Which orchid species did Darwin predict would have a pollinator with a long proboscis?

Darwin predicted that Angraecum sesquipedale, a long-spurred orchid from Madagascar, must be pollinated by an insect with an exceptionally long proboscis. The orchid’s nectar is stored deep inside a spur roughly 20–35 centimetres long, making ordinary moths unable to reach it while contacting the flower’s reproductive structures.

He made the prediction after examining the orchid’s unusual anatomy in his 1862 book On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects. Darwin reasoned that such an extreme floral structure could have evolved alongside an equally extreme pollinator. The prediction was famous because no such insect had yet been observed.

A suitable moth was eventually identified in Madagascar. It was initially treated as a long-proboscis form of Xanthopan morganii and named Xanthopan morganii praedicta, with “praedicta” referring to Darwin’s prediction. Later taxonomic treatments have often recognized the Madagascan moth as the species Xanthopan praedicta. Its long tongue allows it to feed from the orchid’s nectar while transferring pollen.

The episode became a classic example of coevolution and of how anatomical clues can reveal an undiscovered ecological relationship. The orchid is also widely known by the synonym Angraecum sesquipedale, while its common name is Darwin’s orchid.