Stellenbosch University Botanical Garden, South Africa

Common name: Darwin-se-orgidee, Darwin's orchid

Family: Orchidaceae (Orchid)

Synonym: Aeranthes sesquipedales, Mystacidium sesquipedales

Distribution: Coastal Madagascar

Habitat: Hot coastal lowlands usually below 100 m, on tree branches in light shade or on the edge of forests.

IUCN Red list: Not evaluated

Life form: Epiphytic

Comments: In 1862, Charles Darwin received a specimen of Angraecum sesquipedale, an epiphytic orchid from Madagascar. Upon seeing the intriguingly long spurs that the flowers have he predicted that only a moth with a tongue length comparable to the orchid's spur could pollinate it. Ironically, it was mainly entomologists who ridiculed Darwin for this bizarre theory because a moth with a tongue that long was not yet known to exist. However, 135 years after Darwin's famous prediction, Xanthopan morgani var. praedicta, a moth with an unusually long tongue, was confirmed as the only pollinator of A. sesquipedale.
The star-shaped flowers of A. sesquipedale become strongly scented at night to attract its nocturnal pollinator, the Malagasy hawkmoth. The hawkmoth is not only lured by its sweet smell but by the sugary nectar that lies at the bottom of the orchid's extremely long spur. The orchid's spur may reach lengths of 43 cm while the moth's proboscis, a tongue-like structure, averages 22 cm in length. The orchid's spur needs to be longer than the moth's tongue to ensure successful pollination. Besides, the longer the spur, the more nectar it contains so even shorter tongued individuals are rewarded for their efforts. The large bottom petal serves as a landing platform for the hawkmoth, allowing it to reach the bottom most nectar within the spur. This action transfers the pollen from the orchid to the proboscis of the moth, equipping it to fertilize the next orchid it visits.
Two theories could explain the relationship between this long-spurred orchid and long-tongued hawkmoth. It might help to understand that moths whose proboscises exceed orchid spur lengths reaches the nectar but does not transfer the pollen. The pollen sits too low and falls off when the moths roll up their tongues. Shorter tongued moths might be able to pollinate but they won't visit the flowers if they cannot reach the nectar in the spur. Thus, a longer spur length would encourage a longer proboscis length, gradually evolving longer spurs and p