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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.

Case view of a Pycnopsyche guttifera (Limnephilidae) (Great Autumn Brown Sedge) Caddisfly Larva from the Yakima River in Washington
It's only barely visible in one of my pictures, but I confirmed under the microscope that this one has a prosternal horn and the antennae are mid-way between the eyes and front of the head capsule.

I'm calling this one Pycnopsyche, but it's a bit perplexing. It seems to key definitively to at least Couplet 8 of the Key to Genera of Limnephilidae Larvae. That narrows it down to three genera, and the case seems wrong for the other two. The case looks right for Pycnopsyche, and it fits one of the key characteristics: "Abdominal sternum II without chloride epithelium and abdominal segment IX with only single seta on each side of dorsal sclerite." However, the characteristic "metanotal sa1 sclerites not fused, although often contiguous" does not seem to fit well. Those sclerites sure look fused to me, although I can make out a thin groove in the touching halves in the anterior half under the microscope. Perhaps this is a regional variation.

The only species of Pycnopsyche documented in Washington state is Pycnopsyche guttifera, and the colors and markings around the head of this specimen seem to match very well a specimen of that species from Massachusetts on Bugguide. So I am placing it in that species for now.

Whatever species this is, I photographed another specimen of seemingly the same species from the same spot a couple months later.
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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Little Yellow Stoneflies

Like most common names,"Little Yellow Stonefly" can refer to more than one taxon. They're previewed below, along with 5 specimens. For more detail click through to the scientific names.

Stonefly Species Isoperla quinquepunctata

These are pretty much always called Little Yellow Stoneflies.
This species is often the most common Perlodidae or Little Yellow Stone In Northern California, especially in mid-elevation rivers and streams.
Isoperla quinquepunctata (Perlodidae) (Little Yellow Stonefly) Stonefly Nymph from the Yuba River in California
The dorsal patterning is quite unique.

Entoman

Stonefly Genus Isoperla

These are often called Little Yellow Stoneflies.
This is a very common trout stream stonefly genus. It is also huge, with sixty species scattered over the entire continent. Though anglers usually call them Little Yellow Stones or Yellow Sallies, the scientific common name for this genus is Stripetail. The latter is probably better to use as not all of them are necessarily little or yellow.
Isoperla fulva (Perlodidae) (Yellow Sally) Stonefly Adult from the  Touchet River in Washington
Dorsal view of a Isoperla (Perlodidae) (Stripetails and Yellow Stones) Stonefly Nymph from unknown in Wisconsin

Stonefly Species Alloperla caudata

These are often called Little Yellow Stoneflies.

Stonefly Species Sweltsa coloradensis

These are often called Little Yellow Stoneflies.

Stonefly Species Suwallia pallidula

These are often called Little Yellow Stoneflies.
This species keeps turning up when I collect little yellow stoneflies throughout midsummer in Washington and Montana. I suspect it is one of the most ubiquitous of the Chloroperlidae "yellow sallies."
Suwallia pallidula (Chloroperlidae) (Sallfly) Stonefly Adult from the Touchet River in Washington
Dorsal view of a Suwallia pallidula (Chloroperlidae) (Sallfly) Stonefly Nymph from Mystery Creek #199 in Washington
This specimen keys out to Suwallia, for which I did not find any nymph species keys. However, I'm placing it in Suwallia pallidula because I caught a few adults in the same spot that closely resembled the abundant nymphs and keyed them out to species. Features I noted under the microscope when keying this specimen to genus included apical hairs of cercal segments that were directed at posterior angles, and the longest apical hairs of distal segments were shorter than their following segment.
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