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 61 records from GBIF, adults of this species have mostly been collected during May (41%), April (28%), March (15%), and June (11%).
In 37 records from GBIF, this species has been collected at elevations ranging from 20 to 2559 ft, with an average (median) of 600 ft.
Time of day: Dusk
Current speed: Usually medium to fast, sometimes slow
Substrate: Many types, but gravel is preferred
Environmental tolerance: Unlike Ephemerella subvaria, invaria is very tolerant of a wide range of conditions.
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 species of the Ephemerella invaria group; genitalia much as in E. rotunda (now a synonym of E. invaria); venation pale.
Eyes of male probably orange, in life. Head and thorax light reddish brown; pronotum shaded with smoky. Mesosternum rather deeper reddish brown. Legs pale whitish. Wings hyaline; venation pale; stigmatic area opaque whitish. Dorsum of abdomen light smoky to purplish brown; basal tergites darker than the apical ones. Posterior margins rather dark smoky brown; pleural fold yellowish. Ventrally pale yellowish white, the middle sternites semi-translucent; apical sternites opaque, yellowish. Tails white; joinings purplish black, very distinct. Genitalia very similar to rotunda, but with somewhat fewer spines on the penes.
Described as E. fratercula
Body length 7 mm, wing length 8 mm
A species of the Ephemerella invaria group; numerous ventral spines in the apical area of the penes; venation wholly pale. Nymph unknown.
Eyes reddish brown. Head light yellow; vertex shaded with brown. Prothorax smoky brown. Mesonotum light yellow; pleura and sternum deeper yellow; brown shading at bases of wings. Legs light yellow; femur of fore leg deeper yellow than tibia and tarsus, which are slightly darker than the middle and hind legs. Wings hyaline; venation wholly hyaline; cross veins indistinct except in the stigmatic area. Dorsum of abdomen deep smoky brown on the basal and middle tergites, lighter yellowish brown apically; basal tergites deeper smoky than middle ones. Posterior margins narrowly blackish brown. Venter light yellow, the apical sternites opaque and slightly deeper yellow; ganglionic areas smoky brown; no reddish brown shading, as in Ephemerella subvaria. Forceps as in Ephemerella invaria; numerous ventral spines on the apical area of the penes (see fig. 152). Tails pale, the joinings narrowly dark brown.
Described as E. rotunda
Body length 10 mm, wing length 10 mm
A species of the Ephemerella invaria group; venation pale; spines on the penes more numerous than in E. invaria and Ephererella subvaria; nymph dark brown with many fine pale dots, the dorsal spines better developed than in invaria.
Eyes probably orange in life. Head yellowish, shaded with reddish on the occiput. Antennae pale. Thorax light reddish yellow; pronotum shaded with purplish black. A dark brown area on each side of the mesonotum and two narrow blackish submedian streaks anterior to the scutellum. A dark brown line above each of the middle and hind legs; pleural sutures narrowly brown. Legs pale whitish. Wings hyaline; venation very pale yellowish to hyaline; stigmatic area rather opaque white.
Abdominal tergites pale yellowish, each with a broad purplish brown transverse band across the middle. On tergites 1-2 and 8-9, this dark band occupies most of the sclerite. Pleural fold pale. Traces of a pale median line, most distinct on the middle tergites. Ventrally yellowish white, sometimes with faint indications of yellowish brown transverse bands across the middle of each sternite, similar to those on the tergites. Oblique submedian streaks at the anterior margin of each sternite; may be rather indistinct. Tails white, basal joinings distinctly purplish black; beyond the middle these are but faintly indicated. Spines on the penes rather more numerous than in invaria: two small clusters of ventral spines are present, which are not found in invaria. Two to four apical spines are usually present (see fig. 152).
Described as E. vernalis
Body length 9-9.5 mm, wing length 8-10 mm
A species of the Ephemerella invaria group; the second joint of the forceps enlarged distally; venation yellowish to yellowish brown; usually 7 spines only, on penes.
Head light brown; eyes light orange. Thorax brown; pronotum often with a median pale streak; pleura marked with pale yellow; mesonotal scutellum margined with dark reddish brown. Ventrally brown, darkest at the center of the mesosternum. Fore leg yellowish to light brown; purplish streaks on distal portion of femur. Middle and hind legs very similar. Venation very light yellowish brown, the main veins of the costal border yellowish basally. Stigmatic area opaque.
Abdominal tergites 2-7 light purplish brown with a narrow median whitish line; 2-4 washed with brownish, 5-7 paler; posterior margins dark brown, postero-lateral angles and intersegmental rings greyish white; a purplish line margins the pale pleural fold. Tergites 8-10 washed with yellowish brown; a median light brown line is present, and a brownish patch on each side; one or more short dark dots near each stigma. Sternites 1-5 greyish, lavender tinged; 6-9 yellowish brown; on each, a brown line margins the pleural fold, less distinct on 6-9. Posterior margins and ganglionic areas whitish. Tails greyish to light brown, joinings purplish brown. Penes with fewer spines than E. rotunda (now a synonym of Ephemerella invaria), usually only 7, occasionally 10; of these, two may be apical in position, but more often there are no apical spines (see fig. 152).
The nymph is smaller than rotunda (now a synonym for Ephemerella invaria); the dorsal abdominal spines are very poorly developed, and may be almost obsolescent. The color varies from pale yellowish brown heavily sprinkled with brown dots, to specimens which are almost wholly brown with yellowish brown dots. In the paler forms, the brown dots may be distributed rather uniformly, or may be arranged in bands across the anterior portion of the pronotum and between the roots of the wing pads on the mesonotum. Large pale lateral patches may be present on tergites 5 and 6; these patches may run together to form an almost wholly white dorsal area. The pale dots at the location of the dorsal spines are often present, as in E. rotunda.
Described as E. rotunda
The nymph is larger than Ephemerella invaria or Ephemerella subvaria; it is dark reddish brown in color, sprinkled with numerous very small pale dots. Each corner of the pronotum is pale. There are usually other smaller pale areas on each side of the pronotum, including a crescentic mark and two small round spots between this and the median line. Eight to ten somewhat larger pale areas are usually present on the mesonotum, between and anterior to the wing roots. Rather large pale lateral areas may be present on tergites 5 and 6; these may be connected along the anterior margin. Near the middle of each lateral extension is a dark bar. Venter somewhat paler than the dorsum; darkest on the apical sternites and the lateral portions of the middle ones. Traces of a pale median triangle at the anterior margin may be present on the middle and apical sternites. Tails yellowish brown; crossed by two or three dark bands near the middle, and another apically. White spots are present on each tergite, at the base of each dorsal spine. These spines are better developed than in invaria, but less prominent than in subvaria.
Described as E. vernalis
Nymphs resembling E. rotunda (now a synonym of Ephemerella invaria), but less robust. Vary in color from light yellowish brown to dark mahogany; body finely dotted and mottled with numerous small yellowish dots. Legs barred with yellow. A median yellowish dorsal streak is often present on the dorsum of the abdomen. Minute paired dorsal spines are present, and a whitish spot marks the base of each spine. Lateral extensions of the abdomen are more rounded on the outer margin than in rotunda; end spines not incurved; each lateral extension is crossed by a black streak. Indications of a yellowish mid-ventral line and a dark lateral line near the margin; posterior segments usually darker than the preceding ones.