I didn't find a male spinner to associate with this one; I'm tentatively calling it grandis based on size alone. I found it floating down the surface of the Henry's Fork (the only one I saw that day, weeks after the hatch probably peaked) crippled by what appears to be a mass of eggs sticking out of its abdomen.
This mayfly was collected from the Henry's Fork of the Snake River on August 1st, 2020 and added to Troutnut.com on August 18th, 2020.
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