Diel periodicity of emerging mayflies (insecta: Ephemeroptera) in a laurentian stream

The diel periodicity of mayflies was investigated in a Laurentian stream in 1966. The peak period of emergence of mayflies was found to be between 13:00 and 16:00 h (Eastern Standard Time) during the first month of the season and the emergence was more dispersed during the rest of the summer due to the appearance of species emerging around sunset. Most species showed a definite diel pattern of emergence. Males and females of a given species emerged at the same time of the day in most cases, the emergence being perhaps a little more synchronous in females. They exhibited the same general pattern of diel periodicity throughout their seasonal emergence. In males of Paraleptophlebia adoptiva and Leptophlebia cupida however, the peak period moved to an earlier time of day as the emergence season progressed and Baetis pygmaeus emerged in a slightly more dispersed fashion in mid‐summer when days were longer and warmer. Notes on the diel periodicity of the reproduction are included and the possible factors determining the diel periodicity of emergence are briefly discussed.