A MODEL FOR SEASONAL SYNCHRONY IN STREAM MAYFLIES

Many mayfly species have synchronous univoltine life histories over br oad geographic ranges, but the life-history adaptations underlying the ir seasonality remain unidentified. We investigated whether simple ada ptations in the response of development rate to temperature might acco unt for the observed phenology of adult emergence and the 1- and 2-yea r life histories of eight mayfly species studied in Piedmont streams o f eastern North America between 34-degrees-N and 50-degrees-N latitude . We present a model consisting of two sequential life-history stages. Development rate in each stage is a linear function of streamwater te mperature above a lower threshold but, in the first stage, a developme ntal quiescence occurs whenever a maximal temperature is exceeded. The five model parameters are assumed not to vary with latitude. Using da ily field temperatures, the model simulated several successive generat ions beginning from an arbitrary day of the year. With parameters fitt ed from the field data, the model could reproduce both the univoltine life history and the latitudinal variation in the timing of adult emer gence for six species. For two additional species, the model reproduce d a northward transition to a semivoltine life history. The simulation s suggest that nearly all development occurs in spring and autumn duri ng periods of roughly equivalent thermal regime at all latitudes.