A seasonal survey of the abundances of three inquilines species in Sarracenia purpurea L. leaves was conducted by sampling 240 leaves from three pitcher plant populations on Beaver Island in Lake Michigan, United States. Leaf characteristics such as fluid pH, fluid volume, and keel length were quantified. The study focused primarily on the larvae of three dipteran inquiline species – the midge, Metriocnemus knabi Coquillet, the mosquito, Wyeomyia smithii (Coquillet), and the flesh fly, Fletcherimyia fletcheri Aldrich. Dipteran inquiline abundances varied with sampling season. Most pitchers contained one of the three studied inquilines species. Mosquito abundances were low relative to the other inquilines. In samples collected soon after new leaves open, a mixture of mosquito and midge larvae was characteristic. We related inquilines abundances to leaf parameters using generalized linear mixed models. The midge and mosquito responded differently to leaf parameters during different sampling seasons, where as the flesh fly showed no response. The most important leaf parameters were pH and percent of total leaf volume filled with fluid (relative volume). The midge and mosquito were more likely to be present in pitchers with low pH and higher fluid volume. Our results indicate that pitchers of isolated northern populations of S. purpurea are inhabited by a less diverse insect community than reported elsewhere. Our study also suggests that site isolation may play an important role in the seasonal variation of inquiline community structure.