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Thanks John. What part of the insect I have to photograph to separate both genera?

Edit: I had understood you wanted to tell me the possibilities were that the fly was or Hercostomus (two possibles species) or Dolichopus (one species), but now re-reading your answer I think that in fact you are saying that there are different species in the photographs, being Hercostomus in the first and second photographs, and Dolichopus in the third one. I took the photographs in the same area with an interval of twelve minutes only, to different individuals, but I thought all was the same. Can you confirm this last interpretation is correct?