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Thanks Tony :D
I have since seen another photo of the same fly, and it was clear that the apparent proboscis was just a bit of shadow split by the left front leg.
That settles that.
However, I will try to see whether I can visit the spot and see any signs of frequent fly activity about the frogs. It might be a challenging task though. becuse I know that species of frog, and they have nasty suspicious minds and prefer deepish water. :)