A 9‐year study of the life cycle of Ephemera danica Müll. (Ephemeridae: Ephemeroptera) in the River Lambourn, England

Abstract. 1. The life cycle of the mayfly, Ephemera danica , was studied on two contrasted sites on the River Lambourn between 1971 and 1979. 2. Quantitative samples of nymphs were taken on the five major biotopes of the river bed, gravel, silt, Ranunculus, Berula and Callitriche . Exposed areas of gravel and silt held significantly lower densities of nymphs than the three macro‐phytes with their underlying substrata. The sandy substratum underlying beds of Berula frequently held significantly higher densities than the other macrophytes. 3. Monthly samples from March 1971 to April 1972 followed by samples in June and December from 1972 to 1979 indicated that the nymphal phase normally took 2 years in the River Lambourn. 4. All odd‐numbered years from 1971 to 1979 produced weak year‐classes. In contrast, all even‐numbered years between 1970 and 1978, with the exception of 1972, produced relatively strong year‐classes. 5. Meteorological data for the period of flight activity in E.danica indicated that conditions were colder and damper in 1972 than in the other even‐numbered years of the study, when recruitment was more successful.