A species of the higher ranges of the Alpes and Pyrénées. The male upperside is a shining pale blue with strong uneven uph black borders sometimes having detached black spots in each space, and with slight chequering of the fringes. T&L shows it as being very pale and with a medium but constant width upf black border. You would have difficulty identifying these upperside photographs as eros from the T&L illustrations.

The underside is very similar to the Common Blue (P. icarus) even to the extent of having a unf cell spot, which other French Polyommatus blues do not have (excluding the Lysandra species which were - temporarily - reclassified as Polyommatus). The ground colour is usually a cold clean mid-grey.

In the past few years I have found eros at a number of locations, almost all males and nearly always puddling. Only at one location did I see them in good numbers. I have only seen female eros on four occasions.

observations

a male, with a black upf border of neatly consistent width and a uph border just about breaking up into discrete black spots. It has a Safflower Skipper (Pyrgus carthami) for company.

a male, not unlike 22088 but rather lighter in colour, with a Glandon Blue (Plebejus glandon) for company.

a rarely-seen female, only ID'd for certain because I had seen the underside, at least, that's what my notes said at the time I saw it. Male eros were flying within a few metres and no escheri were seen at that location. Since then, I have received comments from a French lepidopterist who is confident it is a female Escher's Blue (P. escheri) for the following reasons:

1. there does not appear to be a cell spot on the unf. It is very difficult to see from this angle, but it appears this is so, in which case escheri is very much more likely.

2. the curvature of the wing shape points to escheri.

3. the colours are "warm", pointing to escheri whereas eros tends to have "cold" colours.

4. the lunules are arched in the normal manner for escheri.

5. it appears that the plant it is on is Astragalus monspessulanus, the larval hostplant of escheri, from what little I can see, and if this is true, then maybe this is compelling evidence.

However, on a subsequent visit to the UK Natural History Museum to study the archives for eros and escheri, 17682 looked identical to the female eros, and all the female escheri were brown with no blue scales and a full set of orange lunules. This mirrored the illustrations in T&L. No female escheri I have ever seen - and I have also seen quite a number since 2009 - has had any blue scales. I also saw, in 2016, 41637, which looks more or less identical to 17682, and this clearly has a unf cell spot. I feel this is fairly conclusive evidence that 17682 is eros. 

a male, typically grey and with rather heavy internal edging to the unf marginal lunules.

a male underside, very similar to icarus, but more lightly marked at the margins.

a male underside, slightly browner than 13216 but still quite a "cold" grey.