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Thanks Tristram. I have the 2002 version of Gregor et al.'s "The Muscidae (Diptera) of Central Europe" where the key also points to H.vespertina having halteres with a yellow knob. However, the text (where I previously missed the description of H. vespertina's halteres) says "knob of halteres dark brown to black" (page155). I assume that was changed by the time the 2016 version of the manual was published.
Thanks again!

Steve