Baetidae, the most widespread ephemeral and with the largest number of species. Fly-fishers know that many of them can be represented by flies gathered in the classic four groups, Pale Olive, Medium Olive, Dark Olive and Iron Blue. The size in spring is larger than in summer and autumn. During intense hatch it often happens that the fish only preys on emerging nymphs or cripple or stillborn. Even the nymph should be interpreted, in colors and sizes, then cast in perfect drift, sometimes made to move and others absolutely not.