The Large blue female lays her eggs on a flower of wild thyme. The first instar caterpillar burrows into a floret and begins to eat all parts of the flower. When the caterpillar reaches its fourth instar it drops to the ground and waits to be found by a passing red ant (Myrmica sabuleti). The caterpillar mimics the chemistry and behaviour of an ant grub. The red ant is deceived, picks it up, takes it to its nest and places it in the brood-chamber. The once herbivorous caterpillar becomes a carnivore and begins to eat the ant brood.
The caterpillar spends the winter underground and resumes feeding in the early spring before becoming a chrysalis. The newly emerged butterfly must climb through the tunnels of the ant nest before emerging into the daylight, inflates its wings and flies off in search of a mate. The life-cycle then starts all over again!