Caenis macrura

One of a group of species known as Angler's Curse.

Careful and close examination of the abdominal tergites failed to reveal a spine on T2, so this should be either macrura or luctuosa. Of these two luctuosa is the one I have identified most frequently based on the shape of the antenna, but on this one the terminal bristle remained narrow right down to its point of insertion into segment 2, and that points to macrura. As it was a male I was able to check the genitalia. This wasn't entirely clear from checking against the diagrams in Elliott & Humpesch but comparing with the photos in Baunernfeind & Soldán it seemed to firmly support my ID of macrura.

male Caenis macrura showing antenna, tip of abdomen (ventral; 2 views) and forewing length, Wendling Beck Environment Project (Norfolk, UK), 3rd August 2023