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your acari belongs to the genus Eylais, but species name impossible without a closer study. There are many species of underwater acari (Hydrachnidia). In The Netherlands about 248 species. Your Ephemeroptera is indeed Cloeon, but be carefull with the name C. dipterum, for it may be a complex of species. Some authors believe that there they include C. intextum and C. praetextum, but others don't. On your photo however, I cannot even decide if it perhaps is C. simile, a true species.