Cold Heat Protection

David L. Denlinger

Ohio State University

As poikilotherms, insects are largely at the mercy of environmental temperatures. There are a few exceptions: some moths, bees, and flies do have the capacity to elevate their body temperature by vigorously contracting their flight muscles to generate heat, and a few species of cicadas, grasshoppers, and other desert species exploit evaporative cooling to lower their body temperature on hot days. The majority of species, however, have a limited capacity to alter their body temperature. Insects survive, perform, and reproduce across a broad temperature range, but they do so with varying levels of success at different temperatures. A thermal performance curve (Fig. 1) can be generated for nearly any quantitative trait. The curve delimits the body temperature at which a certain activity can be performed (tolerance zone). The low extreme is the critical thermal minimum, and the upper extreme is the critical thermal maximum.

Construction of such a curve will demonstrate that any activity has a temperature at which performance is optimal (optimum body temperature). Characteristically, the drop in performance outside the optimum body temperature is more precipitous at the high end of the temperature scale than at the low end. When given a choice, insects will readily select temperatures at which performance is maximized. But, the extremes of the daily temperature cycle and the dominant temperatures that prevail during major portions of the year pose significant obstacles for insect growth, development, and performance. This review describes the nature of the injury inflicted by high and low temperatures and discusses the protective mechanisms used by insects to counter these forms of injury.

Temperature

FIGURE 1 Hypothetical performance curve delineating the tolerance zone, critical thermal minimum temperature, optimum temperature, and critical thermal maximum temperature at which any quantitative trait can be performed. Note that the decline in performance above the optimal temperature is usually more precipitous at the high end of the temperature scale than at the low end.

Temperature

FIGURE 1 Hypothetical performance curve delineating the tolerance zone, critical thermal minimum temperature, optimum temperature, and critical thermal maximum temperature at which any quantitative trait can be performed. Note that the decline in performance above the optimal temperature is usually more precipitous at the high end of the temperature scale than at the low end.

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