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A unique photo, I have not seen the like before.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
I, too have never seen this before.
Clearly the Brimstone had found the optimum basking position and the Bee-fly's instinct told it that it was better than on a leaf. Perhaps the butterfly had to tolerate it as it was not yet warm enough to fly off.
What an intriguing observation - thanks for posting it.
Entomologist and biological recorder
Lat/Lng: 52.602, -1.181
OS grid ref: SK555007
Uni of Leic Attenboro Arb, Knighton, Leicester.