“Over the next 6 months, it will prey on the defenceless larvae until it’s 100 times its original size.” This undercover carnivore may look harmless – but don’t be fooled

The large blue butterfly (Phengaris arion), the largest and rarest of the UK’s blue butterflies, has a remarkable lifecycle. It relies entirely on a single species of red ant (Myrmica sabuleti) to survive.

In the third episode of the BBC TV series Wild Isles, Grasslands, David Attenborough narrates a chilling chain of events.

“This caterpillar is one of the very few in the world that becomes carnivorous,” he says. Less than 1% of the nearly 200,000 moth and butterfly species are carnivores.

The first trick up this caterpillar’s sleeve is a droplet of honeydew which it releases to lure a worker ant to tend to it. Next, it inflates its body to then release the air, making a sound resembling a distress call of a queen ant. During this, it also douses the ant in intoxicating pheromones.

The worker ant picks up the caterpillar and carries it back to the nest, as if rescuing it from whatever distress it could be experiencing. The other ants accept this new arrival.

“Once inside the nest, the caterpillar can go wherever she likes,” continues Attenborough.

Over the next six months, the caterpillar will prey on the defenceless ant larvae until it’s 100 times its original size. Then, it’ll become a chrysalis, to emerge as an adult large blue butterfly – nearly a year later.

The large blue butterfly became extinct in Britain in 1979, but it has since been reintroduced from Europe – now, the UK has one of the densest populations in the world.

This article Magda Patynska was first published by Discover Wildlife on 29 May 2026. Lead Image: Large blue butterfly upper side. Credit: PJC&Co, CC BY-SA 3.0, via Wikimedia Commons.