Thermoregulation

These tropical beetles are unable to sustain the body in flight at the temperature of an English

Our treatment of the thermal physiology of insects the extent of thermal acclimation is often rather can broadly be divided into 'resistance adapta- modest, one reason being that thermoregulation tions', which concern the lethal effects of extreme alters the relationship between environmental and temperatures and are the subject of the previous body temperatures (Kingsolver and Huey 1998). chapter, and 'capacity adaptations', which concern The thermal sensitivity of performance in ecto-the effects of ambient temperature on physiological therms, especially locomotor performance, has performance (Prosser 1986; Cossins and Bowler become an important research focus in evolutionary 1987). Figure 6.1 illustrates how the relationship physiology, and flight performance and thermo-between body temperature (Tb) and performance regulation of insects are inextricably linked. Adein ectothermic organisms is bounded by their quate power output for flight is usually available critical thermal limits. Maximal performance occurs for only part of the temperature range experienced at an optimal body temperature, and the thermal by insects (Josephson 1981), but thermoregulation performance breadth is the range of Tb permitting a prolongs flight activity in suboptimal thermal certain level of performance (Huey and Stevenson environments and enhances flight performance in 1979). Performance curves may be shifted by more favourable conditions.

acclimation, changing in position or shape, but As with studies of insect flight (Dudley 2000), laboratory.

Machin et al. (1962)

Figure 6.1 Relationship between body temperature and performance in ectotherms, showing the optimum temperature for performance (70), the 80% performance breadth (B80),

Source : Huey and Stevenson (1979).

Heterotherms

Figure 6.1 Relationship between body temperature and performance in ectotherms, showing the optimum temperature for performance (70), the 80% performance breadth (B80),

Body temperature

Body temperature max anthropomorphic bias and technical limitations have focused attention on large but unrepresentative insects, most of them heterotherms. There are numerous examples of impressive endothermic abilities being recorded for large tropical insects in the absence of information on their natural history, and thus the ecological significance of the endo-thermy (e.g. Morgan and Bartholomew 1982). The mechanisms involved in insect thermoregulation were comprehensively reviewed by Heinrich (1993), using a taxon-based approach. His conclusion that research in the field had already reached the stage of redundancy may well have been true for the mechanistic details, but in recent years the research approach has broadened considerably into the thermal ecology of insects. In consequence, we emphasize the ecological and evolutionary implications of thermoregulation. Small insects are the majority: the average adult insect body length is around 4-5 mm (Dudley 2000). It follows that most insect species (and, of course, all larval stages) are ectotherms. However, ectothermy does not preclude sophisticated thermoregulation.

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