There are many tricky tests out there for the amateur naturalist or the casual wildlife observer and it can occasionally come down to detail. Sometimes you need microscopic examination or even DNA analysis to tell species apart and it becomes a job for the scientist. Blue damselflies are tricky but at least there are clues that mean scientific knowledge is not essential.
The common blue damselfly (Enallagma cyathigerum), the azure damselfly and the southern damselfly are superficially similar. The fact that individuals can vary does not help but, first and foremost, the key difference in appearance is the 'symbol' that occurs on the first segment of the abdomen. The common blue has what is best described as a black silhouette shape of an egg cup whereas the azure damselfly has a profile of a tumbler glass or perhaps better described as the outline of a square without the top bar, just three sides showing. The southern damselfly has a mark a bit like a Viking helmet!
That is all well and good if the insect is at rest and will let you get close enough to see the marking but I am afraid that is rarely the case! It is at that point that the non-specialist is forced to fall back on fundamental principles, where does it live and statistically how many are there and what are the chances of seeing that species as opposed to others. Whilst your record may not be accepted by the county recorder you can still have a pretty good stab at telling them apart. The southern damselfly is rare, even here in the south. Unless you are at a place that is known to support them you will not see one! The common blue likes open areas of water whereas the azure can be found around damp ditches and small weedy ponds. The azure is often more abundant in spring and early summer with the common blue more abundant later. These are clues and they help but you really need to get a look at the vital segment marking to be sure.
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