Cataglyphis is a fairly homogeneous ant genus which is widespread over the arid regions of the Old World. All Cataglyphis species are thermal specialists which are adapted to extreme environments where they forage at nearly lethal temperatures. This study focusses on two Cataglyphis species which differ considerably in their physical caste systems. These species have developed two alternative mechanisms facing extreme heat. In C. velox, foraging at high surface temperatures is clearly dependent on size: large C. velox workers forage at midday and are able to withstand higher temperatures than small workers. On the other hand, C. rosenhaueri has not developed great physical specialization, but the workers of this species have achieved physiological (such as low cuticular transpiration and metabolic rate), and behavioural adaptations (such as raising their abdomen to protect the vital organs contained in it from high temperatures) to tolerate thermal stress. The result is that small C. rosenhaueri workers may withstand extreme heat conditions in a similar way to large C. velox workers, and much better than small C. velox workers. The different mechanisms used by these two species to withstand extreme heat could reflect fundamental patterns of independent evolution. In some situations, selection may act to promote a relatively narrow size range of adult workers, all of them able to withstand thermal extremes, while in others it may act by producing different worker sizes with different tolerance to environmental conditions.