The shape of the hind margin of the pronotum is diagnostic in blue damselflies. The illustrations below are of male insects however the shape is similar in both sexes.
| Common Blue Damselfly|
The hind margin has a flat, shallow central lobe
The hind margin is broad and flattened
The hind margin has a prominent, narrow central lobe
| Irish Damselfly
The hind margin has a distinct raised lobe
|Nelson, B., Thompson, R. & Morrow, C., 2000 (August 28). [In] DragonflyIreland http://www.ulstermuseum.org.uk/dragonflyireland/|