A quite large and very attractive blue, usually found at altitude, at least that is my experience, although I have also found it at very low altitudes. It seems quite widespread across southern France, but I have never seen more than a few in any one place. As with most blues, the male is encountered more often than the female.

The male upperside is a bright blue, with clearly lighter upf veins around the cell area. The margins of both wings are black, of varying width, and there is quite often uph chequering, sometimes quite pronounced. The male underside ground colour is a beautiful clean pale grey colour with very pronounced submarginal lunules, which are usually large and very pointed, varying in colour from orange to deep brown.

There is no unf cell spot (c.f. Common Blue (P. icarus)).

The female underside is similar but with a clear brown colour, and the spots can sometimes be very large. The less-frequently seen female upperside is brown with a full set of strong orange lunules.
 

This species is referred to as Agrodiaetus escheri in T&L, but is now (and has been for some while) classified as a member of the Polyommatus genus.

observations

a male, showing the strong deep blue, pronounced black margins, and slight uph chequering at the margins.

a male with heavy borders, especially on the upf, and chequering.

a male, with a very pale grey underside ground colour. There are no unf orange submarginal lunules, and the unh lunules are quite brown and very pointed, often the case with escheri but unusually so here.

a male, with a very delicate pale grey colour, making the darker markings stand out.

a male, more typical than 7571 with strong bold markings and a darker grey ground colour. The unh submarginal lunules are darker orange than most Polyommatus species, becoming quite brown as the unf costa is reached.

another example of how beautifully marked the male underside is, especially when as fresh as 16878.

a rather pale male underside, with mid-orange submarginal lunules.