The Azure Damselfly (Coenagrion puella) is a species of damselfly found in most of Europe. It is remarkable for its distinctive black and blue colouring.
Adult male Azure damselflies have a head and thorax patterned with blue and black. They have an azure blue abdomen patterned with black markings. The marking on segment two is U-shaped, usually separated from the segment's narrow terminal black band, but sometimes joined to it by a short central bar.
Segments three to five are blue with broader black terminal bands, lacking the forward-pointing projection the upper surface which adult male Common Blue Damselfly has. Segment six has a similar pattern but with more restricted blue and a broader area of black, and segment seven is mostly black, with just a narrow blue area at the base. Segment eight and much of segment nine are sky-blue, forming a noticeable contrasting patch, but there are small dark markings on the rear upperside of segment nine, which adult male Common Blue Damselfly doesn't possess.
Adult female Azure Damselflies have a head and thorax pattern similar to that of the male, but with dull green replacing the blue colour. The abdominal segments are largely black in colour, with narrow pale markings at the junction between each segment.
Mature adults are seen frequently mating and laying eggs. It usually stays close to the vegetation around the pond or lake and flies from May to September.
This common Damselfly looks very like a Common Blue Damselfly, but a close look can distinguish the two. The behaviour is also different - unlike Common Blues, they rarely fly out over large stretches of water. They are not normally as common around August and September, June and July being the peak of their populations.
Description taken from Wikipedia: http://en.wikipedia.org/wiki/Azure_damselfly