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The causes of thermal tolerance limits in animals are controversial. In many aquatic species, it is thought that the inability to deliver sufficient oxygen at high temperatures is more critical than impairment of molecular functions of the mitochondria. However, terrestrial insects utilize a tracheal system, and the concept of a mismatch between metabolic demand and circulatory performance might not apply to them. Using thermo-limit respirometry, it has been shown earlier in Drosophila melanogaster that CO2 release rates at temperatures above the upper thermal limit (CTmax) exceed the rate at CTmax. The nature of this post-CTmax, or “post-mortal” peak, is unknown. Either its source is increased aerobic mitochondrial respiration (hyperthermic overdrive), or an anaerobic process such as liberation of stored CO2 from the hemolymph. The post-mortal peak of CO2 release was found to be oxygen dependent. As the rate of CO2 emission is a conservative indicator of rate of O2 consumption, aerobic flux at the thermal limit is submaximal, which contradicts the theory that oxygen availability limits metabolic activity at high temperatures in insects. Consequently, the tracheal system should be capable of delivering sufficient oxygen for aerobic activity of the mitochondria at and above Ctmax.
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