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This individual shows quite a dark thorax, which is suggestive of Keeled (all blue in Southern), and there are other features which can be useful including the fact that the broader abdomen and shorter pterostigmas of Southern mean that the abdomen is typically broader than the length of the pterostigmas (the same size or narrower in Keeled) - but this feature really needs to be seen from directly above.
The best feature is usually the number of doubled cells above the radial supplementary vein (see here: http://www.birdforum.net/attachment.php?attachmentid=285766&d=1285604457 ).
Keeled typically has no doubled cells (rarely up to four), Southern typically have at least four doubled cells.
In the linked photos a Southern is on the left, Keeled on the right.
Many thanks Roy. As you know I find veins quite difficult, and your link was really useful.
Lat/Lng: 42.5456, 2.8715