Confidence: I'm as sure as I can be.
Notes: 3 tails of equal length, with dark bands towards the tip and numerous smaller rings around the tails between these bands and the body. This is likely to be Cloeon dipterum, the pond olive but could also be Cloeon simile, the lake olive. What habitat was it found in? Might give a clue as to the species. One useful way of telling them apart is to look at the gill plates, which should be in pairs along the side of the body. In simile each pair will have on large and one small gill, whereas in dipterum both gills in each pairing will be of similar size.