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Black robber fly with golden hairs on legs and abdomen, c 8mm in ancient woodland in SW UK. New to me and a nationally scarce species not yet recorded in Wiltshire if I'm correct, though I may not be. NB UK species records have it as C. marginatus rather than C. marginata.

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Pair mating on same day 26.6.17

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Choerades marginata or femorata. In NL we name it also C. marginatus but the feminine version seems to be correct. http://www.latin-dictionary.net/definition/9462/choeras-choeradis
C. femorata is very similar but has slightly longer antennae than marginata and the shiny facial hair is usually white(more yellowish in marginata). In the near future Die Raubliegen Deutschlands will be published and I think we will find a solution there.

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Many thanks Reinoud. I can't find records of C. femorata in the UK and think it may only be on mainland Europe, so this is likely C. marginata if our recording is accurate. I found this good key to German Asilids http://www.robberflies.info/keyger/htmle/keychoerades.html which gives the key antennal differences, though my photos don't show the antennae clearly enough.

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C. femorata has long been regarded as synonym to marginata, that is probably the reason why it was never taken into account (also in the NL). In the NL probably all/most marginata specimens are actually femorata. Your specimens look more like femorata too because of the white facial hair. Just an observation; in France I collected quite a few specimens with yellow facial hair and all in a fir forest. In NL you will hardly ever find "marginata" in coniferous forests. So, I suspect that marginata is a species of coniferous forests and femorata of deciduous forests.

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The UK list assumes the derivation is from the patronymic -ades in Greek nouns, which is masculine, so it should be femoratus. According to the UK checklist, Choerades was treated as feminine only because species within it were originally considered part of the feminine Laphria genus. The derivation is obscure, but taking it as Choer-ades might give the derivation 'son of a pig'. An alternative would be that it comes from the feminine plural noun choerades meaning 'scrofulous swellings in the neck', which would mean marginata could be right. I suppose it comes down to whether the name was inspired by pigs or scrofulous sores... ;)

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Many thanks to you both for the extra info on C. marginta/us/femorata. Sounds like some taxonomic revision is needed for this Genus/Species to bring some clarity and hopefully the German work will do that. The current taxonomy seems more like a pig's ear than a sore to me at the moment! These links suggest that the status of C. marginatus / C. femorata is currently undecided in the uk.
http://www.dipteristsforum.org.uk/t3261-Request.html
http://www.dipteristsforum.org.uk/t1027-Choerades-marginatus%3F.html
The location was a deciduous woodland ride, and the facial hairs do look white not yellow, so C. femorata does seem the most likely true ID and that name needs to be added to the UK list!

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For the last Choerades paper, we asked people from the International Code of Zoological Nomenclature in order to solve this confusing situation and after consulting the original description of the genus Choerades , this was the answer:

Choerades is the vocative plural of choeras – and is feminine. It refers to a swelling of glands in the neck.

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'Vocative' plural would surely be wrong, unless they have started talking to flies! Nominative plural...possibly.