Valenzuela burmeisteri

Apparently a common species.

I really struggled to identify this. Using the Barkfly Recording Scheme key I did land on the right ID as the most likely ID but it wasn't convincing, as it indicates this species should have a pale line alongside the pterostigma and radial vein. Looking at a variety of magnifications, different lighting and different angles I simply could not make out any hint of this pale line. So with no clear answer from the website I turned to the RES key by T R New. The key here demanded views of two bits of anatomy that I am not familiar with - the distal inner labral sensilla and the lacinia. Unable to see these on the dried specimen I resorted to clearing the head in potassium hydroxide solution, but still couldn't make out either, even under the compound microscope. I left it clearing further in euparal essence for 2-3 days and went back to it. Eventually, I think I worked out where the inner labral sensilla were, but I still couldn't convincingly match up any of the diagrams with what I could see - I think they are just a bit too awkward to see against the outer labral lip. I had a bit more luck this time finding the lacinia, looking at the mouthparts from underneath. This had rounded lobes and even without the labral sensilla that seems to be enough to continue to the next steps in the key. Notably, this key does not require the presence of a pale line alongside the pterostigma and radial vein on burmeisteri, so that now feels like a satisfactory conclusion.

Valenzuela burmeisteri showing wings, lacinia and two views of the labral groove, North Elmham (Norfolk, UK), 22nd May 2020