While separating thermal tolerances and regulation, water balance, metabolism, and growth and differentiation, is convenient, it in no way reflects reality. Insects must solve several environmental problems simultaneously (Park 1962; Scriber 2002). At any given time, they have to obtain sufficient resources for development and/or reproduction, avoid succumbing before they have reproduced, and cope with much environmental variation. For example, during dormancy (in an insect from seasonally dry subtropical South Africa) dehydration is likely to be prolonged and access to water and energy resources extremely limited. Therefore, there is likely to be strong selection for low metabolic rate to conserve energy resources, and for any mechanisms that might reduce water loss. Many arthropods also spend this time below ground, where conditions are at least mildly hypoxic and hypercapnic, so there is also likely to be selection for improved gas exchange capabilities (Chown 2002). In consequence, a particular suite of characteristics, such as discontinuous gas exchange and low metabolic rate in this instance, might have evolved in response to more than a single variable. In this example, the need to conserve water, reduce internal resource depletion, and exchange gases efficiently, are all likely to have promoted discontinuous gas exchange. Environmental covariation (Loehle 1998; Hoffmann et al. 2003b) is also likely to promote cross-resistance, and to favour the co-option of mechanisms promoting survival of one set of circumstances to a host of other tasks. Unsurprisingly, environmental covariation also complicates adaptive inferences based on comparative studies (Davis et al. 2000).
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