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Thanks for the confirmation. This would be very impossible in dutch waters, hence my question.
Adult Tabanids are not aquatic insects. This is the first report known to me of such behavior. It also makes little sense to me. My best guess that we are looking at the result of some kind of accident and / or dying female. Being in very cold water would make it impossible to function normally. The white spots on the wing might be drops of air, but they are definitely not used for breathing, if that would be suggested.

First time for everything, I guess !

Theo