As grasshoppers age and become larger, leg lengths increase five-fold, thorax width increases six-fold, and body mass can increase by as much as 100 times from hatchling to adult. How do these increases in size affect the function of the insect respiratory system? To explore the relationship between development and gas exchange, we used grasshoppers (the American locust, Schistocerca americana) of known age and body size and exposed them to graded hypoxia. We measured ventilatory frequency by counting abdominal pumping and MCO2 (umol h -1 ) using flow-through respirometry. In normal air, size had little effect on ventilation frequency. In response to hypoxia, ventilation frequency increased in large but not small grasshoppers. Larger grasshoppers had much lower critical PO2’s (the ambient PO2 at which MCO2 became significantly lower than that in normoxia) possibly due to their greater ability to increase abdominal pumping frequency in response to hypoxia. Large grasshoppers were able to maintain constant MCO2’s down to a critical point near 3 kPa PO2, whereas progressive hypoxia caused logarithmic decreases in MCO2 in small grasshoppers. These findings support the hypothesis that smaller grasshoppers are more reliant than larger grasshoppers on diffusion for gas exchange when the oxygen delivery system is challenged by hypoxia. However, it is not clear whether such a difference is size-, or simply, age-dependent. Smaller grasshoppers may not yet have developed the neural/signaling mechanisms required for responding to hypoxia. In addition, we studied the effect of within instar development on critical PO2 for grasshoppers in the first, third, fifth, and adult stadia. S. americana substantially increase their body mass and MCO2 during an instar. Since we suspect insects can make only minimal changes in the structure of the major trachea between moults, we hypothesized that animals near the end of an instar may have decreased oxygen delivery capacity relative to their metabolic needs. Therefore, we predicted that insects near the end of the instar would be more sensitive to hypoxia than insects that had recently moulted. We found that animals nearing the end of the instar did have increased ventilatory frequencies and increased critical PO2’s compared to the younger insects. This decrease in oxygen delivery capacity relative to tissue metabolic needs may constrain growth within an instar for insects in general.

Index terms: grasshopper, gas exchange, ventilation, body size, development