The effects of extended photoperiods on the drift of the mayfly ephemerella subvaria McDunnough (Ephemeroptera : Ephemerellidae)

Field drift studies indicated that the nocturnal drift density of E. subvaria nymphs was greater in early May than in early November.Laboratory studies showed that the number of individuals appearing in the drift was a linear function of the duration of the preceding photoperiod. Nymphs had a greater propensity to drift when they were not in a state of active growth than when they were growing. The tendency of individuals in a single laboratory population to drift was observed to change under conditions of constant temperature and randomized photoperiod. This suggests that the shift was due to some internal physiological change rather than to an external cue.It is suggested that drift in E. subvaria functions as a method relocation from fast-water areas to slow-water pools and stream margins. Redistribution to these areas may reduce mortality incurred during spring run-off and during emergence.