It's such wonderful warm weather in the UK at the moment, I thought it was time to celebrate with another butterfly post! I particularly wanted to take a closer look at the butterfly Phengaris arion which is rather unimaginatively known more commonly as the Large Blue.

Unfortunately the Large Blue went extinct in the UK in 1979. It's been successfully introduced in south-west England from Sweden but I've never seen one myself. In order for reintroduction to be successful, the butterfly doesn't just need the correct habitat and foodstuffs to flourish, it also needs the presence of a species of red ant called Myrmica sabuleti.

The butterflies need the ants as they use them for free childcare. The Large Blue caterpillars are neither large nor blue but instead closely mimic the appearance of the red ant larvae. They are also able to mimic the sounds of the ant larvae, and secrete chemicals that make the ants believe that the caterpillar is one of their offspring. As a consequence, the ants will carry the caterpillars back into their nest. Some species of blue butterfly mimic ant larvae so well that the ants will bring them food and care for them until they are ready to pupate and turn into a chrysalis. The Large Blue is slightly less sophisticated. Once inside the caterpillar hangs out around the outskirts of the nest, where it is protected from predators, occasionally venturing into the centre of the nest to feast on ant larvae. It is also able to mimic the sound of the queen ant to prevent it being detected.

Despite being protected from external predators in the safety of the ant nest, this strategy is not a perfect one for survival. The relationship is highly species specific; if the caterpillar is picked up by the wrong species of red ant it is likely to be killed and eaten inside the nest. Even with the correct host there is an incredibly strong selective pressure towards perfect mimicry, as any caterpillars that make the ants suspicious will be identified as intruders and killed. This becomes more likely as the caterpillar grows, as it ends up far larger than the ant larvae it is trying to mimic.

Research indicates that 230 large larvae and a minimum of 354 Myrmica workers are needed to ensure the survival of one Large Blue butterfly! As these are far more than will be found in one nest, it's likely that the caterpillar can move between ant nests, surviving starvation as it travels. Knowledge and research such as this is vital for conservation efforts, as it gives a better idea of the necessary conditions to introduce endangered and extinct species. A healthy large blue population requires a healthy ant population, and in turn will lead to healthier and more diverse ecosystems.


Previous butterfly posts: The wall butterfly, Four legs vs. six legs, Multigenerational migrations

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