As Southern Damselfly "Coenagrion mercuriale" has not appeared on this thread yet, I thought I`d add it.
Southern is the smallest of the blue Damselflies.
It is found on small steams & flushes in southern England, south Wales & Anglesey.
The male has a "Mercury" mark on S2, and spear marks on S3 & S4.
The female,though a blue form exists, normally has an abdomen which is dull green below & mainly black above,with a Thistle shape on S2, there are blue divisions between the last few segments ( Distinct from Small Red "Melanogastrum" form which has red divisions.