In an attempt to define environmental heterogeneity and uncertainty in a meaningful manner, thermal and hydric aspects of the microenvironment of the pitcher plant mosquito (Wyeomyia smithii) were studied. Mechanistic mass and energy balance models were developed to predict surface temperatures in a Sphagnum bog and temperatures and water losses in Sarracenia purpurea pitchers. Field tests indicate that the models predicted surface and pitcher temperatures within 2-3 C, and hourly and daily water losses from pitchers to within 30%, using only basic meterological data as inputs. Experiments and observations in a natural population of W. smithii in northern Michigan, USA revealed significant differences in larval developmental rate, voltinism, and larval mortality due to microclimatic effects. The model predicts larval developmental rates and voltinism for each microclimate within 10%. Water loss simulations predict, and field observations confirm, that pitcher desiccation is a function of microclimate and pitcher size, and that rainfall patterns on the order of 5-30 d determine desiccation patterns. Identification of the spatial and temporal scale of both the environment and the organismic (population) phenomena in question is crucial to constructing a meaningful definition of environmental heterogeneity. Thermal and hydric components of environmental variation may play an important role in the maintenance of fitness variation in W. smithii. These results support the hypothesis of Istock (1978) that environmental uncertainty favors mixed life history strategies in Wyeomyia.