A small crop of a quick photo with the iPhone:
At least it stayed in view while I fetched the Nikon compact camera and managed to take a few more photos. These shots are all small crops from the originals.
I am pretty sure this is a diving beetle going by the size (about 8mm), colour, shape and the length of the back legs. I wonder what it was doing out of the pond. I understand they are good fliers so it probably fancied a change of environment.
