Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
In 25 records from GBIF, adults of this species have mostly been collected during May (44%), March (24%), April (16%), and June (12%).
In 29 records from GBIF, this species has been collected at elevations ranging from 381 to 11453 ft, with an average (median) of 5351 ft.
Time of day: Morning and again at dusk in the West; only dusk in the East, where they're not important
Habitat: See notes
Diet: Detritus and algae
Current speed: Slow to fast in the West; medium to fast in the East
Substrate: All types, but prefer gravel and cobble with weed growth or the edges of weed beds in spring creeks
Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.
A member of the Ephemerella invaria group; second joint of forceps not swollen apically. Eyes (in living male) egg-yellow. Vertex of head, and antennae, reddish brown. Dorsum of thorax reddish brown. Legs pale yellow; first tarsal joint of fore leg, all tarsal joinings of fore leg, and tips of all the tarsi, cloudy. Wings hyaline; venation wholly hyaline; a slight yellowish tinge on the costa of the fore wing. Dorsum of abdomen reddish brown, sometimes almost reddish black. Tails whitish, the joinings brown except near the tips. Second joint of forceps little if any swollen apically; no spines at the apex of the penes.
Described as E. semiflava
Body length 7 mm, wing length 8 mm
A species of the Ephemerella invaria group; second joint of forceps not swollen apically; nymph unknown.
Eyes of living male mustard yellow. Head yellowish, tinged with reddish brown near the ocelli. Thorax olive brown shaded with yellow; light yellowish brown near the scutellum; reddish brown shading anterior to the wing roots. Blackish transverse streak near the posterior margin of the prothorax; pleura and sternum light yellowish brown; an oblique stripe of pure yellow on the pleura from the base of the fore wing to the base of the fore leg. Legs light yellow, becoming whitish on the fore tarsi. Fore tibia about one and one-half times as long as the femur; tarsus about twice the length of the tibia. Wings hyaline; venation hyaline.
Abdomen yellowish olive brown; lateral rows of indistinct oblique brown stripes, more or less fused on tergites 8-10. Tergites 2 and 3 faintly blackish on the posterior margins. Tails white, with faint brownish joinings at the base. Genitalia very similar to Ephemerella excrucians. It is quite possible that this species may be synonymous with excrucians.
The nymph is rather dark reddish brown in color; the head and thorax are irregularly mottled with light and dark areas, which are sometimes rather extensive on the pronotum. The lateral margins of the pronotum are pale. Legs blackish brown, the femora with three large pale areas; tibiae pale at apex and near the base; tarsi pale distally. Spines on the joints are very similar to E. mollitia (now a synonym of Ephemerella dorothea infrequens). Lateral extensions of the abdominal segments very well developed. Dorsal spines absent. A rather large pale spot on each side of the median line, on the posterior margin of each tergite. On the middle tergites the median line is often pale, sometimes wholly whitish. Rather extensive pale areas are often present just above the gills on these tergites, most pronounced on tergites 5 and 6. These markings are variable in different specimens. Tails yellowish brown, with several median and apical dark bands. Ventrally yellowish brown, the apical sternites and the lateral margins of the middle ones blackish brown. Traces of dark submedian oblique dashes on the anterior margins, and of dark dots nearer the center of the sternites, may be present.