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I think I'll retract the Dryinidae suggestion - these swellings are too symetrical. I'd agree with Dima that these are part of the fly - tephritid males are known to use pheromones to attract females, and I suspect that these "balloons" are involved in that activity. There does not appear to be any easily accessed on-line information about this. The most promising Google suggestions all end up at one of those "Pay us some money and we'll let you read the article!" sites. :@
Great pictures from Dima and Ben, though! B)