Map of the Week: North African migrants invade UK
From the Atlas Mountains of Morocco, millions of North African butterflies are arriving in Britain - expected to be the largest migration of the Painted Lady species ever seen in the UK.
With warm, southerly winds over the bank holiday weekend, the extraordinary annual journey of these fragile looking insects suddenly hit Britain and experts think it may break all records.
"We have all been stunned at how quickly it has all happened. We were expecting them to arrive and suddenly with the good weather - Bang!", Dr Martin Warren from the charity Butterfly Conservation tells me.
"All the signs are that this will be the largest ever migration of Painted Ladies to the UK."
They have been spotted as far north as Dumfries and Galloway in Scotland and hundreds have been sighted in central London.
An estimated 18,000 were spotted flying past Scolt Head Island on the Norfolk coast on Monday, passing at 50-a-minute over a 400m front yesterday.
Source: Butterfly Conservation
A Spanish researcher had predicted that numbers could be unusually high.
Constanti Stefanescu reported seeing hundreds of thousands emerging in North Africa in mid February and beginning their long flight north.
They were seen in large numbers in Spain during April and a few weeks later in France.
It is thought that heavy winter rains in Morocco allowed good germination of the caterpillar food plants this year, but experts think that global warming explains increased sightings of the Painted Lady over the past few decades.
The butterflies, with a 3-inch wingspan, manage an average speed of around 30mph. They don't swarm like bees - butterflies tend to be solitary insects which may explain why there is no official collective noun for them.
Suggestions I have seen include flight, flutter, kaleidoscope, rabble and rainbow but perhaps, in honour of this year's invasion, readers might like to propose a suitable word.
The last really big migration to Northern Europe was in 1996 when Painted Ladies were spotted in the North of Scotland and even in Iceland and Greenland.
There is a mystery to be solved too. Apparently no-one has ever witnessed the return migration of the Painted Lady around September/October time.
We assume they do go back, British winters are too cold for them and their genes are said to be needed back in the Atlas mountains.
The returnees would be the children of the spring migrants - their parents, exhausted after their journey, will survive only a few months.
So the appeal goes out - Butterfly Conservation wants people to record their sightings now on the map AND remember to do the same in the autumn if they see these dogged migrants heading south.
The charity says that butterflies are important indicators of the health of an environment. "In profusion they show us that nature is in healthy balance".
Surprising, some might suggest, that so many were spotted in Westminster last weekend.
You can help them track the Painted Ladies here.