VARIABLE RESPONSES OF INSECT SIZE TO ATMOSPHERIC OXYGEN LEVEL ACROSS SPECIES AND POPULATIONS

Atmospheric hyperoxia has been widely hypothesized to have enabled insect gigantism in the late Paleozoic. Does hyperoxic rearing lead to larger insects? In single-generation experiments, some insects (fruitflies, some beetles) grow 2-15% larger when reared in higher-than-normal oxygen level. However, other insects (e.g. grasshoppers, caterpillars) are the same size or smaller when reared in hyperoxia. Effects of hypoxia are more consistent, with most species tested being 5-20% smaller when reared in 10% oxygen. Over seven generations, fruitflies evolve 7-14% larger sizes when reared in hyperoxia (40% O₂) and 14-16% smaller sizes when reared in hypoxia (10% O₂). These data suggest that acute effects of hyperoxia on body size within a species may be taxon-specific, while hypoxia effects may be more general. Across species, effects of atmospheric oxygen on size may be even more substantial. Larger beetle species devote a proportionally larger fraction of their body to the tracheal system, and extrapolation of this trend suggests that the leg orifice of the largest extant beetle species may be nearly full with tracheae. This analysis is consistent with the hypothesis that the current level of atmospheric oxygen limits the size of extant insects. Hyperoxic rearing reduces the size of tracheae, potentially releasing spatial constraints within the insect body and allowing the evolution of larger insect species. An urgent need is to test whether there were actually increases in the size of insects when oxygen levels rose in the Permian and Carboniferous, as well as decreases in insect size during hypoxic conditions in the Triassic. Supported partially by NSF IBN 0419704 and DOD 3000654843.