Wing span 33-41 mm. Looks like the female of Alcon Blue, but is known for its size, shining blue colour, the black wing-spots and the strong markings on the underside. The spots on the upperside may varie and sometimes lack completely.
Its flying time is in July. The habitat is warm heaths, dunes and pastures. It overwinters as a half-grown caterpillar in nests of the ant species Myrmica sabuleti. The fodderplants of the caterpillar are flower heads of thyme (Thymus) or marjoram (Origanum). When little the caterpillar falls to the ground and is found by an ant, who carries it down in its underground nest. The caterpillar produces a sweet secretion for the ants, while it is eating their eggs and larvaes. The caterpillar transforms into a butterfly in the ant-nest and crawls out from the entrance-hole without any attacks from the ants.
The flight of Large Blue is unsettled and low, but can also be quick, high and goal-oriented, if it is scared. It suns itself in bushes and often seeks nectare at thyme, sheepsbit scabious etc.
Large Blue was almost extinct after 1950. Since 1990 seen only in one spot at Møen and a couple of spots in North Jutland. Listed.
Protection of the species: the locality must not be fertilized, and the vegetation has to be kept low by a suitable management. Any intervention in order to protect the species demands the assistance of specialists.
photo 2007 Høvblege, Møn: stig bachmann nielsen, Naturplan Foto