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Hi,

Melanostoma dubium is a synonym of Melanostoma certum, and as far as I know a montane species. Furthermore, Melanostoma species always have yellow markings on the abdomen, unless of course they are melanistic, but then they wouldn't show these greyish dusted spots.
This is indeed a male P. ambiguus, easliy recognized by the "bloated" dusted face and the unmodified foretibia and -tarsi.

Best, Jonne