Evolutionary changes in the seasonal timing of life-history events can alter a population's exposure to seasonally variable environmental factors. We illustrate this principle in Wyeomyia smithii by showing that: (1) geographic divergence in diapause timing reduces differences among populations in the thermal habitat experienced by nondiapause stages; and (2) the thermal habitat of the growing season is more divergent at high compared with low temperatures with respect to daily mean temperatures. Geographic variation in thermal reaction norms for development time was greater in a warm compared with a cool rearing treatment, mirroring the geographic trend in daily mean temperature. Geographic variation in body size was unrelated to geographic temperature variation, but was also unrelated to development time or fecundity. Our results suggest that proper interpretation of geographic trends may often require detailed knowledge of life-history timing.