We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30°N) to Ontario (49°N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30°N) and one northern (49°N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.