The secrets of the UK's migrating painted lady butterfly population have been revealed by scientists who have discovered where they go in autumn.
The insect flies from the continent each summer to the UK, but experts have not known until now whether they headed south again in autumn, like the closely-related red admiral, or if they died in the UK.
With the help of 60,000 public sightings across Europe, including efforts by 10,000 British volunteers, and highly sensitive radar surveying during the mass migration of painted ladies in 2009, the mystery has been solved.
Scientists found the painted lady migrated south in autumn, but high in the sky out of sight of human observers, averaging an altitude of 500 metres (1,600ft) and clocking up speeds of 30mph by selecting favourable conditions.
The species undertakes a phenomenal 9,000-mile round trip from tropical Africa right up to the Arctic Circle, the researchers from Butterfly Conservation, the Centre for Ecology and Hydrology, York University and Rothamsted Research discovered.
The entire length of the journey is not undertaken by a single individual but by successive generations, so that painted ladies returning to Africa in autumn are several generations removed from those which left the continent in the spring.
But Richard Fox, surveys manager at Butterfly Conservation, said many of the individual butterflies arriving in the UK will have come as far as from north Africa and home-grown painted ladies will head there in the autumn.
He said: "The extent of the annual journey undertaken by the painted lady is astonishing.
"This tiny creature weighing less than a gram with a brain the size of a pin head and no opportunity to learn from older, experienced individuals, undertakes an epic intercontinental migration in order to find plants for its caterpillars to eat.
"Once thought to be blindly led, at the mercy of the wind, into an evolutionary dead end in the lethal British winter, this amazing combination of mass-participation citizen science and cutting-edge technology has shown painted ladies to be sophisticated travellers."
He said radar in Hampshire used to track the butterflies could tell the direction they were travelling in, their speed and details of their mass and shape.
Dr Jason Chapman, a researcher at Rothamsted Research which operated the radar, said: "The apparent lack of a return migration of the late-summer generation of painted lady butterflies was one of the greatest enigmas in insect migration ecology.
"But through a combination of traditional monitoring by butterfly enthusiasts and new radar techniques, we have finally solved this long-standing puzzle."