This species seems to be quite widespread in France. I have seen arion in a number of places in southern France, often at high altitudes such as 1800m, but never more than one or two at each location. The underside of fresh specimens is quite breathtakingly silvery blue and it is noticeably larger and darker than most blues, making it quite easy to spot in flight. Unusually, nearly all the specimens I have seen have been females, often egg-laying. High altitude forms are often quite dark, of the form obscura, where the dark upf spots extend and sometimes merge with the dark margins. There are also intermediate forms.

The larval hostplant is various species of Thyme, principally Large Thyme (Thymus pulegioides).

On a related theme, arion has been very successfully re-introduced into the UK using Swedish stock, with 10,000 individuals reportedly flying in 2006, and still doing well as at 2016; UK Butterfly Conservation are to be congratulated on this achievement, especially as arion has a larval stage that is entirely dependent on a complex relationship with ants of the Myrmica genus. In short, no ants, no arion. This was the big breakthrough - manage the habitat for the ants, not for arion or its larval hostplant.

This species was previously known as Maculinea arion.

observations

a rare opportunity to get a shot of a male upperside.

a male, fairly typical arion in terms of markings. At least I have assumed 12817 to be female based on the colouring, but the body length suggests male, so maybe the body length is a more reliable indicator of sex than the colouring. However, on studying the magnified image, the end of the foreleg appears to be hooked, which confirms that this is a male.