Our largest blue, both sexes have large black spots on the face side of the forewings, which are more pronounced in females and may appear to merge into broad stripes. For its development, it is crucially dependent on the presence of Wild Thyme and of ants from the genus Myrmica. This is because it completes its larval stage inside anthills. The caterpillar pays rent to the ants in the form of a mixture of sugars and amino acids, which it excretes from its body. The species has been on the decline all across Europe.