Note the three caudal filaments. Unfortunately this specimen looks like the gills on the left side have been completely lost, and they are not really visible on the right side. My guess is Paraleptophlebia (they have small forked gills that are easily knocked off, or when out of the water they will be stuck to the sides of the body as they seem to be here), and they do look like this. You need to photograph them in the water to see the gills, which are critical for generic placement.

Stoneflies always have two caudal filaments, mayflies usually have three. When they do have two, they always have easily seen abdominal gills.