Which butterfly has the longest migration of any butterfly species?

The painted lady has the longest migration of any butterfly species.

Painted ladies are found on nearly every continent except Antarctica and are especially notable for their long-distance movements. Populations can travel between tropical Africa and Europe, crossing the Sahara Desert and the Mediterranean region. The complete journey may span thousands of kilometres and requires several generations, because individual butterflies do not normally live long enough to make the entire migration.

Unlike monarch migration, painted lady movements can be irregular and strongly influenced by rainfall, plant growth, wind, and population density. Large influxes into Europe sometimes attract widespread public attention. The species is also called the cosmopolitan butterfly because of its unusually broad global distribution.

Painted ladies are sometimes mistaken for monarchs, but their upper wings have distinctive black-and-white markings near the tips and several small eyespots on the hindwings.