Individual growth rates and timing of emergence of four species of Ephemerellid mayflies were examined in relation to temperature among six sites on the Au Sable River near Grayling, Michigan. All species showed definite reductions in specific growth rates at lower temperatures, but incremental growth in mean length was continuous until emergence. These results indicate the degree-day concept is a useful tool in relating insect growth to thermal regime at intermediate temperatures, but is less accurate at temperatures approaching upper and lower development thresholds. Mean larval size and growth rates varied unpredictably among taxa and across sites, and could not be explained by variations in temperature at different sites. Other habitat variables not estimated in this study clearly influenced patterns in growth and timing of emergence of these species.
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