Microdistribution and Density as Factors Affecting the Downstream Drift of Mayfiles

During high volume of flow in a stream that has considerable seasonal fluctuation, the microdistribution of five species of mayflies was determined by displacement of individuals by drift from rapid current areas to those with gentle or no current. The major effect of drift was dispersal, not depletion, of the mayfly population. Occurrence in drift is determined by a species—specific complex of interdependent factors including life cycle, microdistribution (both before and after the effect of spates), and the behavioral characteristic of individual species.