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First published online March 16, 2007
Journal of Experimental Biology 210, 1288-1296 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.001982

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

Kendra J. Greenlee¹, Christina Nebeker² and Jon F. Harrison²

¹ Department of Medicine, Section of Pulmonary and Critical Care Medicine, Baylor College of Medicine, Houston, TX 77030, USA
² School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA

*Author for correspondence (e-mail: greenlee{at}bcm.edu)

Accepted 5 February 2007

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 CO₂ 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 frequencyxtidal 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.

Key words: oxygen delivery, scaling, insect, gigantism

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