LIGHTON, J.R.B.; Univ. of Nevada, Las Vegas: The discontinuous gas exchange cycle in tracheate arthropods: Overview, status and controversies.

The discontinuous gas exchange cycle or DGC decouples O₂ uptake and CO₂ emission in tracheate arthropods, including many insects as well as argasid and ixodid acarines, sulphugids and scolopendromorph chilopods. Partial pressures gradients of O₂ across the spiracles may cycle by as much as 18 kPa, and are regulated at these high gradients for much of each cycle followed by a �burst� of CO₂ emission. Classically, the DGC is hypothesized to be an adaptation to decrease respiratory water loss (RWL); this is the �hygric hypothesis�. More recently, I have proposed the �chthonic hypothesis�: that the DGC evolved in response to hypoxic or hypercapnic microhabitats (this has been misunderstood for some reason as referring to the hygric hypothesis). T. Bradley and S. Hetz propose, instead, that the DGC evolved as a response to oxygen toxicity, specifically reduction of free radical damage; this is the �hypoxic hypothesis�. And adding to the confusion, many insects can exchange respiratory gases continuously in addition to possessing a DGC. Experimental data that bear directly on the RWL correlates of the DGC have only recently been acquired. In this talk I summarize our present knowledge of the field, propose a standardized terminology, and present experimental data pertaining to its main controversies. These data are mostly from studies in which temperature modulation of catabolism, or hypoxia, are employed to eliminate the expression of the DGC and assess the resulting effect on RWL. In each case no reduction in RWL during the DGC was found, relative to RWL during continuous gas exchange. This disproves or at least seriously weakens the hygric hypothesis, emphasizing again that in evolutionary physiology, mere credibility is at best a weak indicator of correctness.