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A comparative phylogenetic approach was used to test the following adaptive hypotheses pertaining to the physiological abilities of the Namib desert tenebrionid beetle genus Onymacris to withstand the hot, dry desert environment: (1) Desert-interior species evolved longer legs (relative to body size) than beetles in the cooler coastal region to facilitate stilting, i.e., elevating their bodies our of the hot boundary layer of air close to the substrate. (2) Wax blooms on the exoskeleton, which reduce evaporative water loss, are more likely to evolve in desert-interior species than in coastal species. (3) The high costs of activity in the extreme climates select for perfect coadaptation of preferred body temperatures (i.e., optimal temperatures for activity) and those they achieve in the field. All three of these adaptive hypotheses were supported by the results of squared-change parsimony and independent-contrasts analyses. Additionally, a parsimony approach suggested that a novel means of obtaining water from periodic fogs, known as fog basking, has evolved independently on two occasions.