The wings of the mature male are a very dark blue, almost black, where the female’s wings are an iridescent green-brown with a false white spot near the tips. The male’s body is a metallic blue-green, and the female’s metallic green with a bronze-tipped abdomen. Body length up to 50mm. Forewing 40mm.
The males are very territorial, and perch on vegetation as look-outs warding off unwelcome visitors. The females can lay up to 300 eggs at a time in emergent or floating vegetation. The larvae develop for over two years before emerging as adults.
It flies May to August, and it is found near slow-flowing streams and rivers, in farmland and woodland. Common and widespread in the south-west of England and Wales and southern Ireland, but absent or local in the rest of England, Scotland, and northern Ireland.