Abstract
the general conditions obtaining in bodies of water, even lakes (Macan 1970). The definition of these habitats has been based extensively on single collections. The findings of a long continuous study of one place now provide information on which the validity of this step can be examined. There are three other objectives. The collections in Hodson's Tarn were started with a view to discovering the effect of predation by Salmo trutta L. on the community, and continued for comparison with catches on artificial substrata. The outcome of the first experiment was described as soon as predation had been reduced to a low level (Macan 1965a, b, 1966). Confirmation of the conclusions is now provided by the subsequent observations. The work with artificial substrata adds to knowledge about the habits and habitats of certain species. Finally changes in the vegetation and in the corixid fauna during the twenty-one years can be related.
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