge

The fact that many adult insects undergo a discontinuous gas exchange cycle (DGC) is well documented, but the selective advantages of the DGC remain controversial. Though usually considered an adaptation that reduces respiratory water loss (the �hygric hypothesis�), the DGC, with its decoupling of oxygen uptake and carbon dioxide emission, may also be advantageous in slightly hypoxic or hypercapnic environments (the �chthonic hypothesis�, which hypothesizes that respiratory water loss reduction is exaptive rather than the primary selective pressure operating on the DGC, at least during its early evolutionary stages). However, no studies of the DGC and allied changes in rate of respiratory water loss in small insects have been undertaken using hypoxia to evaluate the hygric hypothesis. Using gradated hypoxia (< 10 kPa) provides an excellent way to modulate the diffusion gradient for oxygen during the critical fluttering-spiracle or F phase of the DGC during which the primary water loss reduction, relative to simple Kroghian gas exchange through open spiracles, is thought to occur. We demonstrate that carbon dioxide emission during the F phase increases inversely proportionally to oxygen partial pressure until enough carbon dioxide is lost diffusively during the F phase to eliminate the DGC. We present and discuss the respiratory water loss correlates of these changes in light of the prevailing theories of the evolutionary genesis of the DGC.