If you have yet to see them, you soon will. Fluttering gently on the breeze, these delicate insects seem hardly able to get over the next hedge, but they've actually just undertaken one of the greatest insect migrations of Europe: coming all the way from the Atlas Mountains of Morocco, this year's summer influx of Painted Lady butterflies (Vanessa cardui) is the biggest since 1996.

The Isle of Wight, as a staging post for winged arrivals from the south, is often the bellwether for such migrations - such as the harlequin ladybird invasion.

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Sure enough, last weekend reports began to arrive. On May 24th, Islander @ecclestonegeorge reported on Twitter "Painted ladies streaming in from the continent. Warm front a comin!". Before long everyone was seeing them. This was not the normal few stragglers - this was a full-blown invasion. Richard Fox, Surveys Manager at Butterfly Conservation, said: "There are literally millions of Painted Lady butterflies arriving right across Britain. This is a spectacular phenomenon".

Patrick Barkham explains in the Guardian:

Painted ladies reach our shores every summer, but the last major migration was in 1996. This year, rumours of an impending invasion began circulating in late winter. A Spanish scientist, Constanti Stefanescu, reported seeing hundreds of thousands of them emerging in Morocco in mid-February after heavy winter rains in north Africa triggered the germination of food plants devoured by its caterpillars. Aided by favourable winds and unimaginable reserves of stamina, large numbers were seen in Spain during April. A few weeks later, they had reached France.

Now the little creatures are as far as Scotland and still going. They're likely to raise a couple of generations of caterpillars here in what is for them the far north, before the majority of adults die. Each season a new flight will arrive from Africa - although normally not in these numbers.

So why do they do it? It has been suggested that they are blown in on the wind, and whilst wind certainly helps and their flight gives this appearance this is not the whole story. Other butterflies and winged insects live in Africa and indeed on continental Europe, yet they don't get blown up to the UK in clouds. This is more likely to be a behaviour which is part of the painted lady's survival strategy. By periodically breeding far more individuals than can be locally supported, a viable population can set off en masse to find a new breeding ground. This way new populations can become established, and the original population continues. It's not dissimilar to the way in which a colony of bees will periodically swarm and set up a new colony. In our lifetimes Northern Europe is unlikely to be suitable for the butterflies to survive and overwinter. But if global warming continues - as seems inevitable - then maybe eventually the painted ladies will arrive, find the habitat suitable, and stay for good. Then, presumably, they would send occasional clouds of painted ladies off to Russia or Greenland...