Twin-striped Dark Splay Erioptera bivittata

This is a Red Data Book species that is normally associated with brackish water though is apparently tolerant of lower salinity with varied freshwater flora (per Stubbs in British Craneflies). So this record in mid Norfolk, about 18 miles from the coast, is unexpected. Is it a wanderer from the coast, or is it tolerating the freshwater carr it was found close to? There appear to be a number of records from Wicken Fen in Cambridgeshire, so perhaps it is plausible that it might be breeding in the wet woodland here in mid Norfolk too.

I am of course assuming my ID is correct, but I struggled to identify it. My first hiccup was at Chioneinae couplet 3 in Stubbs - the basal flagellar segment seemed to be closer to the "elongate conical" shape illustrated for Crypteria and Neolimnophila than any of the shapes described for the alternative, but vein M₁ clearly wasn't forked beyond the discal cell so it couldn't be either of those. Then at couplet 15, the lack of strong wing markings appeared to point away from Ilisia, but vein M₃ was a little curved, and the shape of the apex of Cu wasn't far off what was illustrated for Ilisia, so I had to rule out the possibility of it being a weakly-marked Ilisia. Then at couplet 17 the wing membrane was hairy (so not bare as one option required) but it wasn't exactly covered with long hairs as the alternative demanded. Indeed, the alternative led to Ormosia fascipennis which appeared to be known in the UK only from the high massif of the Cairngorms!

There were some similarities between the genitalia of Ormosia fascipennis and my insect, but it wasn't close enough. I ended up flicking through the genitalia diagrams until I found a match, and there it was, strikingly spot-on, Erioptera bivittata. Turns out I needed to key it as membrane bare even though the text in the species account says, "there are hairs even on the wing membrane". Indeed the description of bivittata matched my insect in all respects. The wing length was about 5.4mm.

At one point when I was struggling to reach a satisfactory conclusion I turned to Boardman's Shropshire Craneflies. This often helps but in this case it didn't - it keyed best to genus Scleroprocta there but clearly wasn't either of the species in that genus. Of course while the vast majority of Norfolk's craneflies are included in Shropshire Craneflies, this coastal species isn't.

male Twin-striped Dark Splay Erioptera bivittata showing wing, antenna, top of thorax from above and genitalia (3 views - 1 dorsal as I found it, 1 dorsal with the two sides prised slightly apart for a clearer view and one view from below), Wendling Beck Environment Project (Norfolk, UK), 13th May 2024