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Abstract

Summary 1. Continuous catches of drifting larvae during two consecutive development cycles of Isoperla goertzi have been analysed. a) Each year there have been caught 1000-2000 larvae drifting in a distance of 600 mtrs. from the spring. The yearly drift-rate must be essentially higher because the sieves used for catching covered only 1/5 to 1/10 of the stream's width. b) The activity of drifting shows a regular annual and diurnal periodicity. c) This activity and mean growth of the larvae as well are influenced by the water-temperature. 2. Influence of light on the diurnal periodicity has been tested under permanent light and obscuration conditions. a)Permanent light delays the drift. Consequently no periodicity could be recognized. b)During permanent obscuration the diurnal periodicity remains. However the main maximum of drifting Isoperla occurs 2 hours earlier.