Common Blue Damselfly

The photo below is of an immature (i.e. not yet fully coloured) Common Blue Damselfly, Enallagma cyathigerum

The principle character is the 'lollipop tree' or club shaped mark on the top of the 2nd abdominal segment (learnt that one from Alistair Driver on Sunday's walk in the Thames Valley Nature Reserve) and also segments 8 and 9 are all blue, or very light blue in this immature specimen.