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Thanks for the idea support Andre, only downside of this idea was the danger of the flies setting themselves on fire from the lensing effect of the drops :D
Only other theory I could think of was that they were aerating their stomach contents to allow aerobic bacterial digestion, but this would not explain the high frequency of observation in cold weather/ bright sun, mind you I'm not sure how much or whether a drop would warm up in the sun.
Brian V.