Biology

The large blue butterfly emerges in late June or early July and is on the wing for three to four weeks. The females lay their eggs on wild thyme (Thymus serpyllum) and the caterpillars feed on the soft tissue of the flowers. In common with many other blue butterflies the caterpillar then enters a fascinating and, until fairly recently, little understood phase of its life-cycle. After moulting, the caterpillar drops to the ground and waits for the particular species of red ant to find it. This ant, (Myrmica sabuleti), is attracted by a gland on the caterpillar that secretes a sweet liquid. After the ant has fed on this liquid for some four hours the caterpillar inflates the skin behind its head, mimicking the behaviour of an ant grub. The ant, encouraged by the caterpillar's mimicry, takes it underground to its nest and places it amongst the ant colony's own brood. Here, the caterpillar eats the ant grubs.The blue butterfly caterpillar hibernates in the ant's nest. It then pupates just beneath the surface and it emerges as an adult butterfly in June. Whilst in the ant's nest the caterpillar may eat as many as 500 ant grubs.