The Willow Emerald Damselfly (Chalcolestes viridis, formerly Lestes viridis) is a species of damselfly in the family Lestidae, the Spreadwings.
Spreadwings hold their wings at about 45 degrees to the body when resting. 
This distinguishes them from most other species of damselflies which hold the wings along, and parallel to, the body when at rest.
The Willow Emerald Damselfly can be distinguised from the other Spreadwings (Lestes species) by its yellow/brownish pterostigma (wingspot) and by having a little green marking in the shape of "thorn" on the side of its thorax.

Haff Réimech Nature Reserve, Luxembourg, 13 August 2024