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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.

Silver Creek in Idaho

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Onocosmoecus (Great Late-Summer Sedge) Caddisfly Larva Onocosmoecus (Great Late-Summer Sedge) Caddisfly Larva

Lateral view of a Onocosmoecus (Limnephilidae) (Great Late-Summer Sedge) Caddisfly Larva from the Yakima River in Washington

This specimen keys pretty easily to Onocosmoecus, and it closely resembles a specimen from Alaska which caddis expert Dave Ruiter recognized as this genus. As with that specimen, the only species in the genus documented in this area is Onocosmoecus unicolor, but Dave suggested for that specimen that there might be multiple not-yet-distinguished species under the unicolor umbrella and it would be best to stick with the genus-level ID. I'm doing the same for this one.

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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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From Slough Creek to the Henry's Fork

From Slough Creek to the Henry's Fork

By Troutnut on July 30th, 2020

I really wanted to photograph some bugs in the evening, so I didn't fish in the morning. Instead I got up early, broke camp, and hiked about ten miles out the well-maintained trail to the trailhead from 8:30 to 11:50 am. I stopped in a fly shop in West Yellowstone to replace a couple minor supplies, while trying to keep my breathing to a minimum in the crowded store during a pre-vaccine Covid peak. I would have good odds of surviving the virus, but it would have good odds of screwing up my fishing plans, so better safe than sorry.

I arrived at the famous Harriman Ranch section of the Henry's Fork around 3 pm, but nothing was really hatching until 5. As clouds rolled in, PMDs started hatching (Ephemerella excrucians), followed eventually by Flavs (Drunella flavilinea) and Gray Drakes (Siphlonurus occidentalis), mixed with lots of caddisflies. I caught four small rainbows, then a 15-incher and a 17-incher (my best yet on this reach) that made a great cartwheeling leap high in the air. The bug collecting was even better than the fishing.

Photos by Troutnut from Slough Creek and the Henry's Fork of the Snake River in Wyoming and Idaho

Two whitetail deer in one of the meadows along Slough Creek. I watched them all morning before I hiked out.

From Slough Creek in Wyoming

The Henry's Fork of the Snake River in Idaho

The Henry's Fork of the Snake River in Idaho

These were the contents of my bug net after several swoops through the grass along the Henry's Fork.

From the Henry's Fork of the Snake River in Idaho

The Henry's Fork of the Snake River in Idaho

The Henry's Fork of the Snake River in Idaho

Closeup insects by Troutnut from the Henry's Fork of the Snake River in Idaho

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Male Ephemerella excrucians (Pale Morning Dun) Mayfly Dun

Lateral view of a Male Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Dun from the Henry's Fork of the Snake River in Idaho

5
Male Ephemerella excrucians (Pale Morning Dun) Mayfly Spinner

Lateral view of a Male Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

17
Leptoceridae Caddisfly Adult

Lateral view of a Leptoceridae Caddisfly Adult from the Henry's Fork of the Snake River in Idaho

This specimen keyed easily to Leptoceridae, but I hit a bit of a dead end in the genus key, because I can't seem to find the cross-veins in the wings that are supposed to help narrow it down. I took pictures of all the relevant features under the microscope; maybe somebody with more experience with the family can help.

9
Female Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

Lateral view of a Female Siphlonurus occidentalis (Siphlonuridae) (Gray Drake) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

8
Male Hydropsyche (Spotted Sedge) Caddisfly Adult

Lateral view of a Male Hydropsyche (Hydropsychidae) (Spotted Sedge) Caddisfly Adult from the Henry's Fork of the Snake River in Idaho

6
Male Ephemerella excrucians (Pale Morning Dun) Mayfly Spinner

Lateral view of a Male Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

7
Hydropsyche (Spotted Sedge) Caddisfly Adult

Lateral view of a Hydropsyche (Hydropsychidae) (Spotted Sedge) Caddisfly Adult from the Henry's Fork of the Snake River in Idaho

This species was really common along the Henry's Fork at the end of July. I photographed it along with a male that appears to be the same species, and another male of the same genus but a seemingly different, less common (on this day) species.

