As well as sun-seeking people, insects move between the Palaearctic and the tropics every year. With isotope marking, we can now reveal the routes and distances of these species. The painted lady butterfly, Vanessa cardui stands out as one of the most welcome and pretty migrators of this type and was studied in this paper as well as in a forthcoming BBC programme.(This prog. should be available after 2100 on Monday, Greenwich Mean Earth Time!)

6 generations carry this species around Europe and Africa, even south of the Sahara, in a round trip. Southern European insects in spring are adults which fed up in North Africa. Then the European generations largely take over in another northward migration.

In Africa, there is an opposite trend. European migrants arrive in October on their southward migration, then breed locally. South of the great Sahara, these European migrants also appear. Densities of population are low, until October, when the desert must be crossed by the wave of migration from Europe. The distance of individual migrations when winds were favourable, was 4000km (2485miles,) presumably at high altitude. The Sahel suffers high productivity after the rainy season, so we can assume the butterfly is adapting to this brief situation, along with a total of 3500-4500million birds of many species!

When the offspring of this southern migration hatch, the larvae face rapid worsening of conditions. The surviving adults migrate north across the desert to the Maghreb, against the prevailing wind or Harmattan. Locusts apparently perform this migration too. The butterflies surviving this flight are observed in southern Morocco in late autumn. Recognising this adaptive migration, the authors have named their paper,Long-distance autumn migration across the Sahara by painted lady butterflies: exploiting resource pulses in the tropical savannah. Their advanced techniques utilise stable hydrogen istopes from wing chitin. This data was linked to isoscapes of hydrological hydrogen isotopic distribution in rainfall. Constanti Stefanescu, David X. Soto, Gerard Talavera, Roger Vila and Keith A. Hobson from the Natural History Museum of Granollers, Institut de Biologia Evolutiva and CREAF, Spain; Environment Canada and the University of Western Ontario and Harvard University, US, publish in the Biology Letters of the Royal Society today.