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The form of light reflected or diffracted from the fly would be in part determined by intermittently bright speckled sunlight. Colour response by the camera at low light intensities is not necessarily accurate. That being said, I’m not quite ready to give up on the idea that, in the darker image, parts of the fly are actively glowing.
Thanks for the ID. I'll see if the fly turns up again
Lat/Lng: 51.4337, -2.7624
OS grid ref: ST470707