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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 Neoleptophlebia (Leptophlebiidae) Mayfly Nymph from the Yakima River in Washington
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
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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Leskorcala has attached these 2 pictures to aid in identification. The message is below.
Posts: 16
Leskorcala on Apr 7, 2020April 7th, 2020, 3:41 pm EDT
Love to pick few brains to ID this black and white mayfly nymph I just collected one on Bitterroot river in Montana April 7th I found it under tree trunk
Healdsburg, CA

Posts: 344
Millcreek on Apr 8, 2020April 8th, 2020, 5:28 am EDT
Definitely a member of Heptageniidae. Might be a Rhithrogena. If you still have the specimen, put it on it's back and see if the gills form a "suction cup", or if they meet in front and back, like in this picture.

"If we knew what it was we were doing, it would not be called research, would it?"
-Albert Einstein
Posts: 16
Leskorcala on Apr 8, 2020April 8th, 2020, 5:08 pm EDT
Thank you for your help fortunately i did not collect the specimen, i will try next time out.Very different in black and white or cream markings which i have not seen often.
Healdsburg, CA

Posts: 344
Millcreek on Apr 8, 2020April 8th, 2020, 5:25 pm EDT
I'm guessing at Rhithrogena. Seems likely because of the general aspect of it. The color is unusual for Rhithrogena but there are several undescribed species of western Rhithrogena nymphs.
"If we knew what it was we were doing, it would not be called research, would it?"
-Albert Einstein
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Posts: 498
Konchu on Apr 10, 2020April 10th, 2020, 6:50 am EDT
That looks like my search image of a living Rhithrogena. I'm only aware of R. undulata having been found in the Bitterroot R in MT, but like Millcreek said, we don't know as much about the nymphs as we'd like. The record I have was from a June emergence, FWIW.

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