page_title: Angraecum sesquipedale
section_title: Coevolution model
hierarchical_section_title: Angraecum sesquipedale / Reproduction / Coevolution model
caption_attribution_description: Sphinx Moth Fertilizing Angraecum Sesquipedale in the Forests of Madagascar
context_page_description: Angraecum sesquipedale, also known as Darwin's orchid, Christmas orchid, Star of Bethlehem orchid, and king of the angraecums, is an epiphytic orchid in the genus Angraecum endemic to Madagascar. The orchid was first discovered by the French botanist Louis-Marie Aubert du Petit-Thouars in 1798, but was not described until 1822. It is noteworthy for its long spur and its association with the naturalist Charles Darwin, who surmised that the flower was pollinated by a then undiscovered moth with a proboscis whose length was unprecedented at the time. His prediction had gone unverified until 21 years after his death, when the moth was discovered and his conjecture vindicated. The story of its postulated pollinator has come to be seen as one of the celebrated predictions of the theory of evolution.
context_section_description: Angraecum sesquipedale is best known within the botany community for its association with the naturalist Charles Darwin.  After being sent several flowers of A. sesquipedale by James Bateman, Darwin noted the defining characteristic of the species, its extremely long spur. From his observations, Darwin surmised, in his 1862 publication On the Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects, and On the Good Effects of Intercrossing, that there must be a pollinator moth with a proboscis long enough to reach the nectar at the end of the spur. He arrived at this conclusion after attempting in vain to remove the pollinia of the flower using needles and bristles. Only after placing a cylinder with a diameter of ​¹⁄₁₀ of an inch (2.5 mm) down the full length of the spur was he able to detach the pollinia upon retracting it. The viscidium attached to the cylinder as he removed it. Darwin surmised that during the moth's attempt at getting the nectar at the end of the spur, the moth would get the pollinarium attached to itself.  The next orchid it visited would then be pollinated in the same manner.
  For some time after this prediction the notion of a pollinator with a 35 cm long proboscis was ridiculed and generally not believed to exist. After Darwin's publication, George Campbell, 8th Duke of Argyll published a book in 1867 titled, The Reign of Law, in which he argued that the complexity of this species implied that it was created by a supernatural being.  Alfred Russel Wallace replied in the same year with a paper he titled "Creation by Law", setting out in detail a sequence through which the moth and the flower could have coevolved with no guidance other than natural selection.
  
  In 1903, such a moth was discovered in Madagascar by Walter Rothschild and Karl Jordan. This confirmed Darwin's prediction. The moth was named Xanthopan morganii praedicta.  It is possible that the subspecific epithet praedicta was given in honor of the fact that Darwin predicted its existence, but there is no reference to Darwin in the paper that described the moth. A more conservative explanation is simply that the existence of the moth had been predicted and widely accepted before it was discovered.
   In 1873 William Alexander Forbes wrote an article in the journal Nature asking readers if they knew of the moth predicted by Darwin. A reply to the question was first made that same year by Hermann Müller. He announced that his brother Fritz Müller had discovered a moth with a proboscis of 30–33 cm (11.8–13.0 in) long, but it was discovered in Brazil and so was not a candidate for pollinating A. sesquipedale. Although Darwin learned of Müller's finding he did not live to see the discovery of Xanthopan morganii. Even after the 1903 discovery however, news of Xanthopan morganii praedicta was not immediately disseminated. A second inquiry into the existence of the pollinator moth was made in the 30 January 1907 issue of the journal Nature by E. W. Swanton. Presumably still unaware of Rothschild and Jordan's discovery, Wallace responded stating that he didn't know of a suitable pollinator in Madagascar, but that he had heard of one from East Africa with a long enough proboscis.
  Both Darwin and Alfred Russel Wallace had suggested that the evolutionary basis for how the odd relationship between the sphinx moth and A. sesquipedale evolved over time could be understood by considering one orchid with a long spur and another with a short spur.  If a moth goes to fertilize a flower with a short spur its proboscis would easily reach all the way to the bottom of the spur and it would get the nectar.  However, since the proboscis of the moth is longer than the spur of the flower, the head of the moth would not touch the flower obtaining the pollinarium and so the flower would not be fertilized. The orchid with the longer spur on the other hand would be able to be fertilized since the entire length of the proboscis fits within the spur and thus allowing the head of the moth to touch the flower and become connected to the pollinarium
caption: An illustration by Thomas William Wood, based on Alfred Russel Wallace's description, showing a moth pollinating A. sesquipedale. This was Wallace's conception of the event since it was drawn in 1867 before the moth was even discovered.[54]
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