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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

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.

Lateral view of a Clostoeca disjuncta (Limnephilidae) (Northern Caddisfly) Caddisfly Larva from the Yakima River in Washington
This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Mayfly Species Epeorus hesperus

Where & when

In 3 records from GBIF, adults of this species have been collected during August (100%).

In 1 record from GBIF, this species has been collected at elevation of 502 ft.

Species Range

Physical description

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.

Male Spinner

Described in Needham et al (1935) as Iron sancta-gabriel
Body length: 10 mm
Wing length: 10-12 mm

A yellowish species, allied to I. youngi (now a synonym of Epeorus albertae), also to I. lagunitas (now a synonym of Epeorus lagunitas) and I. albertae (now a synonym of Epeorus albertae); dark markings on thoracic pleura and at postero-lateral angles of tergites.

Head yellowish. Thorax light red-brown, mesonotum quite distinctly deeper in color, especially anterior portion and lateral margins. Anterior and posterior margins of pronotum narrowly black; posterior portion shaded with blackish; two curved black submedian streaks terminate at median line near posterior margin. Posterior margins of mesonota and metanota, including tips of scutella and depressions on each side of these, blackish. A blackish streak extends from lateral margin of pronotum to base of fore leg. A small black spot below root of fore wing; another anterior to middle coxa; two others, one above and one posterior to hind coxa. A small black dot on each coxa; a black streak on inner margin of each trochanter, also two minute dark dots at apical angles of each. Legs yellowish; fore tibia and all tarsi tinged faintly with olive brown. A round black spot at middle of each femur, also a subapical blackish band. Apex of fore tibia, all claws and tarsal joinings, blackish. Fore claws similar, blunt. Wings hyaline, appearing to have a very faint yellowish tinge. Costa, subcosta and radius yellowish brown, other longitudinal veins dark brown. Cross veins darker than longitudinals, those of costal margin almost blackish; slightly thicker than longitudinals. Humeral cross vein black in posterior half, pale next to costa. All costal cross veins well developed; 7 before bulla, 2 to 4 between bulla and stigmatic area, 9-10 stigmatic veins.

Abdominal segments 2-7 semi-hyaline, yellow; tergites faintly smoky. Posterior margins of tergites narrowly black. An oblique black mark cutting off posterolateral angle of each, divides to enclose a whitish area at spiracle, as in I. hesperus (now a synonym of Epeorus hesperus) and I. youngi. A black line in the same position on tergites 1 and 8. Segments 8-10 opaque, rather dark red-brown. Tails light red-brown with yellowish tinge; in basal portion, narrowly darker at joinings. Genitalia much as in I. albertae but differing in the forward-projecting apical margin of forceps base and the recurved lateral spines on each division of penes (see fig. 107).

Female Spinner

Described in Needham et al (1935) as Iron hesperus
Body length: 10 mm
Wing length: 12 mm

Only the female imago of this species in known. Pale yellowish in color. Curved black mark on vertex between eye and lateral ocellus; black mark near posterior margin of head between eyes, a short extension from this along median line. Anterior and posterior margins of pronotum narrowly black; a black streak extends from edge of pronotum downward along edge of coxa and trochanter. Scutella of mesonota and metanota darker than other portions of thorax. Pleura yellowish; a black spot anterior to middle coxa, and black mark at base of middle and hind coxae. Legs pale yellow; dark median and apical mark on each femur, the median spot large, black. Claws black. Basal joint of fore tarsus “is rather shorter than the second, which is barely longer than the third” (Banks). Basal joint of hind tarsus “a little longer than the second, and the second is a little longer than the third”’ (Banks). “Wings hyaline; venation yellowish, the cross veins more brownish; in costal space near base is a prominent black comma-shaped mark.” One of us (Needham), after examining the type specimen, notes that the longitudinal veins appear brown, the cross veins somewhat heavier than the longitudinals. Cross veins of stigmatic area not anastomosed; this area rather narrow, about 2/3 as wide as space below it. About six basal costal cross veins, four beyond bulla anterior to stigmatic area.

Abdominal segments pale. At the spiracular area, two oblique black streaks, joined together at the ends, surround each spiracle, on segments 2-7. The enclosed pale area is largest on tergite 2, smaller on the following segments. Sternites immaculate; “last two segments fumose” (Banks.). Apical margin of subanal plate with a median notch.

Female Dun

Described in Needham et al (1935) as Iron sancta-gabriel

A female subimago is marked much like the male. An oblique dark mark on head between lateral ocellus and eye; posterior margin of head and extension along median line, dark brown.


Described in Needham et al (1935) as Iron sancta-gabriel

Nymphs taken are rather similar to I dispar (now a synonym of Epeorus dispar) in postero-lateral spines, femoral flanges and gills of first and seventh pairs. This species appears to be closely allied to I. hesperus of which only the female is known.

Start a Discussion of Epeorus hesperus


  • Needham, James G., Jay R. Traver, and Yin-Chi Hsu. 1935. The Biology of Mayflies. Comstock Publishing Company, Inc.

Mayfly Species Epeorus hesperus

Species Range
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