Homeostasis and Polymorphism in Vernal Development of Chaoborus Americanus

Last instar larvae of Chaoborus americanus overwinter in a state of developmental arrest (diapause) but remain otherwise active and capable of feeding. The diapausing larvae are developmentally responsive to long—day photoperiod and food. A portion of a wild—caught population responds rapidly to these conditions; the remainder of the population is more conservative. For either morph, the closer an individual approaches adult emergence, the more its rate of development is retarded by a small drop in water temperature. This response to temperature both promotes synchronous adult emergence and insures favorable climatic conditions at that time. Even this individual thermal homeostasis does not ensure survivorship throughout the total range of vernal climates encountered in a single shallow pond. The fast—responding morph predominates following warm, continuous springs; the more conservative morph predominates following a spring where the initial thaw is succeeded by a refreezing of the pond. Response time polymorphism is thus envisioned as an adaptive extension of the individual thermal homeostasis and serves as a model for persistent polymorphism.