This is one of the most attractive blues, probably the most. But someone once asked a famous expert what was his favourite butterfly. His answer "whichever one I'm looking at that moment".

It is distinctly larger than most blues, which is quite apparent when they are on the wing. The male upperside is a silver-greyish blue, often, but not always, with a darker upf border of broadly consistent width (not borne out by these photographs, though). The female upperside is brown often with a near-full set of uph orange lunules and sometimes some vestigial ones on the upf, although not always so. The male underside is quite characteristic and cannot really be confused with any other species. The female underside is similar with a consistent rich creamy brown colour and the same strong unh lunules and black post-discal series of spots often touching, and usually forming a clear right-angle.

It is a spring butterfly in my experience, emerging at the end of May and not being apparent after early June except at higher altitudes. It is not common but usually seen at medium altitudes of around 1000-1300m, although I had found colonies (which never comprise more than a few individuals) at two locations at around 400m, which is just about the lower end of its altitude range. However, these lowland colonies have since been wiped out by horse grazing.

It is strongly attached to its larval hostplant, Tufted Vetch (Vicia cracca) as indicated on several of the above photographs.

observations

a male with a dark border which is tapering from the apex, not of constant width.

another beautifully fresh gleaming blue male.

a male, quite typical.

a female, unmistakeable from other female blues by virtue of its size.

a male, roosting for the night on V. cracca. It is actually on the flimsy flowers, not on the stalks or leaves, and this is the actual angle of resting, so can someone please explain how it does not fall off?

a male, puddling. The photograph has been rotated 90 degrees anti-clockwise to fit the frame.

a male taking salts from a manure heap, with quite a light silvery-grey ground colour. It was one of about twenty males in this locality, by far the most I have ever seen in one place.

a very strongly marked female underside. Quite breathtakingly beautiful. Sitting, as 10674, on V. cracca flowers.

a mating pair, obligingly stationary for a photograph and nicely in the same plane so that depth of field was not a problem. The male is above. It is a pity that a bird had taken a chunk out of the female's forewing tip.