Increasing temperature directly affects the developmental rates of ectotherms, accelerating sexual maturity and leading to smaller adult body size. Since weapon size has a positive allometric relationship with body size, high temperature may negatively influence weaponry exaggeration. However, temperature also influences productivity and ultimately food availability. High food availability may promote larger bodies and weapons because these traits are often condition-dependent, especially in males. Thus, the indirect effect of temperature (via food availability) on weapon size should be positive. Here we contrasted predictions about direct and indirect effects of temperature on the degree of weapon exaggeration in the earwig Labidura xanthopus. Our dataset comprises 21 populations along a 30° latitudinal range of the eastern coast of South America. We found strong direct negative effects of temperature on mean body and forceps length, but variation in these traits is better explained by indirect effects related to food availability. We also compared the degree of condition dependence of forceps length between two populations in the extremes of the latitudinal gradient. In the laboratory, we tested whether weapon size is more sensitive to food availability in males than females and whether this is more evident in the population from colder and more variable climate (south). The results support both predictions, indicating predictable interpopulation variation in the degree of condition dependence of weaponry exaggeration. We discuss our findings in a broad macroecological perspective, stressing the importance of physiological and ecological information to make solid predictions on the patterns of geographical variation of sexually selected traits.