Lethal temperature limits

There is in most animals which have been subjected to experimentation with temperature, a range of several degrees in which the animal is not markedly stimulated ...As the temperature is raised or lowered from such a condition, the animal is stimulated.

Shelford (1911)

Temperature has an effect on most physiological processes. The ways in which insects modify their response to temperature effects, over the range of temperatures at which they are generally active, are known as capacity adaptations. Responses to what otherwise might be lethal effects of temperature extremes have been termed resistance adaptations (for further discussion see Cossins and Bowler 1987). In both cases the term 'adaptation' is something of a misnomer because the responses are often a consequence of phenotypic plasticity or flexibility, or because they might not be adaptive at all. In addition, these responses are often intimately related and cannot be clearly separated. For example, in marine species, and arguably also in several terrestrial groups, lethal temperature limits (typically considered resistance traits) result from insufficient aerobic capacity of mitochondria at low temperatures, and a mismatch between excessive oxygen demand by mitochondria and insufficient oxygen uptake and distribution by ventilation and circulation at high temperatures. These are typically traits associated with capacity (Portner 2001). Nonetheless, distinguishing between resistance and capacity responses is often convenient, and both kinds of responses are characteristic of insects. Here, we deal with those responses that occur when potentially lethal temperature extremes are encountered, leaving what are more typically considered 'capacity' responses to the relevant sections in other chapters.

A convenient way to think about the responses of insects to potentially lethal temperatures is the thermobiological scale presented by Vannier (1994) (Fig. 5.1). At both ends of the scale, continuation of the change in temperature results first in knockdown of the insect (stupor in Vannier's terms), then in prolonged coma, and finally in irreversible trauma and death. At low temperatures, insects show a wider variety of responses to sublethal and potentially lethal temperatures than they do at high temperatures, including responses to non-freezing temperatures and those made in preparation for the decline of temperatures below the freezing point of water (Fig. 5.2). It is the ways in which insects alter the relationship between the temperatures they are experiencing and their survival probability, the costs of these changes, and the similarities and differences between the responses to upper and lower lethal temperatures (LLTs) that form the substance of this chapter. How insects alter the temperatures they experience (i.e. thermoregulate) is dealt with in Chapter 6.

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