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With respect to Tephritidae i have come across two other explanations of wing swaying (all these expl do not exclude each other):

- spreading of pheromones into the air

- deterring jumping spiders. These spiders themselves have stripes on their front legs which they sway in mating behaviour.
i don't have the refs at hand now, but i'll see if can look sth up later. VCheers, Louis