There’s a painted lady basking on the footpath. Her orange, black-tipped, white-spotted wings, a little worn after her long journey, blend with shadows and sun-flecks on heatwave-baked mud, so she’s almost under our feet before she takes flight. And here’s another, nectaring on a dandelion; and another; then several more. I can’t recall ever seeing so many so early in the year.

Waiting for the arrival of these migrant butterflies is akin to anticipating the first swallow. Tantalising mid-April sightings from Wales and Cumbria were reported on social media, but we waited until mid-May before finding our first in Weardale.

It’s claimed that some of the earliest fly directly from Morocco, a marathon journey, wafted by southerly winds; but most arrive in relays, crossing the Mediterranean to breed in France and Spain. With their short life cycle – egg to imago in six weeks – their numbers multiply exponentially as they move northwards, a rolling, swelling, multigenerational wave of butterflies that breaks on our shores from midsummer onwards.

A newly arrived painted lady with ragged, worn wings. Photograph: Phil Gates

Spectacular “painted lady summers” are the stuff of lepidopterists’ legend. I recall walking along the coast near Whitby in 1996, surrounded by hundreds of them settling to feed in flowery clifftop grassland. That invasion reached Orkney and Shetland. The most recent mass migration that I remember here was in 2009, but the size and frequency of such events are subject to favourable winds and clement weather.

What does the future hold for the painted ladies we watched today? They have time to leave two generations of descendants, with their caterpillars feeding on thistles, before autumn frosts arrive. Until 2012 the assumption would have been that they would all perish in our wet, freezing winters, but in 2012 their autumn reverse migration was discovered. They’ll head back towards Africa, flying at altitudes beyond the gaze of ground-based observers. But there is another possibility. How long before our warming climate allows some to overwinter in England’s milder southern counties?

As lovely as they are, painted ladies’ mystique lies in their epic migration that begins in Morocco. Would that frisson of anticipation, that heart-flutter at the first sighting, be quite the same if their journey started in the Mendips, not Marrakech?