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Moscow region, 13 july, 1,5mm, on fallen on pond leaf.
All dance from meeting till copulation takes 20-30 seconds.
Pair was collected afterwards so I hope to ID species.

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Hydrellia sp.

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Great series of photos!
As well as the distinct "lunule" above the antennae, many species of Hydrellia have bright silver or gold faces. Your photos show the importance of head-to-head meetings, when the face colour will be used to aid species recognition. The bright yellow halteres often appear to be used during these dances too. Everything has a function - even if we haven't discovered what it is yet! :)

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Thank you Tony.
According your lesson I choised male and tried to key it.
Palp yellow, cx black, antennae 3-d - yellow, T5=T4, thorax metallic.
Hydrellia flavicornis?

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Who knows. There is no key (well, no key I know of) covering all central european Hydrellia :(. Personally I name names only after checking male genitalis - and no figures are available for some species, making even this a bit risky. The old key of Nartchuk in Bei-Bieko is not reliable.

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Got to agree with Kahis - I will not identify Hydrellia positively except by examining male genitalia. (There's every possibility of turning up new species.) If you can dissect out the genitalia and clear them, then get a good microscope shot, post the picture and we'll check it against the available drawings. You might very well be right with your identification, but I think it's best to check the genitalia every time. :|

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Thank you Kahis and Tony.
I'll show collected couple to Marina Krivosheina;)
Nikita

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Dancing couple ID by collected flies by Marina Krivosheina as
Hydrellia albilabris.

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Yes, I agree, a very nice series documenting this interesting behavior. Nikita, can you post a photo of the face?

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No, I can't or I can, but of Hydrellia sp.
Nikita