Which butterfly species is known as the large blue?

Phengaris arion is the butterfly species known as the large blue.

This blue butterfly is found across parts of Europe and Asia, but its life cycle is unusually dependent on ants. Young caterpillars initially feed on thyme or oregano. Later, they leave the plant and produce chemical and acoustic signals that can cause red ants of the genus Myrmica to carry them into an underground nest.

Inside the nest, the caterpillar survives by receiving food from the ants and by consuming ant brood. It eventually pupates there before emerging as an adult butterfly. This relationship is a specialized form of social parasitism, not ordinary pollination or simple sheltering.

The species suffered serious declines in parts of Europe when traditional grassland management changed. Conservation programs have restored suitable habitat and the ant communities required for its complex life cycle. The name “large blue” can be confusing because the species is not necessarily the largest blue butterfly everywhere it occurs.