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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

Dorsal view of a Holocentropus (Polycentropodidae) Caddisfly Larva from the Yakima River in Washington
This one seems to tentatively key to Holocentropus, although I can't make out the anal spines in Couplet 7 of the Key to Genera of Polycentropodidae Larvae nor the dark bands in Couplet 4 of the Key to Genera of Polycentropodidae Larvae, making me wonder if I went wrong somewhere in keying it out. I don't see where that could have happened, though. It might also be that it's a very immature larva and doesn't possess all the identifying characteristics in the key yet. If Holocentropus is correct, then Holocentropus flavus and Holocentropus interruptus are the two likely possibilities based on range, but I was not able to find a description of their larvae.
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 denningi (Yellow Sallies)

Species Range


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

Diagnosis. The larval male lacinia of Isoperla denningi is distinct from other Isoperla species with 1 – 4 submarginal (A + B) setae in that the dorsal lacinial surface has no fine setae (must focus down through ventral surface to see) and ventral surface setae (D) are concentrated on the apical half (Fig. 7 f). The larval dorsal head pigment pattern is striking in this species, which has a well defined median longitudinal light band extending from the median base of the M shaped mark, almost reaching the anterior unpigmented frontoclypeus area (Figs. 7 a- d).

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 11 – 12 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 usually not connected to light frontoclypeus area, median longitudinal light band thin with parallel margins, lateral thin arms directed posterolaterally, extending to antennal bases; posterior ocelli with completely enclosed small light areas along outer lateral margins; interocellar area variable, usually its entire area lighter than surrounding dark pigment, partially filled with indistinct light brown pigment, extending past posterior ocelli and connected to light area below occipital spinulae band, occasionally interocellar area partially filled with surrounding dark pigment and not connected to light area of posterior head capsule; occiput with irregular spinulae band extending from below eye to near median epicranial suture, partially enclosed by light brown pigment (Fig. 7 a). Lacinia bidentate, total length 756 – 1093 µm (Figs. 2 e, 7 e- h, Tables 2 - 4); submarginal row (A + B) with 3 – 4 setae, groups A-B interrupted by gap below subapical tooth (SAT) inner margin (Fig. 7 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, submarginal seta (A) or broken, and 1 dorsal seta (DS) located below SAT inner margin, partially obstructed by SAT or submarginal setae (B) (Figs. 7 g-h); 2 – 3 submarginal setae (B) located past SAT inner margin (Figs. 7 g-h); 9 – 13 marginal setae (C) initially long-stout and widely spaced, last few shorter and widely spaced, sometimes borne from ventral or dorsal surface or broken with visible sockets (Fig. 7 e); 11 – 34 ventral surface setae (D) scattered below submarginal and marginal setae, ending posteriorly at approximately ¾ the inner lacinia margin length and concentrated in apical half, occasionally a few setae located below submarginal row striated (Fig. 7 f); dorsal surface setae (DSS) usually absent, sometimes last few marginal setae (C) form a short dorsal row (Fig. 7 f – arrow). 15 – 25 minute scale-like spinulae on and along dorsal inner-lateral margin in basal half. Galea with 35 – 48 setae in thick ventral band, apex with 4 – 5 setae. Maxillary Palp segments 2 – 3 with curved, apically pointed setae. Pronotum with thin, median light area bordered by irregular dark longitudinal bands with adjacent light rugosites; discs each with dark comma shaped lateral areas and fine dark clothing setae restricted to areas of dark pigmentation, lateral margins with broad light bands (Fig. 7 b). Mesonotum and metanotum with contrasting pigment pattern and fine dark clothing setae (Fig. 7 c). Legs with numerous fine golden clothing setae and scattered erect spines on outer surface of femora, erect spines longest and concentrated on dorsal surface; fine silky setae numerous and continuous on dorsal surfaces of femora and tibia (Fig. 20 e); tibia with faint transverse bands near proximal end. Abdominal terga with two distinct longitudinal dark stripes; wide light median longitudinal band bisected with indistinct light brown median pigment band; lateral pair of dark longitudinal stripes not extending to lateral margins; numerous fine light clothing setae and erect spines scattered dorsally; posterior margin with scattered long and numerous short spines in a concentrated row (Fig. 7 d).

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

Male. Aedeagus: sclerotized posterior process present; body with two posterobasal lobes, one large posteroapical lobe, and one dorsolateral pair of rounded lobes (Fig. 5 a); sclerotized process length <0.5 mm, recurved and rod-like, apex clavate in lateral view (Fig. 5 a), bifurcate in posterior view, lateral apices short and rounded (Fig. 5 b), and width of expanded apicolateral lobes 0.09 - 0.10 mm. Abdominal terga 8 - 9, 9, 9 - 10: without stout spinulae or long stout setae. 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 gradually to blunt apices (Fig. 5 c). Vesicle: pedunculate, length subequal to width, constricted near base with curved lateral margins, wider and rounded at apex (Fig. 5 d).

Start a Discussion of Isoperla denningi

Stonefly Species Isoperla denningi (Yellow Sallies)

Species Range
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