Large Blue (Maculinea arion)
Swedish name: Storfläckig blåvinge.
Here this female is laying her egg on the flower of the larval host plant, Wild Thyme.
But after it hatches, the little larvae will only feed on the flower for a short time, as it secrets a sugary substance to attract a species of ant (Myrmica sabuleti). The ant then takes it to its nest where the larva grows by feeding on the ant grubs. At the same time it continues to secrets the sugary substance and smell like its host ants. In other words this beautiful butterly is a parasite!
This butterly is rare in most Europe and is dependent on its host ant species as well as the presence of wild thyme. It is red listed both in Sweden and the rest of Europe.

This photo was taken near my home town, Åhus, that is still blessed by the presence of this butterfly. You can see more images of this species in my Swedish Butterflies gallery