D55 Measurement of metabolic rates

Topic

Measuring the metabolic rates of this specific insect—the very one serving as a model in this context—combines gas respirometry with both direct and indirect calorimetry to quantify the insect's final energy production across its various states: at rest, in flight, or during peak activity.

The insect's discontinuous-flow respirometry is particularly informative: at rest, the insect periodically opens its spiracles in a distinct burst, followed by a phase where the spiracles either close or flutter. While this discontinuous pattern reduces water loss, it complicates continuous measurement; the flow and analysis system must therefore have sufficient temporal resolution to capture each of the insect's bursts. A finer temporal resolution in the system results in a more accurate capture of each burst.

Kleiber's allometric relationship—another model relevant here—predicts that a significantly heavier insect has a higher metabolic rate, though the rate increases less than the body mass does; consequently, a small insect is metabolically more active per unit of mass than a significantly heavier one. The fact that this insect has an even smaller body mass means that, according to Kleiber's allometric relationship, its metabolic rate per unit of mass is consequently even higher.

Conversely, the insect's metabolic thermal coefficient indicates that a significant rise in its body temperature doubles or triples its metabolic rate; this has a direct impact on both its heat vulnerability and the temperature-induced acceleration of its metabolism. A further increase in the insect's temperature causes its metabolic rate to accelerate even more, in accordance with this thermal coefficient.

Therefore, this model—based on the energy production of an insect at rest, in flight, or during peak activity—determines the insect's flight power by the difference between its metabolic rate during flight (measured in a flight chamber or during instrumented free flight) and its basal metabolic rate. The more the insect's metabolic rate during flight exceeds its basal metabolic rate, the greater its flight power becomes.

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