Temperature-dependent oxygen limitation in insect eggs.

Most terrestrial insect embryos support metabolism with oxygen from the environment by diffusion across the eggshell. Because metabolism is more temperature sensitive than diffusion, embryos should be relatively oxygen-limited at high temperatures. We tested whether survival, development time and metabolism of eggs of a moth, Manduca sexta, were sensitive to experimentally imposed variation in atmospheric oxygen availability (5-50 kPa; normoxia at sea level is 21 kPa) across a range of biologically realistic temperatures. Temperature-oxygen interactions were apparent in most experiments. Hypoxia affected survival more strongly at warmer temperatures. Metabolic rates, measured as rates of CO2 emission, were virtually insensitive to hypo- and hyperoxia at 22 degrees C but were strongly influenced at 37 degrees C. Radial profiles of P(O2) inside eggs, measured using an oxygen microelectrode, demonstrated that 3-day-old eggs had broad central volumes with P(O2) less than 2 kPa, and that higher temperature led to lower P(O2). These data indicate that at realistically high temperatures (32-37 degrees C) eggs of M. sexta were oxygen limited, even in normoxia. This result has important implications for insect population ecology and the evolution of eggshell structures, and it suggests a novel hypothesis about insect gigantism during Paleozoic hyperoxia.