Osiris is very similar to Small Blue (C. minimus) based on the underside alone. Details of the differences are given on the minimus page. It is probably much easier to differentiate them in the field where the blue upperside of the male osiris is clearly apparent, and I believe the females can be identified by the visible uppersides in flight as well. Osiris seems to be closely tied to its larval hostplant Sainfoin (Onobrychis viciifolia), especially the female, so if it's sitting on Sainfoin, the odds are that it is osiris. The converse is also true. The theory about the series of four unh post-discal spots being in a straight line in osiris seems only to hold true for the female in my experience. The males can vary significantly, as indicated by the comments on the specific individuals below.

Osiris is considered to be uncommon, but I am coming to the conclusion, after six seasons in Var, that it is more widespread than is generally reported in the books, although in low densities. I suspect it is often missed as it is very similar to several other small bright blues, notably Chapman's Blue (Polyommatus thersites), which has a similar deep blue colouring and well-defined black border, and perhaps also to Common Blue (P. icarus). It is about the same size as thersites and icarus, maybe sometimes slightly smaller. It is an early season butterfly in Var, and I have not seen it later than about the first week in June, although it does have a second brood in other areas.

observations

quite a bright deepish blue that could easily be confused with thersites, as noted above.

a courting pair, male above. The female is the same as in 10264/10280, but I included the photo because it captured both sexes in the same shot.

a sprinkling of blue scales on the basal area of the forewing.

very fresh and appealing with nice basal blue flush. The unh series of four spots: the fourth spot in s2 is missing, not unusual, and the third spot in s3 is vestigial. The unf series is nice and straight, a good indicator of osiris - but not definitively: see the minimus page.

a very clean underside with a strong basal blue flush. As noted above, there are usually four post-discal spots in a straight line, but for some reason this specimen only has the top two in s4 and s5, with no sign of even vestigial lower spots in s2 and s3. The just-visible blue of the uph confirms osiris, rather than minimus.

a typical osiris, except that the fourth post-discal spot is clearly displaced internally.

the underside of 10264.

a female, egg-laying on Sainfoin.