DISSERTATION RESEARCH: Effect of Body Size and Development on Gas Exchange Mechanisms in Insects: Diffusion vs. Convection

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DISSERTATION RESEAERCH: Effect of Body Size and Development on Gas Exchange Mechanisms in Insects: Diffusion vs. ConvectionJon F. HarrisonKendra J. GreenleeDelivery of oxygen in organisms occurs by two basic mechanisms, convection and diffusion. Determining the relative importance of these mechanisms is fundamental to understanding gas exchange in any organism. Convection occurs when pressure generated by respiratory muscles causes bulk movement of air. Diffusion occurs by passive movement of oxygen molecules from areas of high concentration to areas of lower concentration. For insects, the relative role of convection and diffusion in gas exchange is highly controversial. Past evidence supports the notion that some insects can sustain their energy demands through diffusion of oxygen alone. However, many insects exhibit pressure-generating behaviors, such as abdominal pumping in grasshoppers. In particular, it is thought that the relative importance of convection may increase with insect body size. In this study, the relative importance of diffusion and convection to insect gas exchange will be quantified by manipulating the ability of oxygen to diffuse using different carrier gases. Generally, air contains 21% oxygen and 79% nitrogen. Substituting helium or sulfur hexaflouride for nitrogen alters the oxygen diffusion coefficient, making it easier or more difficult, respectively, for oxygen to move by diffusion. The grasshoppers' metabolic response to lowered oxygen levels will be measured in all three carrier gases. If grasshoppers are breathing by diffusion, their metabolic responses should be strongly affected by these manipulations. Conversely, if grasshoppers breathe by convection, their metabolic responses to hypoxia will not be affected by variations in carrier gas. To test whether the relative importance of diffusion and convection varies with size, these experiments will be performed with juvenile (small) and adult (large) grasshoppers. To further test the importance of diffusion and convection in insects, grasshoppers will be anesthetized with ketamine to prevent abdominal pumping movements. If convection does become increasingly important in larger insects, ketamine is predicted to have a much stronger effect on metabolic responses to hypoxia in larger grasshoppers. Together, these experiments will provide the first empirical tests of the relative importance of diffusion and convection in insects of different sizes.