Common in most of France, sometimes occurring in thousands, but it can sometimes be difficult to differentiate from the Idas Blue (P. idas). The argus upperside usually has a broader black border (clear in 12058) than idas, often with marginal black uph spots. My feeling is that the unh black interiors to the marginal lunules are quite pointed for idas and more rounded for argus. However, the Lafranchis ID book describes both argus and idas as "sagittate". It also says that the argus unf spots are more arched, having a "?" shape, than idas - this is clear in 13647 and 16859 although not really so in 21405. In my experience, argus is fairly consistent in terms of size and noticeably smaller than idas, but this would only be a guide at best - the Lafranchis France book gives the argus wingspan at 10-16mm and idas 11-16mm. Argus also seems more rounded, whereas idas seems to me to be slightly more angular.

According to the leading books (see the introductory page), argus can be identified (with respect to idas) by the presence of a small forward-pointing spine at the base of the foreleg tibia. H&R shows this clearly (page 277), the tibia being the middle of the three leg sections. However, you would need to be very close to see it, or take a close-up photo with high resolution. When I looked into this, it did not initially seem obvious to me. I looked at the expanded photos of 2756 and 3364 above, neither of which were actually the spine in question. This is a summary of my path to the correct realisation:

1) does the spine occur on the male only? T&L, Lafranchis France, Lafranchis ID and H&R all say "male". The enlargement of 2756 (above) shows a very strong nail-like backward-pointing spine on the mid-leg. This is not the spine in question, so its appearance on females does not contradict the books. Both sexes of both argus and idas have this spine.

2) which leg does it occur on? They have three pairs of legs. The spine on 2756 and the smaller backward-pointing spine on 3364 are clearly on the mid-leg. The spine on 3364 occurs on both argus and idas (as shown here) and is not the correct spine.

3) several authors refer to it as a "hook". When I looked at the close-up of 11898 above I could clearly see a curved spine on the fore-leg (indicated by the blue line), which was closer to the description as a hook. However, it was not forward-pointing and is not the spine in question, as the book illustrations in H&R and Lafranchis ID show it pointing forward.

So all three of these spines turn out to be red herrings with regard to the definitive spine used to ID argus with 100% confidence. The actual spine in question may just be the one barely visible on the close-up of 11898, thin and black and laying parallel to the leg (as indicated by the white line), but I am not 100% certain of this. Clearly, it is hardly visible when the lower two leg sections are parallel to each other, and would become much more apparent if these sections were at an angle. Whether it would be possible to get a clear photo of the spine in the field without causing distress (by netting/bottling), I do not know and I am personally not prepared to ID argus in this way.

My conclusion is that if the spine is this obscure i.e. barely visible even using cameras with powerful macro lenses, it is not of much value in a field guide. It is really only of value if the specimen is caught and examined under a microscope or strong lens, in much the same way as the genitalia. My views on this are that only accredited scientists and entomologists who NEED to know the ID should be catching and examining specimens in a way that may cause distress if not death. I exclude netting, examining and releasing which, if done with care, does not cause distress.

My thanks to Olli Vesikko for clarifying the spine question and in showing me a photo of the actual spine on a real argus.

observations

a very light border for male argus. Where the uph border is light, it tends to break up into black spots rather than a smooth border.

a more typical broad male argus border.

a typical male, strong blue colour, wide dark upf border and the uph border breaking up into discrete round marginal spots.

a very lightly marked female, with a narrow forewing.

a well-marked female, with strong arched orange lunules on both wings and a spattering of blue scales on the uph.

a less well-marked female with rather indistinct and faint orange lunules on the upf.

a typical male underside, pale silvery-grey ground colour and rounded black edging to the unh orange lunules.

a male, puddling.

a strongly-marked female underside, with good arching of the unf post-discal spots. The strong brown ground colour sets off the white areas inside the unh lunules.

I originally had this on the idas page but was never entirely convinced that this mating pair was idas rather than argus, and a comment from an expert has persuaded me to move it to the argus page. The wings looked rounded, suggesting argus, and the black chevrons were not clearly sagittate (which would indicate idas), especially in the male (on the left). The female is quite fresh, much fresher than the male as is usual in any mating pair.