I saw my first Willow Emerald damselfly Chalcolestes viridis last year and was very excited to finally catch up with this recent colonist. There were very few twentieth century records, until it was recorded in numbers from southeast Suffolk during 2009, with outlying sites in southeast Norfolk and north Essex. Since then it has gradually spread, occupying most wetland sites with fringing willows in Norfolk, Kent and Cambridgeshire and it now seems that it's well-established in Lincolnshire, as there were significant numbers flying round the tops of the white willows at Deeping Lake NR this morning.
Because they spend so much time in the tree tops, they're easily overlooked. This one was perched on a dying branch of hawthorn and was a bit lower than the rest, though annoyingly it was angled so that I couldn't get all its body in focus, wherever I tried to stand. It's probably most easily distinguished from Common Emerald damselfly by its behaviour, but it also has a pale strip on the side of the thorax and a pale pterostigma which can be seen in this image.