GREENLEE, K.J.*; HARRISON, J.F.; EGBERT, K. : How does the respiratory response to hypoxia change throughout ontogeny in a grasshopper and a caterpillar?

As insects age, body size can increase dramatically. For example, grasshoppers (Schistocerca americana) grow to 100 times their original mass, and larval tobacco hornworms (Manduca sexta) increase an impressive 7,000-fold from hatchling mass. How do these increases in size affect the function of the insect respiratory system? To explore the relationship between body size and gas exchange, we used insects (S. americana and M. sexta) of known age and body size, exposed them to graded hypoxia, and measured MCO₂ (umol/h) using flow-through respirometry. We hypothesized that larger insects would tolerate hypoxia better than smaller insects due to an increased usage of convective gas exchange. As predicted, larger grasshoppers had much lower critical PO₂‘s (the PO₂ at which MCO₂ became significantly lower than that in normoxia) and had an increased ability to enhance abdominal pumping in response to hypoxia. The MCO₂ of small grasshoppers decreased almost logarithmically with PO₂, supporting the hypothesis that smaller grasshoppers rely more on diffusion for gas exchange. In contrast, caterpillar body size had no effect on critical PO₂, suggesting that tracheal oxygen delivery capacity matches the increased tissue needs for oxygen across all sizes. We also hypothesized that caterpillars would have relatively high critical PO₂‘s due to their lack of visible respiratory movements and because reported critical PO₂‘s for fly and beetle larvae are greater than values for adults and pupae. However, critical PO₂‘s for M. sexta were low and similar to those found for S. americana adults (near 5 kPa O₂).