A colleague in Canada has informed me that most of the Melangyna (Austrosyphus) spp. (at least all the ones around Canberra), have been determined to all be the one species, of which M. (A.) viridiceps (Macquart, 1847) has priority. Not sure when this will be published, but we can probably assume that we don't need to worry about differentiating M. (A.) collatus and M. (A.) damastor.