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The role of the broadly polymorphic worker caste of the thermophilic ant Cataglyphis velox is discussed. Two different hypotheses have been tested: 1) worker polymorphism allows colonies to collect different-sized prey more quickly and with greater energetic efficiency; 2) it implies greater variability of responses to temperature, which increases the overall period of external activity, and enhances colony success. The relationship between worker polymorphism and size matching was not confirmed by the data: large C. velox workers retrieved neither proportionately nor absolutely heavier loads than did small foragers, they did not collect greater amounts of prey or with greater efficiency than smaller foragers, and they were only more effective than smaller ones in a reduced percentage of their foraging trips. On the other hand, thermal tolerance of C. velox workers was size-related: large workers withstood temperatures 6 to 8 degrees C higher than smaller ones. This led to a diurnal rhythm in which the mean size of workers foraging outside the nest al different times of the day increased with temperature. Small workers foraged preferentially when temperatures were lower, while large workers preferentially foraged at high temperatures. At the colony level, this resulted in a longer daily activity period, and an increased benefit in prey collection for C. velox colonies. From this point of view, large C. velox workers represent a specialized group which preferentially forages at temperatures that smaller workers cannot tolerate.