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Moscow region, 21 july, 5mm.
Wings obviosly dark, without akcilar corner, face silverish-white.
By Bey-Bienko I've got Dolichopus longitarsis. Only female collected.

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The female is a typical Dolichopus longitarsis.

The male is not a Dolichopus at all but Gymnopternus chalybeus (in genus Hercostomus in your key).

Both species are common in herb-rich fens. G. chalybeus loves the same habitat as the plant Comarum palustre (= Potentilla p.) and the fly is almost always found among dense stands of this plant.

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Thank you Kahis.
Tomorrow I'll come back for Gymnopternus chalybeus, it is good oppotunity to understand this genus! Place is exactly as you discribed.
Nikita