Large Blue - Azuré du serpolet

This species has an elaborate lifecycle. The larva is parasitic and feeds on the grubs of a red ant, Myrmica sabuleti, on whom its existence depends. From Wikipedia: "Large Blue caterpillars feed on wild thyme or marjoram flowers for the first few days of development. Once well fed, the caterpillar secretes sweet fluids to attract red ants of the species Myrmica sabuleti. The ants then take the caterpillar back to their nest and proceed to feed from these secretions. The caterpillar then hibernates inside the ants' tunnels. On emerging from hibernation, the caterpillar begins to eat the red ants' eggs and larvae, which it does for up to three weeks. It then suspends itself by its legs from the roof of the nest and pupates. The caterpillar spends a further three weeks transforming into the Large Blue butterfly adult. After its change from caterpillar to butterfly, the insect emerges from its chrysalis and leaves the ant nest to find a mate. Usually, red ants will escort the newly emerged butterfly to the surface, taking it to a low plant or shrub nearby. The red ants will encircle the butterfly and ward off any predators that attempt to attack the butterfly as its wings unfold and harden. When the butterfly is ready to fly, the ants will retreat back into their nest".
Identification: a distinctive feature of this species is two distinct rows of underside submarginal spots.