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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.

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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Stonefly Species Isoperla mormona (Yellow Sallies)

Species Range


Source: The Isoperla Of California (Plecoptera: Perlodidae); Larval Descriptions And A Key To 17 Western Nearctic Species

Diagnosis. Mature Isoperla mormona male larvae are the smallest western Isoperla species (7 – 9 mm) and can be separated from other similar species that possess 2 – 4 submarginal setae (A + B) by having 7 – 14 ventral surface (D) setae (Fig. 12 f), fine silky setae sparse on dorsal surface of femora, numerous and continuous on tibia (Fig. 20 j) and interocellar area partially light (Fig. 12 a).

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.

Source: The Isoperla Of California (Plecoptera: Perlodidae); Larval Descriptions And A Key To 17 Western Nearctic Species

Male larva. Body length of mature larva 7 – 9 mm. Dorsum of head with contrasting pigment pattern and fine dark clothing setae, anterior frontoclypeus margin unpigmented; light M shaped pattern anterior to median ocellus connected to light frontoclypeus area by a thin median longitudinal light band, lateral thin arms directed posterolaterally, extending to antennal bases; posterior ocelli with partially enclosed large light areas along outer lateral margins; interocellar area variable, usually partially light, completely enclosed by dark pigment with light area extending to posterior margin of posterior ocelli, or sometimes mostly light and connected to posterior margin of head capsule by a thin median light band; occiput with irregular spinulae band extending from below eye to near median epicranial suture, not enclosed completely by dark pigment (Fig. 12 a). Lacinia bidentate, total length 525 – 666 µm (Figs. 2 j, 12 e-h, Tables 2 - 4); submarginal row (A + B) with 4 setae, groups A-B interrupted by gap below subapical tooth (SAT) inner margin (Fig. 12 g); 1 submarginal seta (A) inserted at base of apical tooth (AT) inner margin, plus 1 thin marginal seta (TMS) adjacent to AT inner margin, sometimes obstructed from view by AT or broken, and 1 dorsal seta (DS) located below SAT inner margin, partially obstructed by SAT (Figs. 12 g-h); 3 submarginal setae (B) located past SAT inner margin (Fig. 12 h); 6 – 9 marginal setae (C), initially long-stout and widely spaced, last few shorter and widely spaced, blending into and difficult to differentiate from dorsal surface setae (Fig. 12 g); 7 – 14 ventral surface setae (D) scattered below marginal setae, ending posteriorly at approximately ¾ the inner lacinia margin length and concentrated in posterior half (Fig. 12 f); dorsal surface setae (DSS) continue from last marginal setae (C) as a single, laterally protruding, submarginal row (sometimes 2 – 3 setae thick) along inner-lateral margin, ending before posterior-most ventral surface setae (Fig. 12 f). Galea with 12 – 17 setae in sparse ventral row, apex with 2 – 3 setae. Maxillary Palp segments 2 – 3 with curved, apically pointed setae. Pronotum with median light area bordered (at least partially) by thin, irregular dark bands; discs each with partially enclosed light areas or “windows”, fine dark clothing setae and lateral margins with broad light bands (Fig. 12 b). Mesonotum and metanotum with contrasting pigment pattern and fine dark clothing setae (Fig. 12 c). Legs with numerous fine dark clothing setae and scattered erect spines on outer surface of femora, erect spines longest and concentrated on dorsal surfaces; fine silky setae sparse on dorsal surface of femora, numerous and continuous on tibia (Fig. 20 j); tibia with faint transverse bands near proximal end. Abdominal terga with three distinct longitudinal dark stripes; wide light median longitudinal band bisected with thin dark median longitudinal stripe; lateral pair of dark longitudinal stripes about as wide as median dark stripe, not extending to lateral margins; numerous fine dark clothing setae and erect spines scattered dorsally; posterior margin with scattered long and numerous short spines in a concentrated row (Fig. 12 d).

Source: The Isoperla Of California (Plecoptera: Perlodidae); Updated Male Descriptions And Adult Keys For 18 Western Nearctic Species

Male. Aedeagus: sclerotized posterior process absent; body a long recurved tube with one small dorsoanterior lobe, tapering to a multi-lobed apex (Figs. 10 a-b); a thin light dorsoapical band of fine spinulae concentrated somewhat above bi-lobed tip (Figs. 10 a-b). Mesoposterior area of ninth tergum with bipartite patches of stout dark spinulae and long fine scattered setae, tenth tergum without spinulae (Fig. 10 c). Posterolateral margins of at least abdominal segment 8 with scale-like setae clustered in brushes of several setae. Paraprocts: curved dorsally, length if straightened subequal to combined first and second cercal segments, tapering abruptly to blunt apices (Fig. 10 c). Vesicle: trapezoid shaped, length subequal to width, constricted near base with tapered and straight lateral margins (base wider than apex), apex truncate (Fig. 10 d).

Start a Discussion of Isoperla mormona

Stonefly Species Isoperla mormona (Yellow Sallies)

Species Range
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