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Hello Juergen,
Well, your first picture is not a male Melanostoma, but a male Platycheirus! Something like P. scutatus or P. splendidus, but not to distinguish from this photo!
Second photo is indeed a male Melanostoma, in this case M. mellinum! Males of mellinum have a relatively short abdomen, which does NOT extend beyond the wings!
Third picture is a female of Melanostoma, in this case I think it is mellinum! You can just see that the frons (above the antenna) is shining!
So, to explain difference between scalare and mellinum:
Males from mellinum have relatively short straight abdomens, not extending beyond the wings. Males of scalare have an elongated almost lineair abdomen which is extends beyond the wings.
Female mellinum have a shiny face and frons, and a bare arista on the antenna (which is totally useless in a picture! ):D
Female scalare have a dusted face and abdomen, and their arista is short haired.
In older works it is usually also the shape of the spots on the abdomen which is used, but this is often quite difficult to see!
And yes, your last picture is indeed afemale of dasysyrphus albostriatus!
Greetings