Life Cycle, Energy Fluctuations and Sexual Differentiation in Ephemera danica (Ephemeroptera), a Stream-Living Mayfly

Larval growth of Ephemera danica Muller in a South Swedish stream was calculated from changes in the size distribution of the larvae between monthly samples. Part of the population completed a generation in two years, whereas the rest of the individuals spent three years as immatures. Adult and larval sex ratios equalled 3:2 with an excess of males. Stepwise development of external male characters accompanied by decreasing relative energy content of the nymphs took place in two periods (late spring and autumn). It is suggested that such a development of adult characters provides a mechanism allowing nymphs of different age to mature and emerge synchronously.