Hestesko-vandnymfe (dk)Caballito del Diablo Azure (es)Ljus Lyrflickslända (se)
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One of the commonest bluets in Denmark and Europe, together with Common Bluet.
Imagos perhaps flying a bit later than also common species Variable Bluet which it looks a lot like.
Azure has short black lines on the side of the thorax and the shape of the black marking on S2 is resembling a horseshoe, hence the Danish name. Also it generally does not have a pale bar between the eye-spots, but this feature is variable. The S2 marking is too, but probably to a lesser degree and therefore more reliable for identification purposes. The shoulder (antehumeral) stripe is rather broad but narrows toward the rear end while staying intact, whereas on Variable it often breaks; and looks like an exclamation mark.
The pronotum of the bluets is a diagnostic feature which among females often is the best way to separate them: Azure has a gently sloping curve with a wide central lobe.
631 (1 photos) 2021-06-07 Store Hareskov, Sjælland, Denmark
Imago. Male. Here an individual with pale bar between the eye-spots. That the S2 marking looks like a horseshoe, may not always be so obvious in photos.
Imago. Male. Don't know if I should be worried when so many of the individuals I id as this species show the pale blue bar between the eyes? The S2 marking looks very good for the species, as does the shoulder stripe, so I guess I have to conclude for now that the eye bar is a very variable character.
Imago. Female. Maybe it is just dirt at the distal part of the abdomen, otherwise a regular female with all black upper side to abdomen and no bar between the eyes.
Imago. Female. Immature, half pale. Assuming this species over Variable. The lack of a pale bar between the eye-spots support this (according to Smallshire and Swash), though I remain to find this character reliable since I've seen many Azure that has it. Perhaps it is population distinct feature, mostly shown by British animals?
Not sure if it is of this species, or Variable. I can't figure out the shape of the hind-border of the pronotum. This supposedly diagnostic feature is often not possible to see on my photos, and perhaps not always possible at all. Perhaps only on adult looking animals?