The Large Blue

The 1.5-Centimeter Catastrophe: How Science Solved Britain’s Strangest Extinction

On a sun-drenched hillside in Devon in the summer of 1979, a small band of conservationists stood in a field they had spent years defending. They were waiting for a butterfly that never arrived. For a century, the Large Blue had been vanishing from one English hillside after another, and the consensus of the era was clear: the culprit was the collector. To save the species, this last stronghold had been fenced, the gates locked, and volunteers had patrolled the boundaries through every flight season to ward off any man with a net or a killing jar.

The defenses worked perfectly. No collector reached the colony. Yet, the butterfly went extinct anyway. That summer, the Large Blue was declared gone from Britain, vanishing from the chalk downs and limestone banks it had inhabited for millennia. The people who guarded it were left baffled. They had followed the best science of their day, yet they had failed because the answer was buried a few centimetres beneath their boots, in a subterranean chamber they had never thought to open.

The Butterfly is a "Wolf in Sheep's Clothing"

To understand why the Large Blue vanished, one must first realise that it is barely a butterfly at all. For the vast majority of its existence, it is a specialised subterranean predator.

The life cycle begins with deceptive simplicity: in late June, adults fly over short grassland, and females lay eggs on the flower buds of wild thyme. For three weeks, the caterpillar behaves like a model insect, eating flowers. But once it reaches the size of a grain of rice, it drops to the ground and waits. It is waiting for a red ant of the genus Myrmica to find it. Through a complex chemical deception, the caterpillar’s skin secretes a signature that mimics the smell of ant brood. Tricked by the scent, a worker ant concludes it has found a lost larva and carries the predator home, down into the heart of the brood chamber.

"The large blue is the biggest of the British blues... about the size of a postage stamp, dusted a smoky violet with a row of elongated black spots along the forewing that no other British butterfly carries."

Once inside, the caterpillar does not eat plants again. For ten months, it consumes the ants' own offspring, growing enormous on a diet of the host’s children. It is a staggering biological gamble—an improbable, high-stakes existence that leaves the butterfly entirely dependent on the internal sociology of the ant colony it infiltrates.

It Doesn't Just Mimic Ants; It Rules Them

Research by Jeremy Thomas and an international team revealed that the deception goes far deeper than mere scent. They discovered that Myrmica ants communicate through "stridulation"—rubbing body parts together to produce faint pulses of vibration.

Crucially, queen ants have a distinct acoustic call that commands heightened attentiveness and protection from workers. The Large Blue caterpillar has evolved to reproduce this specific royal signal. It has smelled its way through the door and then sung its way to the throne. In a disturbed nest, workers will carry the caterpillar to safety before their own brood. If food runs short, the impostor is fed first while the colony starves. However, this extreme specialisation makes the species fatally vulnerable: its survival depends on the presence of one very specific host.

The "Visible Culprit" Was a Distraction

For decades, conservationists focused on the most obvious threats, assuming that if they stopped people from catching the butterflies, the populations would stabilise. This focus on "visible culprits" proved to be a catastrophic distraction from the reality of the landscape.

• What people thought was the problem: Collectors with nets, pesticides, and the ploughing of meadows.

• What the actual problem was: Soil temperature and the specific height of the grass.

The irony of the 1979 extinction is that the guards were looking upward for humans with nets, while the real tragedy was unfolding in the dark. The "fingerprints" of the disaster were invisible because they didn't look like damage; they looked like a quiet, ungrazed nature reserve.

The 1.5cm Death Sentence

The breakthrough came from the meticulous field ecology of Jeremy Thomas, who refused to rely on assumptions.

"Thomas... spent six consecutive summers [on Dartmoor]. He did not sample, he did not estimate; he found the eggs on the thyme and he marked them and he followed them—more than 1,300 of them, one at a time, through their lives... it was almost unbearably patient work."

He discovered that while four species of Myrmica ants would pick up the caterpillars, the butterflies could only survive to adulthood in the nests of Myrmica sabuleti. In any other nest, the deception would eventually fail, and the ants would kill the intruder.

Myrmica sabuleti is a sun-loving ant that requires bare, sun-warmed soil to keep its brood chambers warm. It is a fragile dominance; the ant is outcompeted by its cooler-tolerant relatives the moment the ground goes into shade. Thomas found that Sabuleti needed the grass kept at a height of roughly 1.5 cm—the depth of a thumbnail. If the grass grows even slightly taller, the soil temperature drops by a few degrees. The necessary ants quietly retreat, replaced by nearly identical red ants that the caterpillars cannot fool. The thyme remains, the butterflies still lay eggs, and the ants still carry them underground—but every caterpillar is killed in the dark. The colony fails in silence.

The Paradox of "Protective" Fencing

The extinction was the result of human decisions with the fingerprints wiped off. First, the arrival of myxomatosis in the 1950s decimated the rabbit population, removing the "mowing machines" that had kept the turf short for centuries. Second, the conservationists inadvertently sealed the butterfly’s fate.

By fencing off reserves to keep collectors out, they also kept livestock out. Without sheep or cattle to graze the grass, the sward thickened, the soil cooled, and the competitive exclusion of M. sabuleti began. The very act of "shutting the gate" to protect the butterfly was the mechanism of its extinction.

Evolution in Real-Time

Because the British genetic stock was gone, the recovery began with "immigrant" butterflies from the Swedish island of Öland. It was not a simple "set and forget" reintroduction; a fourth release into the Cotswolds failed because the climatic difference was too great, proving that you cannot simply put animals back without perfect conditions.

Success required a "grazing prescription": bringing back cattle and ponies to hold the turf at the precise height the ant needs. By 2006, 10,000 adults were flying across 11 sites. Today, Britain holds the world stronghold for the species. Remarkably, the butterflies are adapting to the modern landscape. Within 12 generations, the population produced a more mobile phenotype, dispersing twice as far as their ancestors to cross the "gaps" of fragmented farmland between habitats.

However, this is a "gardened" nature. This recovery is an "island of success in a falling sea," and it is not self-sustaining. If the grazing stops, the sward closes, and the butterfly vanishes again.

Understanding vs. Protection

The story of the Large Blue is a humbling lesson in the limits of protection without understanding. Even today, the work is never finished. We now know that the "carrying capacity" can vary 100-fold between two sites that look identical to a human walker, and climate change is already shifting the target grass height, making old management plans obsolete.

We saved the Large Blue not by building a fence, but by understanding a centimetre of grass and a two-degree shift in soil temperature. It leaves us with a sobering question: as we look out at our dwindling wild spaces, what other 100-fold variations and invisible ecological shifts are we missing because we are looking for dramatic damage, rather than the subtle, silent retreat of a single ant?