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The tails are quite short - but this is partly because this is the dun (or subimago) stage of the mayfly lifecycle. Ephemeroptera are unique among insects in having two winged stages in their lifecycle. The duns hatch from the water and make their way to the bank, where they will transform into the spinner (or imago), which is the true reproductive adult. Spinners have longer tails, and also have clear wings. The cloudy wings of the dun are sheaving the wings of the spinner.
Thanks for this Inopinatus. Yes the photo seemed very like those of subimago E. danica I viewed earlier online. I was aware of the 2 winged stages, but I didnt realise the cloudy wings of the fresh new adult were concealing the clear wings of the reproductive adult, it makes perfect sense.
I will post an image of spent mayfly post reproduction, which I took the same day, I think they are probably E. danica also.
Lat/Lng: 53.825, -7.31
OS grid ref: SA507496