JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Caste theory states that worker size distributions in ant colonies have evolved for efficient division of labor and predicts that they should vary with habitat and have an effect on fitness. We tested these predictions in a comparative study of worker size variation and colony fitness in Florida and Long Island populations of Trachymyrmex septentrionalis. Colonies from the two populations were excavated shortly before the time of the mating flights, so that workers and reproductive forms could be censused. We measured worker head widths in each colony and analyzed the resulting size frequency distributions using the mean, standard deviation, skewness, and kurtosis, and also principal components derived from these parameters. Size frequency distributions were unimodal and close to normal, with a tendency to negative skewness. Colonies in the Long Island population had both larger workers and greater size variation than those in Florida. Florida colonies followed a growth trajectory in which the mean and standard deviation of worker head width both increased with colony size, whereas in Long Island there was an inverse relationship between the mean and standard deviation that was independent of colony size. Analysis based on the principal components showed that worker size variation significantly affected fitness in both populations, and this was not a secondary effect of colony size. Population differences in colony life history strategies may account for the observed patterns: the Long Island population appears to be adapted for surviving the winter diapause, while the Florida population has experienced different selective pressures, possibly for rapid colony growth. Size variation that has evolved in the context of life history strategies may serve as a preadaptation for division of labor.