Effects of Insect Body Size on Tracheal Structure and Function

Abstract

Fossilized insect specimens from the late Paleozoic Era (approximately 250 million years ago) were significantly larger than related extant species. Geologic estimates suggest that atmospheric oxygen in the late Paleozoic Era was 35%. These findings have led to a prominent hypothesis that insect body size may be limited by oxygen delivery. Empirical evidence from developing Schistocerca americana grasshopper experiments suggests that larger/older animals are not more sensitive. Larger/older S. americana grasshoppers have a greater tidal volume at rest in hypoxia as compared to smaller animals. During jumping, larger S. americana grasshoppers have increased fatigue rates but the jumping muscle also consumes significantly more oxygen than smaller animals, suggesting that the tracheal system does not limit oxygen delivery. Larger/older grasshoppers were also found to have more tracheoles in their jumping muscle to promote increased diffusive oxygen delivery. Using real time x-ray synchrotron phase-contrast analysis, we have found that larger/older grasshoppers also have a greater proportional volume of abdominal tracheae and air sacs per body mass than smaller/younger grasshoppers to enhance convective oxygen delivery. To better understand if internal P_O2 changes may be related to the increase in tracheal structure of larger/older grasshoppers, we have begun to use electron paramagnetic resonance to measure internal P_O2 in the femoral hemolymph at rest and recovery during jumping. We have demonstrated that the femoral oxygen stores are signifi- cantly depleted during the on-set of jumping in adult S. americana grasshoppers. If larger S. americana grasshoppers have proportionally more respiratory structures throughout their body to help maintain their internal P_O2, the greater relative amount of body mass dedicated to respiratory structures may inhibit overall insect body size by reducing the amount of energy or space dedicated to other tissues. However, future interspecific studies are needed to better separate the effects of development and body size per se on the insect tracheal system.