A steep, tussocky slope, nestling amidst the gentle folds of the Polden Hills, just a mile or so outside of the sleepy Somerset town of Street, seems an unlikely spot to encounter the UK’s rarest butterfly.

But this unassuming scrub-edged site with its distinctive short-cropped turf is the most famous haunt of the Large Blue in the UK.

It was here around 13 years ago that the general public first glimpsed the return of a native. The Large Blue became extinct in the 1970s, but a partnership of scientists and conservation organisations including Butterfly Conservation, successfully re-introduced the butterfly from Swedish stock to a number of top-secret sites in the south west.

After the re-introduction proved successful, Collard Hill, a site owned by the National Trust, was chosen as the first place to unveil the returnee to the public.

Every June and July ever since, thousands of wildlife lovers have made the pilgrimage to this south-facing, sun-blasted hillside to bear witness to a re-birth.

National Trust wildlife adviser Matthew Oates explains: “Above all, Collard Hill epitomises the all-inclusive openness of modern butterflying. Gone is the intense secrecy about localities that prevailed during the collecting era.  Collard Hill welcomes experts and beginners alike, and has become an integral part of the Somerset scene at midsummer, a wildlife equivalent of Glastonbury. It lights up in midsummer.”

The Large Blue is thriving on the site – as it is at a number of other locations in the Poldens and Gloucestershire.

And so it was that I found myself braving the midday sun on a sweltering July day alongside other pilgrims seeking an audience with Maculinea arion. We didn’t have to wait long. Within minutes a patrolling male skittered into view. Duskier than our other blues and with a meandering flight, the butterfly’s beguiling magic was only revealed once it opened its wings – displaying the distinctive inky fingerprint on the top of the forewing.

But what was even more interesting than the butterfly was the weird and wonderful place it had chosen for its home. To the first-time visitor, Collard Hill can appear a little like the surface of an alien planet from a dodgy 1970s sci-fi film. The butterfly’s favourite part of the site is an unforgivingly steep, sun-baked, green moonscape, littered with outlandish humps and adorned with flushes of purest purple.

To understand this landscape is to unlock the secrets of the Large Blue – which has perhaps the most fascinating life cycle of any insect.

The purple haze is the first clue – Wild Thyme is food source for adult and caterpillar alike. The plant’s nectar provides sustenance for patrolling males searching for mates. Females deposit their eggs onto the Wild Thyme’s purple flower buds, which the freshly emerged caterpillars devour with glee.

So far, so normal. But it is at this juncture that typical butterfly behaviour disappears down the looking glass

The caterpillar feasts for several weeks in the flower head. Then, one evening, it abandons the thyme and drops to the ground. The caterpillar chooses this specific time for it is waiting to be adopted by a species of red ant – Myrmica sabuleti that patrols between 5pm and 7pm.

By releasing a chemical scent, the caterpillar tricks the patrolling ants into believing it is actually a lost ant grub. The caterpillar is then carried to the heart of the ant nest. But little do the ants know that they are harbouring a cuckoo. The caterpillar swiftly sets about devouring grubs in the brood chamber and swells many times it original size. Upon pupation the caterpillar may have eaten as many as 1,200 ant grubs. As a chrysalis the Large Blue emits a sound from its abdomen mimicking a queen. The ants, churned up into a protective frenzy by the noise, accompany the emerging butterfly out of the nest and up above ground. The adult Large Blue climbs a grass tussock and waits for it unfurling wings to set - it's subterfuge complete.

The abundance of ant species on Collard Hill is revealed by the lumpy nests, protruding like green footballs from the ground. Although Myrmica sabuleti doesn’t make these nests its presence is revealed by the short cropped, sun drenched slopes.

The ant needs the warm conditions provided by short grass and short grass on south facing hillsides. If the grass is too long, the ground temperature cools and the ants, and in turn the Large Blue, disappear.

Providing tightly grazed, south facing slopes planted with plenty of Wild Thyme was key in returning the ant to the landscape and the Large Blue to the UK.

The Large Blue is a conservation success story to celebrate. The butterfly is on the wing until mid-July so if you have the chance, make a pilgrimage to Collard Hill.

Previous Large Blue work in the Polden Hills has been funded by SITA Trust Current funding from Biffa Award is helping to secure the future of this butterfly.

The Large Blue Project is underpinned by science, carried out by Oxford University and CEH and implemented by a collaborative partnership between Butterfly Conservation, CEH, Gloucestershire Wildlife Trust, J&F Clark Trust, National Trust, Natural England, Network Rail, Oxford University, Somerset Wildlife Trust, South Somerset District Council and private landowners.

 By Liam Creedon, editor of Butterfly @liamcreedon