despite the difficult angle of this (otherwise great) photo, there might be just enough information to allow for identification. The long abdomen and markings are suggestive of Ameletus or Siphlonurus, but I am not very familiar with the Western species of either genus. A side view of the wings would certainly have helped to narrow the possibilities. When I get a chance to dig through some of the original descriptions of those Western species, I might stumble upon one that fits.

a male imago. The long forelegs (not antennae) that can be seen in your other photo are an indicator of the imago stage in males. I have no guess as to genus/species--a side view would be helpful. Also, is your measurement (2.5cm +) of the body length (head to tip of abdomen) or does it include tails and/or forelegs? (Other than Hexagenia, very few mayflies would approach a body length of 2.5cm).

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