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Hi, thank You all ! The place where I have been watching these flies is full rotting grass etc in high heaps. They make soil in there and have heaps of horse dung too. When all this gets rotten the top will dry and all the liquids runs out from the lower part. This bad smelling liquid is the place. They are running on it and flying very low over it. If they are eating or not, I don't know. Ovipositing, maybe but there are a lot of males too. It's easy to have tens of them in the net with just one sweap. I ?must try tomorrow to take a picture of the whole thing.