Members of this genus are found in great abundance and occupy a wide variety of habitats. Ephemerella subvaria starts the season in April, with other species hatching continuously until October. Due to their manner of emergence, most species are extremely vulnerable to the trout. At hatch time, the nymphs leave the bottom and rise to the surface. Normally, the subimagos emerge from the nymphal skin several inches below the film, making the emerging pattern a very deadly imitation. Once on the surface, the floating duns often require thirty to sixty seconds to get airborne. Members of this genus are similar in outline and appearance, and once familiar with them, they can be recognized at a glance. The various species of Ephemerella are sometimes similar, but knowledge of the minor differences can be valuable to the angler.
Ephemerella nymphs vary widely as to body shape and form. Some are fairly slender and streamlined, while others have flattened bodies and legs. Some species are smooth, some have conspicuous projections or spines, and others are covered with fine hairs. Dorsal platelike gills are present on segments 3 to 7 in many species, but on 4 to 7 in a few others. In the latter situation, the gills of segment 4 are sometimes enlarged and lidlike, concealing most or all of the other gills. Nymphs have three equal tails that are normally about the length of the body and are often hairy. Mayflies of this genus vary greatly as to size, ranging from 4 to 15 mm in body length. The subimagos are quite variable in coloration and markings, but most imagos display shades of brown or olive. Eyes of the female are small and located on the sides of the head, while those of the male are much larger, normally meeting on top of the head. All tarsi are four-jointed, except in the foreleg of the male, which has 5 segments. Three tails of approximately equal length are retained in the adult stage.