The large blue butterfly, which went extinct in the UK in 1979, is making a dramatic comeback after the species was reintroduced by conservationists.
By Paul Rincon
BBC News Online science staff
Numbers of the beautiful insect last year soared to about 6,000, which is possibly more than have been present in Britain at any time since the 1950s.
Efforts are underway to help them gain a foothold at 25 UK sites, nine of which now host thriving colonies.
Large blues from Sweden were used to rebuild the UK population in 1983.
Dr Jeremy Thomas, of the Centre for Ecology and Hydrology in Dorset, told BBC News Online: "We're going two or three steps forward to every one back these days - 20 years ago it was the other way around."
There are six large blue species worldwide, of which only one, Maculinea arion, is native to Britain. All six are in serious decline globally and are classified as either endangered or vulnerable by the IUCN - the world conservation union.
Of the nine thriving UK colonies, about six are unequivocally established. A further three need more monitoring before they can be considered "safe".
"There's one site in Somerset where almost everything comes together as being their idea of heaven," Dr Thomas said.
However, the large blue is not doing so well in Europe, where it is declining or extinct in 21 out of 28 countries for which there is data.
Dr Martin Warren, chief executive of Butterfly Conservation, told BBC News Online: "There is a lot more positive conservation being done across Europe, but the economic pressures to change land use are growing very rapidly,"
"We're very concerned about what's going to happen in eastern Europe when they join the EU and their markets get opened up. Butterflies quite like the old farming ways - light grazing and no fertiliser.
DECLINING UK BUTTERFLIES
Large blue (officially extinct)
Large tortoiseshell (extinct)
High brown fritillary
Source: Jeremy Thomas
"The fear is that agriculture will intensify across Europe and we'll see the same problem we've suffered in other countries."
Intensive farming over the last two centuries in the UK destroyed much of the large blue's favoured habitat.
Just under half of the sites populated by the large blue 150 years ago were ploughed up or spread with fertiliser, according to Dr Thomas.
The rest were abandoned because they are not economic to farm. As a result, they are no longer grazed by animals and have become overgrown.
This rendered this latter habitat unsuitable for the large blue because it eliminated red ants, which play a key role in the butterfly's extraordinary life cycle.
Red ants - particularly the species Myrmica sabuleti - are attracted to the sugary substance produced by large blue larvae and carry them off to their underground nests where the larvae can feed on ant grubs and pupate.
Myrmica sabuleti require the warm conditions provided by a south-facing slope with very short grass. If the grass is too long - even 2 or 3cm tall - in Spring and Autumn, it shades out the Sun and the ants rapidly die out.
The large blues now living in the UK were taken from a population native to the island of Oland in Sweden.
There have been no attempts to re-introduce the UK's other "extinct" butterfly, the large tortoiseshell. This is because the reasons for its original decline are unknown.