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Summary
Ephemeroptera are not only a very important group of Insecta from the entomological point of view but also from the limnological standpoint. They still show many taxonomical difficulties, especially in the family Baetidae. The European species of the genus Baetist are elucidated in my revision (Müller-Liebenau 1970 (1969)). Since then I have received much material of Baetidae from nearly all the continents, so that the worldwide investigation of this most interesting family is feasible. The revision of the European species of the genus Baetis leads to a grouping of the 26 known species in 11 different groups of closely related species. A first survey of the worldwide material showed that many species can be arranged in the European species. Other species are more problematic, and further investigation will surely lead to a change of the present concept of genera within the family Baetidae. As an example, the degree of development of the hinawings in the imagines and the length of the terminal filament in the nymphs have been used as characters in establishing new genera. But as is shown in Figs. 1 and 2, in the well known genera Pseudocloeon and Acentrella and in still undescribed species the abovementioned characters are found in different combinations as well as combined with other morphological characters. They occur indepedently in species of the genus Baetis (worldwide) and likewise in different species belonging to different genera not yet discribed. Further investigations will show that many characters will be specific only in combination with others. Beside the morphological characters, physiological, anatomical and ecological characters will be of taxonomic importance when better known in the future.