Abstract
1. The threshold value of the absolute change in light intensity ΔI necessary to evoke a phototactic reaction in Daphnia magnaStraus was estimated at various adaptation intensities between 3.4×100 and 3.4×104 erg cm−2sec−1. 2. The results point at the existence of two distinct functions: for I < 40 erg cm−2 sec−1: ΔI = 0.21I0.49; I ≧ 40 erg cm−2 sec−1: ΔI= 0.03I1.06. 3. Starting at 40 erg cm–2 sec−1 the threshold of the relative change in light intensity increases for lower as well as for higher adaptation intensities. 4. Neither Weber's Law nor Hecht's exponential function (Hecht, 1934) seem to cover the results sufficiently. These are, however, highly comparable with those obtained by Heberdey and Kupka (1942) working with Daphnia pulexDe Geer. Our interpretation differs from theirs, however. We think of two different phototactic mechanisms. 5. The significance of the results for diurnal vertical migration of Zooplankton is pointed out.
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