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In many arthropods, female clutch size is positively correlated with female size but the benefits of increased size are presumed to be counterbalanced by the mortality risks incurred through delayed maturation. The orb-weaving spiders of the genus Nephila are characterized in part by large female size, and among species female size declines with increasing latitude, suggesting that one important mortality risk associated with delayed female maturation is the end of the growing season. The best-studied species inhabit strongly seasonal habitats. To explore the hypothesis that female size is limited by season length, I collected data concerning size-dependent growth and development rates, prey capture success, predation risk, and female fecundity from a population of N. maculata inhabiting a nearly aseasonal habitat. Comparisons of these data with observations from another N. maculata population and from congeners inhabiting more seasonal sites support the hypothesis that female size in this genus may be limited primarily by seasons length. Combined with recent phylogenetic evidence that male size is an ancestral characteristic, these results indicate that sexual size dimorphism in this group has evolved due to fitness advantages of delayed female maturation.