Meeting Abstract
We previously demonstrated that the upper thermal limits of several species of terrestrial insects were not the result of insufficient O2 delivery, raising the possibility that some other physiological limitation must be the cause of death. In the present study we measured thermotolerance (at 50˚C; 2L min-1 air flow) in over two-thousand hissing cockroaches, Gromphadorhina portentosa, exposed to different relative humidity (RH) levels (0, 23, or 90%), ambient CO2 levels (0, 5, 10, or 15%), or antioxidant treatments (glutathione, mercaptopropionyl glycine, ascorbic acid, or Trolox). The insects were injected with 1 or 10mMol of individual antioxidants or mixtures of antioxidants (0.3ml) 30 minutes before the thermotolerance trials. Contrary to our predictions, Kaplan-Meier log rank survival analyses revealed no diminished thermotolerance with elevated CO2 – even at 15%, suggesting hypercapnia was not a critical issue. We were also unable to detect improved performance using any of the antioxidant treatments, suggesting reactive O2 species were not the direct cause of death; although, we did confirm that treatments with H2O2 hastened death. We predicted that dehydration might be a contributing factor at these high temperatures. Surprisingly, thermotolerance at 0% RH was no different from 23% RH and the times required for half of those populations (LT50) to succumb were ~35 minutes. Contrary to our expectations, thermotolerance was the lowest in the high humidity treatment where the LT50 was ~20 minutes. We do not attribute this response to reduced evaporative cooling, but rather to the fact that water vapor has a specific heat twice that of air and the increased net heat flux caused the cockroach to achieve thermal equilibrium sooner than in the other treatments.
