Body size-independent safety margins for gas exchange across grasshopper species

Why is maximal insect body size relatively small compared to that of vertebrates? Possibly insect body size is limited by the capacity of the tracheal respiratory system to delivery oxygen down longer and longer tracheae to the tissues. If so, one possible outcome would be that larger insect species would have a smaller safety margin for oxygen delivery (higher critical P(O2), P(c)). We tested this idea by exposing inactive adult grasshoppers of a range of species and body sizes (0.07-6.4 g) to progressively lower oxygen atmospheres and measuring their ventilation frequency and their ability to maintain metabolic rate (indexed by CO(2) emission rate). We analyzed effects of body size on these parameters by simple linear regressions, as well as methods to control for phylogenetic relatedness among species. We found interspecific variation in P(c), but P(c) did not significantly correlate with body mass (average P(c) across all species = 4 kPa). Maximal tracheal system conductance scaled approximately with mass(0.7), and estimated ventilation in hypoxia (ventilatory frequency x tidal volume) scaled directly with mass, suggesting that convection is the major mechanism of gas exchange in all these species. These comparative data strengthen the growing body of evidence that body size does not affect the safety margin for oxygen delivery in insects.