Puddles of rain water on the surfaces of rock exposures are a little known but very common habitat for freshwater-dwelling animals. In Africa, these are inhabited by the larvae of two taxa of fly unique to these pools. One of these includes species able to survive dry periods in situ; the other includes species that must reach adulthood and migrate to survive periods when the pool is dry. Hence, the opportunity exists for a comparative study of adaptation among these species. Since puddles are small, our principal method in the study of adaptation has been the experimental manipulation of puddles and their faunas in the wild. Using this method we were able to identify the spatial consistency of pools and their unpredictable duration during the rainy season as the main selective pressure shaping adaptation. Adaptations include diapause and adaptive adjustment of the life cycle. It is the second of these that provides the focus of our interest here.