India, Nov. 30 -- There is a strong statistical relationship between the arrival dates of the butterflies and the combination of minimum temperature and precipitation during December

Each year around mid-summer, somewhere between December and mid-January, the skies of South Africa's Gauteng province, including the city of Johannesburg, fill with small white butterflies.

Some land in people's gardens, allowing a closer look at the thin brown markings on their wings. Those markings give the butterflies their name: the brown-veined white butterfly ( Benenois aurota ).

Their annual migration takes between 80,000 and 155,000 butterflies per hour from South Africa's Kalahari region to Mozambique, a journey of hundreds of kilometres via Gaute...