8
Lepidoptera (Moth) Insect Adult

Lateral view of a Lepidoptera (Moth) Insect Adult from the Henry's Fork of the Snake River in Idaho

In the field, I accidentally mistook this moth for a weird-looking caddisfly. Only later did I notice the moth mouthparts and that the "hair" on the wings is actually tiny, flat scales.

6
Female Ephemerella excrucians (Pale Morning Dun) Mayfly Spinner

Female Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

5
Female Ephemerella excrucians (Pale Morning Dun) Mayfly Dun

Lateral view of a Female Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Dun from the Henry's Fork of the Snake River in Idaho

5
Lepidoptera (Moth) Insect Adult

Lateral view of a Lepidoptera (Moth) Insect Adult from the Henry's Fork of the Snake River in Idaho

6
Female Acerpenna pygmaea (Tiny Blue-Winged Olive) Mayfly Spinner

Lateral view of a Female Acerpenna pygmaea (Baetidae) (Tiny Blue-Winged Olive) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

6
Female Baetis tricaudatus (Blue-Winged Olive) Mayfly Spinner

Lateral view of a Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

8
Male Hydropsyche (Spotted Sedge) Caddisfly Adult

Lateral view of a Male Hydropsyche (Hydropsychidae) (Spotted Sedge) Caddisfly Adult from the Henry's Fork of the Snake River in Idaho

This species was really common along the Henry's Fork at the end of July. I photographed it along with a female that appears to be the same species, and another male of the same genus but a seemingly different, less common (on this day) species.

6
Female Ephemerella excrucians (Pale Morning Dun) Mayfly Spinner

Lateral view of a Female Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

5
Formicidae (Ant) Insect Adult

Lateral view of a Formicidae (Ant) Insect Adult from the Henry's Fork of the Snake River in Idaho

This seems to be what a lot of anglers call a "Cinnamon Ant," caught floating down the Henry's Fork.

5
Female Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

Lateral view of a Female Siphlonurus occidentalis (Siphlonuridae) (Gray Drake) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

This specimen was identified by association with this male.

6
Male Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

Lateral view of a Male Siphlonurus occidentalis (Siphlonuridae) (Gray Drake) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

This specimen was easy to key to species using the old keys in < id="115" />, and was the basis for IDs of several other Siphlonurus specimens of the same species collected on the same day.

6
Female Ephemerellidae (Hendricksons, Sulphurs, PMDs, BWOs) Mayfly Dun

Lateral view of a Female Ephemerellidae (Hendricksons, Sulphurs, PMDs, BWOs) Mayfly Dun from the Henry's Fork of the Snake River in Idaho

7
Female Helicopsyche borealis (Speckled Peter) Caddisfly Adult

Lateral view of a Female Helicopsyche borealis (Helicopsychidae) (Speckled Peter) Caddisfly Adult from the Henry's Fork of the Snake River in Idaho

This one keys pretty easily to the Helicopsychidae family, of which there is only one genus in North America, and one species reported in this general area.

5
Female Ephemerella excrucians (Pale Morning Dun) Mayfly Dun

Lateral view of a Female Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Dun from the Henry's Fork of the Snake River in Idaho

6
Male Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

Lateral view of a Male Siphlonurus occidentalis (Siphlonuridae) (Gray Drake) Mayfly Spinner from the Henry's Fork of the Snake River in Idaho

6
Female Drunella flavilinea (Flav) Mayfly Dun

Lateral view of a Female Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Dun from the Henry's Fork of the Snake River in Idaho

I can't be certain of the ID of this female dun, but I'm calling it Drunella flavilinea for three reasons: 1) the known abundance of "flavs" on the Henry's Fork, 2) clear differences in coloration from my confirmed specimens of Drunella coloradensis, which is the main lookalike, and 3) the habitat (a sizable river in a wide mountain valley, rather than a small, high-altitude stream) suggests it's not coloradensis.

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