A basic problem facing all desert organisms is the maintenance of an equable temperature without using an excessive amount of water for evaporate cooling. In smaller animals such as arthropods and most reptiles, whose surface to volume ratio is correspondingly high, this problem, always present on land, becomes even more acute (Cloudsley-Thompson 1988a). Ambient temperature is important to living organisms because many of the chemical, physiological and physical processes upon which life depends are temperature-dependent. Extremes of temperature - either too high or too low - can be lethal, and animals subjected to wide changes in the temperature of the environment must, if they are to survive, be able either to regulate or to control their body temperatures adequately. Small terrestrial animals exchange heat with the environment by conduction, convection, radiation and evaporation. In the case of most day-active arthropods and reptiles, heat is gained mainly by radiation and lost chiefly through convection (Willmer 1982). The obvious way of avoiding radiation is to keep in the shade.