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Ukraine, Rivne region, on August 26.
Is it possible to determine the species of Melanostoma for this image?

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I am afraid we cannot find out with certainty. However, I tend to M. scalare. The triangular dust spots on the front are a bit big for M. mellinum, and they seem to be laterally confluent with the dusting of the face, which is not the case in M. mellinum. The antennae of M. mellinum should be darker. The halteres of M. scalare do not necessarily have to be greenish. Unfortunately, we cannot estimate the abdomen shape viewed from above. So I would suggest Melanostoma cf. scalare.
Regards, Sundew

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On some pictures of Melanostoma mellinum, I see yellowish antennas with darkness at the end and rather large dark spots on the face:
https://macroid.r...hoto=45685
https://diptera.i...d_id=78000

The view from above is, but it is obscured

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As I said, it's difficult with pictures of females! Some characters match one species, some the other. And who knows the whole range of character variability?!
I rely on the book "Britain's Hoverflies" by Ball & Morris 2015 (see screenshot below). A helpful paper is also given by Haarto & Stahls 2014 (https://www.resea..._Syrphidae) - see descriptions and comparing pictures. And there are good websites by Steven Falk for M. mellinum https://www.flick...491551024/ and M. scalare https://www.flick...855917333/. He points out that both species may prove to be species complexes, so we have to wait for further enlightening studies...
I am sorry that I cannot contribute more decisive knowledge. Regards, Sundew

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Thanks ! I do not speak English fluently, so I first took dusty spots in the dark) I think the best thing in this case would be to sign Melanostoma sp.