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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.

Stream photos

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From Slough Creek to the Henry's Fork

From Slough Creek to the Henry's Fork

By Troutnut on July 31st, 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

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

7
Male Ephemerella excrucians Mayfly Dun
6
Male Siphlonurus occidentalis Mayfly Spinner
5
Male Ephemerella excrucians Mayfly Spinner
17
Leptoceridae Caddisfly Adult
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.
6
Male Ephemerella excrucians Mayfly Spinner
5
Female Ephemerella excrucians Mayfly Dun
7
Hydropsyche Caddisfly Adult
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.
9
Female Siphlonurus occidentalis Mayfly Spinner
6
Female Ephemerella excrucians Mayfly Spinner
8
Lepidoptera Insect Adult
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 Baetis tricaudatus Mayfly Spinner
8
Male Hydropsyche Caddisfly Adult
5
Formicidae Insect Adult
This seems to be what a lot of anglers call a "Cinnamon Ant," caught floating down the Henry's Fork.
5
Female Siphlonurus occidentalis Mayfly Spinner
This specimen was identified by association with this male.
6
Female Ephemerellidae Mayfly Dun
7
Female Helicopsyche borealis Caddisfly Adult
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.
6
Female Acerpenna pygmaea Mayfly Spinner
6
Male Siphlonurus occidentalis Mayfly Spinner
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 Drunella flavilinea Mayfly Dun
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.
5
Lepidoptera Insect Adult
5
Female Ephemerella excrucians Mayfly Dun
8
Male Hydropsyche Caddisfly Adult
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 Mayfly Spinner
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