Paul writes: Demoiselles are large damselflies and are easily told from other species by their coloured wings. There are two species in Britain, the Beautiful Demoiselle (Calopteryx virgo) and the Banded Demoiselle (Calopteryx splendens). The Beautiful Demoiselle, especially this male, is exactly what its name suggests. It is best seen on a sunny day; in flight with its iridescent blue-green wings lit up by the sunshine.
They are inhabitants of fast flowing rivers and streams. Males are territorial so will find a perch on some bankside vegetation and wait for passing insects to catch or a female to chase. They often return to a favourite spot, so once located are reasonably easy to observe. They are quite common in south west England and Wales, elsewhere they are localised. In North Yorkshire they occur in several locations. I am lucky that there is a small colony on the River Hartford, not far from my home, which is where I saw this one, along with several others.