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I agree with Menno that it appears to be selenitica, but officially the form name unicolor is described for S. pyrastri and treated as synonym for that species.

Females with ovaries have been described for many more species and no doubt the phenomenon will occur in selenitica as well. In some of those species these 'females' are considered to be intersexes because they have intermediate characters between males and females and no developed genital structures. The intermediacy is especially clear in species of Platycheirus and Pyrophaena. In other species it usually appears as if the frons is narrower than in ordinary females.
In your specimen the frons does not really seems to be narrower, and since vague markings are visible (though much darker than normal) I am not yet convinced that this is a true 'unicolor'-like specimen.

For 'unicolor's also see images linked on http://www.bioima...R38418.HTM.