Flying insects have the highest mass-specific metabolic rate of all animals. Oxygen is supplied to the flight muscles by a combination of diffusion and convection along the internal air-filled tubes of the tracheal system. This study measured maximum flight metabolic rate (FMR) during tethered flight in the migratory locust under varying oxygen partial pressure ( ) in background gas mixtures of nitrogen (N), sulfur hexafluoride (SF) and helium (He), to vary O diffusivity and gas mixture density independently. With N as the sole background gas (normodiffusive-normodense), mass-independent FMR averaged 132±19 mW g at normoxia ( =21 kPa), and was not limited by tracheal system conductance, because FMR did not increase in hyperoxia. However, FMR declined immediately with hypoxia, oxy-conforming nearly completely. Thus, the locust respiratory system is matched to maximum functional requirements, with little reserve capacity. With SF as the sole background gas (hypodiffusive-hyperdense), the shape of the relationship between FMR and was similar to that in N, except that FMR was generally lower (e.g. 24% lower at normoxia). This appeared to be due to increased density of the gas mixture rather than decreased O diffusivity, because hyperoxia did not reverse it. Normoxic FMR was not significantly different in He-SF (hyperdiffusive-normodense) compared with the N background gas, and likewise there was no significant difference between FMR in SF-He (normodiffusive-hyperdense) compared with the SF background gas. The results indicate that convection, not diffusion, is the main mechanism of O delivery to the flight muscle of the locust when demand is high.
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