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I was just alerted to this list and this fly behavior. I agree with the suggestion that data be taken on the orientation of the fly and drop relative to the sun as the thermoregulatory idea may have some traction. I have never seen the behavior, but the photos are reminiscent of behavior of nectar sipping Hymenoptera that regurgitate a droplet of nectar thus exposing it to the air for evaporation of water and concentrating its contents.