Trypeta zoe

I used the RES key to identify this (both the original key published by RES and Mike Hackston's version of it). It was straightforward enough to get it to genus but then became problematic:

• Crossvein r-m was before the apex of R, but only a short way before. If I took it to be before this should make it zoe (which occurs in Norfolk), but if I took it to be "at" (i.e. level with) then it would be either immaculata (which may not occur in Norfolk - at least there are no records at the NBN Atlas) or artemisiae (the species with largest number of dots on the map in Norfolk at the NBN Atlas).
• The postgenal setae (or at least what I thought must be the postgenal setae) were black, not white. They should be white on zoe or black on immaculata and artemisiae. Now I'm not quite sure which the postgenal setae are, but I thought all the most likely candidates were black. There is a small row of setae immediately behind what I believe to be the genal seta (longer and more robust than the others) - I'm not sure if those are the postgenal setae or if they're extra genal setae; then there is another longer row of setae along the back of the eye - maybe these are the postgenal setae? Either way they were clearly black and not white. At 63x magnification I could not see any other setae that could be described as postgenal except perhaps a few small scattered ones round the back of the bottom of the head (these were pale, though not white). If I identified the postgenal setae correctly then this should rule out zoe.
• The preapical and discal cross bands were not fused in cell cua₁ (or remotely nearly so) which should rule out immaculata.
• Vein R4+5 had absolutely no dorsal setae beyond r-m crossvein in either wing. I suppose it's possible they'd broken off but they were still present and easy to see along this vein up to r-m crossvein (one was so close to r-m that the tip of the seta was beyond r-m, but I assume it means the setae should emerge from a point along the vein distal to r-m crossvein). This should be ok for immaculata but rule out artemisiae which should have at least 3 such setae. The key does not state how many such setae zoe should have, if any.
• The forewing length of 5.0mm is within range for zoe and for immaculata but below the bottom of the stated range for artemisiae.

At this point therefore there were clear contradictions with the criteria given in the key whichever of the three species it was (assuming it was one of the three) and none of the photos of any of the three species I could find online exactly matched mine. In case I had miskeyed in getting to genus I checked through all the wings shown in the RES key and examined images of the most similar species online, but I cannot find anything else that has similar wing pattern. I'm assuming there isn't a new species present that is omitted from the key as there are no other species listed in e.g. NBN Atlas.

Finally to resolve the conundrum I checked the aculeus, and the shape of this appears to confirm that the fly is Trypeta zoe. Comparing it with the diagrams in the original RES key the point is too long and attentuated to be artemisiae and too rapidly widening away from the point to be immaculata - the shape exactly matches the diagram for zoe. The preapical serrations also match the diagram for zoe best.

female Trypeta zoe showing head from side, sternopleura, hind femur, postgenal setae and aculeus (with close-up of tip and closer-up of preapical serrations), North Elmham (Norfolk, UK), 13th April 2024