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I am hopeful that this ID is correct. The female has an insect in her clutch, and the femora are all orange.
What stunning photos, especially the first - worthy of a prominent location on the new Dipterists Forum website. I have downloaded a copy of the first picture for use (with due acknowledgment) in any presentations I might be asked to give. Having only seen E. opaca once or twice I can't swear to their identity. What about the prey - Sciaridae perhaps?
As to ID of the Empis, I saw that there are 2 similar species with dark wings, and they both give the female a "gift" before attempting to mate. This one is distinctive in having red-orange femurs. It's not your proper watertight identification, but it's rarely practical to use a key in these sort of circumstances. I am still open to correction.
Not sure about the small fly - I have only recorded one species of Sciaridae, and that had dark wings. I suspect they all do, but this one looks too transparent and pale to me.
I have now revised my opinion of the species.
I used the Mike Hackston Empidiae on-line key, via Google.
I don't get to E. opaca, because before that there is a selection based on :-
12. Face hairy, wings in female very broad and brown.
It's tricky being sure that the hairs are not simply around the head, rather than actually on the face as well. But I think so. Certainly the 2 latter points apply. Which makes it E. borealis, not E. opaca.
Checking images, my female specimen certainly has sufficiently broad wings to comply. What had put me off was the habitat, being both northern and upland. It is "northern", but definitely lowland, though it's otherwise similar to many upland locations in being boggy, sphagnum moss, etc, etc.
So Phil, I hope you agree it's E. borealis !!
I certainly can't gainsay you, having never come across E. borealis. The Collin book and internet pictures certainly indicate that the broad brown female wings are very distinctive, as seen in your second picture.
I'm pretty confident (for a change) that I've got this one right.
But I had been fooled by the assertion that there were only 2 species where the male offered the female an insect. There are clearly more than that. An interesting angle to this specific species is described at:
http://diptera.myspecies.info/taxonomy/ … scriptions
There's a similar species account via the new NBN Atlas.
There were several "possibles" about, but I didn't see swarms, or any other interesting behaviour actually happen. At least I did see something though. At Duddon Mosses NNR. A new species for the site, and the SW corner of Cumbria.