Blue Emperor
AKA
Emperor Dragonfly
Anax imperator
male
Lagoa do Negro,
Gruta Natal,
Terceira Island,
Azores,
19 July 2013

This species is fairly common and widespread all across the island of Terceira at any farm pond, forest area, marshy spot, etc. It is widespread in Europe:
http://en.wikipedia.org/wiki/Anax_imperator
It is one of only three species of odonates found on the island of Terceira and there are only 4 or 5 species found in all the Azores.

It is similar to the North American Common Green Darner but has a darker thorax, a different spot on the top of the frons and blue patches on the top of the thorax just forward of the wing junction.