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Yes, John is right: they belong to the subgenus Asemosyrphus of Lejops. The cause of my mistake seems to be that the genus Lejops is apparently (and erroneously) missing in the key to Neotropical syrphid genera of Thompson (1999). I had used this to identify a series of specimens from Mexico in my collection which look very much like the specimens on the picture. As Lejops is not included in the key, they keyed to Dolichogyna...