A large blue caterpillar feeds on wild thyme or marjoram flowers for around three weeks, then leaves the plant and waits on the ground. Foraging Myrmica ants find it, and the larva secretes a sugary substance and carries a cuticular chemical profile close enough to the ants' own that workers pick it up and carry it into the nest. Once inside, it becomes a predator, consuming ant larvae through autumn, winter and spring before pupating and emerging the following summer.
The deception extends to sound. Large blue larvae and pupae produce calls resembling those of the ant queen, and worker ants respond to them with the attendance normally given to a queen. This raises the caterpillar's standing within the colony above that of ordinary brood.
The association is narrow. In Britain the successful host is Myrmica sabuleti, which requires very short turf to keep the soil warm. When grazing declined and myxomatosis destroyed rabbit populations, the sward grew, the ant was replaced by cooler-tolerant congeners, and the butterfly died out in 1979. Reintroduction succeeded only after grazing was restored.