Serene as the myriad butterflies that have lately been fluttering like confetti around Gauteng may seem, there actually is something quite desperate - and sad - about their flight.
Many of them will keep flying until, eventually, they plunge into the Indian Ocean from exhaustion and die. They are often found floating in the coastal waters off Durban and in the Mozambique Channel.
Theirs is not exactly a migration, as might seem from the way they are all moving in an easterly direction. It is more a dispersal, says Steve Woodhall, the author of the authoritative and beautifully illustrated Field Guide to Butterflies of South Africa.
It is a lemming-type flight. Like the mouselike North American rodent that is popularly, but apparently incorrectly, believed to commit mass suicide in some years by tumbling over cliffs into water, many of the butterflies keep going east.
What is driving them, as apparently with the lemmings, is their own population pressure. So hard-pressed are they to get out of each other's way in their habitat - the drier westerly areas of Gauteng, North West, Limpopo and Botswana - that they hardly mind about moving right through the centre of towns and even the cities of Johannesburg and Pretoria.
On their way they get nectar from flowers, but they fly on until they find a suitable food source, which is one of the trees their larvae feed on. Then they mate, lay eggs and die. Many of those who do not find such a food source keep going until they end in the ocean.
The butterfly is the brown-veined white (Belenois aurota), called so after the dark brown and black markings on its otherwise white wings.
They have a normal lifespan of well over a month and usually move across the country at this time of year in search of lebensraum. But in good rainy seasons, like the present, they do so in spectacular numbers.
Woodhall says that when Torben B Larsen, a renowned international butterfly expert, lived in Botswana, he estimated one year's swarm at 2 billion.
Why the butterflies fly east, he cannot say for sure. It could be some inborn instinct, or it could be that they follow wind directions. However it has happened, though very rarely, that the butterflies, after a good breeding season in the east, would reverse their flight pattern by moving west.
It is a complete misconception that the brown-veined white is a menace to farmers or gardens, or the origin of the feared commando worm. Their larvae feed on specific trees, most preferably the shepherd's tree (Boscia albitrunca), which is widespread in the drier areas.
Woodhall says the butterfly numbers increase faster than those of their predators, including spiders, but mostly wasps and flies, which parasitise them by laying their eggs in them. The butterflies seem to be able to detect when the food source will be over-exploited by the next brood's larvae.
Curiously, the next brood then consists of larger and more robust butterflies that will be able to withstand the rigours of migratory flight. Their bodies have large food stores, which accounts for the greasy smear they leave on windscreens.