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Looking at this photo on a better screen, I think I favour the equally-matched proboscis-fighting explanation, though I suspect they are actually Desmometopa. I suppose that any small fly that spends its time stealing from spiders is likely to be a bit prone to physical confrontation - stress and so on. ;)
In any case it's a remarkable picture. B) Is there any way we can get to download the film? - it would be wonderful to see the action!