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Thought I would continue the discussion here. To sum up so far: I think it is possible to i/d trivittaus from above (without seeing the 'face' or more accurately facial stripe) from the colour of the frons just above the antennal insertion.
Here is an excellent photo showing the feature: http://www.diptera.info/photogallery.php?photo_id=7849.
The Falk plate shows this yellow area quite clearly from above. I think it is just a continuation of the facial stripe above the antennal insertion - maybe?
You can't always see this clearly in photos posted here, but if this is black it is usually obvious and I think rules out trivitattus. If it looks vaguely pale buff or yellow it might be just hidden in the 'dusting'.
Here is an interesting EOL photo labelled H. pendulus, which I think is actually trivittatus: http://media.eol.org/content/2010/03/24/05/88154_orig.jpg
There is one exception I have found so far: when the facial stripe is reddish-brown (as stated happens occasionally in Stubbs & Falk) this does continue to give a brownish insertion. The face is just as confusing in that case.
Why is this important? I think it is a good 'field character' that has not been mentioned elsewhere. The accepted wisdom seems to be that you can't i/d trivittatus without seeing the face, while I think this is as good if you can actually see it.