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Thanks to Mr. Kahis I know that it is Dolichopus simplex (I hope so).
On the picture Dolichopus simplex eats tiny Collembola (Sminthuridae) on the pond's beach

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great picture and interesting observation! louis

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Thank you Louis. But I still do not understand how they (Dolichopodidae) eat something more than collembola larvae.

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Well, they have no piercing mouthparts but the mouthparts seem to transformed into a big sucker, like one on an octopus' legs. I think they put a tremendous sucking force behind it after they pick up their prey, breaking its exoskeleton and emptying their liquid body contents. I have not checked literature, it is just something I surmised from seeing Medetera trying to crack the first instar of a true bug.

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Thank you Paul.