Critical Role of Body Temperature in Insect Biology and Development

Temperature directly affects virtually all biological rate processes in insects (Logan et al., 1976; Wagner, 1984a, 1984b; Chappell and Whitman, 1990; Stamp and Casey, 1993; Lactin et al., 1995; Lactin and Johnson, 1996b, 1998a, 1998b), and population responses may vary dramatically in response to climate change (Coxwell and Bock, 1995). Shifts may favor increased food consumption and digestion, more rapid development and faster growth rates, increased survival, or higher fecundity (Huey and Kingsolver, 1989; Kingsolver, 1989; Law-ton, 1991; Yang and Joern, 1994b; Harrison and Fewell, 1995; Lactin et al., 1995; Woods and Kingsolver, 1999; Peterson et al., 2000). It is key to remember that the small size of insects makes it more important to recognize microclimate rather than macroclimate when assessing responses to global climate change (Casey, 1993; Lactin and Johnson, 1998b). Operative thermal environments relevant for insects may vary in ways that are not obvious from general descriptions of local weather conditions. For example, what is the effect of a 2°C increase at the global level to an insect living within the boundary layer (~5 mm) of a corn leaf?

Insects are ectotherms of generally small size, and must rely on external heat sources and sinks to control body temperature (Tb). If they cannot thermoregulate, Tb equals the temperature of the surrounding environment. Some insects can exert significant control through physiological and biochemical means, but with energetic costs (Heinrich, 1993). More often, however, insects control Tb using a variety of anatomical and behavioral means called thermoregulation (Casey, 1981, 1988; Chappell and Whitman, 1990; Casey, 1992; Lactin and Johnson, 1996a, 1996b, 1997, 1998a, 1998b) to keep Tb around 38°C for as much of the time as possible. These include manipulating body temperatures using incoming solar radiation and through microhabitat selection (Anderson et al., 1979; Chappell and Whitman, 1990; Aarssen, 1992; Casey, 1993). The quantity of solar radiation absorbed is relatively independent of body temperature, but strongly linked to absorptivity by the animal's surface, typically on the order of 70% to 75% for grasshoppers (Porter, 1969; Anderson et al., 1979; Wilmer, 1981). By manipulating the surface area and portion of the body that is exposed to the sun's rays coupled with judicious use of shade or microhabitats at varying heights above the ground (with and without wind), body temperatures can be significantly regulated within narrow limits (Casey, 1988, 1992). These relationships can now be predicted in the field (Lactin and Johnson, 1996). Because insects have so much control over body temperatures, and microclimates have so much impact, broad-scale predictions about the impact of temperature changes must be tempered and scaled to this much finer level.

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