A full-factorial study of the effects of rates of temperature change and start temperatures was undertaken for both upper and lower critical thermal limits (CTLs) using the tsetse fly, Glossina pallidipes. Results show that rates of temperature change and start temperatures have highly significant effects on CTLs, although the duration of the experiment also has a major effect. Contrary to a widely held expectation, slower rates of temperature change (i.e. longer experimental duration) resulted in poorer thermal tolerance at both high and low temperatures. Thus, across treatments, a negative relationship existed between duration and upper CTL while a positive relationship existed between duration and lower CTL. Most importantly, for predicting tsetse distribution, G. pallidipes suffer loss of function at less severe temperatures under the most ecologically relevant experimental conditions for upper (0.06 degrees C min(-1); 35 degrees C start temperature) and lower CTL (0.06 degrees C min(-1); 24 degrees C start temperature). This suggests that the functional thermal range of G. pallidipes in the wild may be much narrower than previously suspected, approximately 20-40 degrees C, and highlights their sensitivity to even moderate temperature variation. These effects are explained by limited plasticity of CTLs in this species over short time scales. The results of the present study have broad implications for understanding temperature tolerance in these and other terrestrial arthropods.

Physiologists have long appreciated that environmental conditions and their variability have an influence on phenotypic plasticity (see Section 4). It is widely thought that acclimatization is more likely in species from temperate than those from less variable tropical and polar environments (Spicer and Gaston, 1999; Ghalambor et al., 2006), and less likely in stenothermal (narrow temperature tolerance) species (Somero et al., 1996; Pörtner et al., 2000), although tropical species might be more eurythermal (wide temperature tolerance) than their polar counterparts (Somero, 2005). More generally, the environmental circumstances under which adaptive population differentiation, phenotypic plasticity, or some combination thereof arise form the subject of a large and growing theoretical field (e.g. West-Eberhard, 2003; Berrigan and Scheiner, 2004; Pigliucci, 2005). Somewhat surprisingly, this field and work examining the evolution of thermal physiology remain reasonably distinct (though see Lynch and Gabriel, 1987; Gilchrist, 1995), even though the physiological models often struggle to explain the high frequency of eurythermic strategies (see reviews in Angilletta et al., 2002, 2003, 2006). Hence, we focus on the former plasticity models, noting parallels with the thermal physiology models where appropriate. Many investigations have shown that greater environmental variability tends to favour phenotypic plasticity within populations, as long as cue reliability and accuracy of the response (which is a function of environmental lability and unpredictability, and of the extent to which the response lags behind the environmental change) is high, and the cost of plasticity is low (Lively, 1986; Moran, 1992; Scheiner, 1993; Tufto, 2000). This conclusion holds for both optimality and quantitative genetic (environmental threshold) models (Hazel et al., 2004). Recent modelling work has also shown that the likelihood of this outcome is affected strongly by migration between different populations (Tufto, 2000; Sultan and Spencer, 2002). With little or no migration, and different environments, adaptive differentiation between populations in each of these environments readily evolves. Increases in migration rate, by contrast, lead to fixation of the plastic phenotype even though it might not be the best type anywhere (i.e. relative to adaptively differentiated habitat specialists) (Tufto, 2000; Sultan and Spencer, 2002). Nonetheless, if response accuracy is low (i.e. no better than random for at least one environmental state), the specialist phenotype is favoured, and the same is likely to be true if the global cost of plasticity is high (though evidence for the latter is scarce) (Van Tienderen 1991, 1997; Moran 1992; Sultan and Spencer, 2002, but see also Relyea 2002; van Kleunen and Fischer, 2005). In addition, environmental-threshold models show that with low cue reliability and low frequency of benign patches, a reversed (counter-intuitive) conditional, but unstable, strategy is favoured (Hazel et al. 2004).

Summary 1.Biologists have long been concerned with measuring thermal performance curves and limits because of their significance to fitness. Basic experimental design may have a marked effect on the outcome of such measurements, and this is true especially of the experimental rates of temperature change used during assessments of critical thermal limits to activity. To date, the focus of work has almost exclusively been on the effects of rate variation on mean values of the critical limits. 2. If the rate of temperature change used in an experimental trial affects not only the trait mean but also its variance, estimates of heritable variation would also be profoundly affected. Moreover, if the outcomes of acclimation are likewise affected by methodological approach, assessment of beneficial acclimation and other hypotheses might also be compromised. 3. In this article, we determined whether this is the case for critical thermal limits using a population of the model species Drosophila melanogaster and the invasive ant species Linepithema humile . 4. We found that effects of the different rates of temperature change are variable among traits and species. However, in general, different rates of temperature change resulted in different phenotypic variances and different estimates of heritability, presuming that genetic variance remains constant. We also found that different rates resulted in different conclusions regarding the responses of the species to acclimation, especially in the case of L. humile . 5. Although it seems premature to dismiss past generalities concerning interspecific and acclimationrelated variation in critical thermal limits, we recommend that conditions during trials be appropriately selected, carefully reported and rigorously controlled.

Plastic responses figure prominently in discussions on insect adaptation to climate change. Here we review the different types of plastic responses and whether they contribute much to adaptation. Under climate change, plastic responses involving diapause are often critical for population persistence, but key diapause responses under dry and hot conditions remain poorly understood. Climate variability can impose large fitness costs on insects showing diapause and other life cycle responses, threatening population persistence. In response to stressful climatic conditions, insects also undergo ontogenetic changes including hardening and acclimation. Environmental conditions experienced across developmental stages or by prior generations can influence hardening and acclimation, although evidence for the latter remains weak. Costs and constraints influence patterns of plasticity across insect clades, but they are poorly understood within field contexts. Plastic responses and their evolution should be considered when predicting vulnerability to climate change-but meaningful empirical data lag behind theory.

Temperature has dramatic evolutionary fitness consequences and is therefore a major factor determining the geographic distribution and abundance of ectotherms. However, the role that age might have on insect thermal tolerance is often overlooked in studies of behaviour, ecology, physiology and evolutionary biology. Here, we review the evidence for ontogenetic and ageing effects on traits of high-and low-temperature tolerance in insects and show that these effects are typically pronounced for most taxa in which data are available. We therefore argue that basal thermal tolerance and acclimation responses (i.e. phenotypic plasticity) are strongly influenced by age and/or ontogeny and may confound studies of temperature responses if unaccounted for. We outline three alternative hypotheses which can be distinguished to propose why development affects thermal tolerance in insects. At present no studies have been undertaken to directly address these options. The implications of these age-related changes in thermal biology are discussed and, most significantly, suggest that the temperature tolerance of insects should be defined within the age-demographics of a particular population or species. Although we conclude that age is a source of variation that should be carefully controlled for in thermal biology, we also suggest that it can be used as a valuable tool for testing evolutionary theories of ageing and the cellular and genetic basis of thermal tolerance.