INVITED: DOES ATMOSPHERIC OXYGEN LEVEL LIMIT MAXIMAL INSECT SIZE?

The correlation between historical atmospheric oxygen levels and size of insect fossils has led to the widely quoted hypothesis that elevations in atmospheric oxygen level allowed the evolution of gigantic insects in the Paleozoic. It is possible that evolutionary processes have changed insect tracheal systems. However, given the similar external morphology of Paleozoic and extant insects, it seems reasonable that if this hypothesis is correct, that the size of extant insects should be influenced by oxygen levels. One way oxygen might limit insect size is that it might be more challenging for larger insects to deliver oxygen. However, a variety of morphological and physiological measurements examining the effect of size within and across insect species suggests that size does not affect the safety margin for oxygen delivery in insects. Larger insects achieve similar safety margins for oxygen delivery by expanding the relative size of their tracheal system, and by increasing use of convection. Conceivably, maximal size could be limited by space conflicts within insects; increasing tracheal system size could reduce space available for other tissues. Alternatively, insect body size may be affected by developmental effects of oxygen. Increases in insect mass within an instar must occur with a given size of tracheae and spiracles, and increases in body and tracheal system dimensions can only occur with molting. Growth thus leads to strongly declining safety margins for oxygen delivery within an intermolt period, perhaps explaining why increased oxygen level allows greater intermolt periods and growth rates in some insects. Some insect species (eg. fruitflies and mealworms) but not all (grasshoppers) show positive correlations between body size and rearing oxygen level. Fruitflies evolve larger body sizes in higher atmospheric oxygen levels, and atmospheric oxygen level can constrain the evolution of large size during artificial selection experiments. Together, the bulk of these data support the general hypothesis that historical fluctuations in atmospheric oxygen levels may have had major effects on insect biota.