Effects of ontogeny on the oxygen sensitivity of jumping performance in the American locust (Schistocerca americana)

S. D. Kirkton, N. Petrie & J. F. Harrison

Dept. of Biology, Arizona State Univ., Tempe, AZ 85287-1501, USA

As insects grow, the distance between the spiracle and the tissue increases such that diffusive gas exchange capacity could be reduced. For example, leg length increases five fold and body mass increases 100 fold during development from juvenile to adult in the American locust (Schistocerca americana). We tested the hypothesis that older, larger insects have more difficulty supplying oxygen to body tissue during locomotion. We forced second, fourth, and sixth instars and adult S. americana to jump in five different oxygen atmospheres (ranging from 5% to 60% oxygen). Each grasshopper experienced only one oxygen concentration (balance nitrogen, normobaric). We measured the initial hop rate, endurance time up to 20 minutes and total hops to exhaustion for each individual. Animals were considered exhausted when 30 seconds passed between jumps. Smaller, younger grasshoppers in normal (21% oxygen) atmospheres jump at higher rates, have greater endurance times, and more jumps to exhaustion. Hyperoxia increased initial hop rate and the number of jumps to exhaustion only in the largest (sixth instar and adult) grasshoppers suggesting oxygen limitation of performance. Hypoxia decreased initial hop rate and the total number of jumps until exhaustion more strongly in larger grasshoppers. These data support the hypothesis that increased body size reduces the tracheal oxygen delivery capacity relative to leg muscle metabolic needs. Body size also changes within an instar, such that grasshoppers approximately double their mass, but do not change the structure of their primary trachea. Therefore, tracheal oxygen delivery capacity relative to leg muscle needs may be reduced within an instar. Also air sac size may decrease during and instar, reducing the internal oxygen stores. To test these possibilities, we compared the jumping performance and oxygen sensitivity of performance of grasshoppers early and late in an instar. Early within an instar, grasshoppers had higher initial hop rates, greater endurance times and more jumps to exhaustion. These results were supported both in early (fourth instars and in late instars (sixth) and adults. These results support the hypothesis older grasshoppers, within a moult, have a reduced capacity to deliver oxygen during locomotion as compared to younger ones. The results are also consistent with the idea that hyperoxia in the Paleozoic Era permitted for the evolution of gigantic insects.

Index terms: gas exchange, body size, grasshopper, age, locomotion

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