John Curd recently published this very interesting article (edited) and chart on how to identify Willow Emerald Damselflies:

Willow Emerald Identification Chart

Recently, there have been a few posts concerning Willow Emerald Damselflies (Chalcolestes viridis). From a probable bridgehead near Ipswich in 2009, they have become established and are inexorably spreading across the country, firstly filling Suffolk whilst spreading into Kent and Norfolk. Last year (I believe) they reached Surrey. Within the last week or two, they have been recorded for the first time in my own Bedfordshire and, I think, Lincolnshire.

Some of the recent posts have indicated a bit of uncertainty about identifying them – not unnaturally, since they are less familiar to many of us. Since more and more of us will begin encountering them, though, given their successful colonization, I thought I’d put some identification clues together compared to the more familiar and widespread Emerald Damselfly (Lestes sponsa).

Some talk centred around the colour of the pterostigmas, which is a good clue. The most usual distinguishing feature, though, is the very distinctive so-called “spur”, forward-pointing on the side of the thorax. That works for both sexes. Think of the Willow as being “a thorn in the side”. 😉

Once sexed, there are also useful differences to be seen in the genitalia of both sexes. Again, once sexed, male eye colour is also a good feature.

The so-called Scarce Emerald/Robust Spreadwing (L. dryas), is restricted to the south-east of Great Britain, largely East Anglia, with other populations in Ireland. It is very habitat specific. Its range actually appears to be shrinking, perhaps because of habitat change. Since 1991. its former northernmost, westernmost and southernmost populations on GB have apparently disappeared. [See the BDS Atlas]. L. dryas may be readily confused with L. sponsa.

Much more restricted – again very habitat specific – is another recent colonist, the Southern Emerald/Migrant Spreadwing (L. barbarus). It almost cerrtainly has an established colony at Cliffe in Kent [BDS Atlas again], where it has been recorded yearly since 2010, and perhaps Winterton Dunes in Norfolk. IMHO, L. barbarus is very distinctive – see its antehumerals, rear of the eye (the so-called “jowels”) and pterostigmas