This is the large blue, Phengaris arion.

It is a species of butterfly in the family Lycaenidae, or, the gossamer-winged butterflies. It is the largest member of this family, with a wingspan of 5 centimeters.

The upperside of its wings are a light, metallic blue, with black edging, and a row of black spots. The underside is highly distinctive - a multitude of black dots on a background of dusky grey which shifts to a bright, powder blue towards the base.

The large blue could historically be found throughout Europe and Asia, but has disappeared from much of its former range. It is one of the rarest known butterflies.
It has a complex life cycle, which has only become clear in the last few decades, and has made conservation efforts difficult.

After hatching, the first 3 of its 4 larval stages progress rapidly, with the caterpillar feeding on various species of wild thyme, but growing little in size. At the start of the fourth stage, the caterpillar stops eating its host plant, and drops to the ground.
It then attempts to attract one kind of red ant, Myrmica sabuleti, by secreting a sugar-rich liquid. As the ant is pacified by the liquid, the caterpillar swells its thoracic segments, creating a specific turgidity which tricks the ant into identifying it as an escaped member of its own brood. The ant dutifully carries the caterpillar into its nest.
Once inside, the caterpillar quickly employs a mix of acoustic and chemical mimicry to avoid detection. In many species of ants, queens rub their legs together to produce sounds which signal her dominance over the nest. The caterpillar is able to emulate this sound, prompting subservience in the worker ants. They even defend the caterpillar if the nest comes under attack.

It also camouflages itself with the nest’s unique combination of odours. Some of these scents are synthesised by the caterpillar in special organs, others are acquired through direct contact with the ants. It then seeks out a rarely visited, peripheral cell of the nest, which will be its lair for the next 10 months.

From its hiding place, it makes frequent trips to hunt for young ants, which it carries to its den and devours. The caterpillar grows slowly in size, and after 9 months lurking in the nest it is ready to pupate. A few weeks later, it emerges as a butterfly. It fights its way out of the nest, and takes flight. Once on the wing, the butterfly has just a few days to find a mate and lay eggs before it dies.

At each stage of the large blue’s development, therefore, it relies on very specific conditions. Fortunately, these stages are now well understood by scientists. Habitats have been effectively restored in many areas by ensuring the presence of both host plant and red ant species in grazed pastures. With meticulous planning, the species has even been successfully reintroduced to some regions where it had previously gone extinct.

Most insect species are not so lucky. Insects are the least-studied animal group - 80% of them have not even been named. 40% are at risk of extinction. While the beautiful large blue and its fascinating life cycle have attracted much scientific attention, many other species are disappearing without ever being noticed.