|Natural History Museum of London|
Rothschild & Jordan, 1903
Xanthopan morganii, or Morgan's sphinx moth, is a very large hawk moth from East Africa (Rhodesia, Nyasaland) and Madagascar. It is the sole member of its genus, and little is known of the biology, though the adults have been found to visit orchids (see below).
In January 1862 while researching insect pollination of orchids, Charles Darwin received a package of orchids from the distinguished horticulturist James Bateman, and in a follow up letter with a second package Bateman's son Robert confirmed the names of the specimens, including Angraecum sesquipedale from Madagascar. Darwin was surprised at the defining characteristic of this species: the "astonishing length" of the whip-like green spur forming the nectary of each flower, and remarked to Joseph Hooker "I have just received such a Box full from Mr Bateman with the astounding Angræcum sesquipedalia with a nectary a foot long— Good Heavens what insect can suck it"[?] The spur of the flower is 20–35 cm (7.9–13.8 in) from its tip to the tip of the flower's lip. The name "sesquipedale" is Latin for "one and a half feet," referring to the spur length.
From his observations and experiments with pushing a probe into the spur of the flower, Darwin surmised in his 1862 book Fertilisation of Orchids that there must be a pollinator moth with a proboscis long enough to reach the nectar at the end of the spur. In its attempt to get the nectar at the end of the spur the moth would get pollen rubbed off on its head. The next orchid it visited would then be pollinated in the same manner.
A few years later in 1867 Alfred Russel Wallace published an article in which he supported Darwin's hypothesis, remarking that the African hawkmoth Xanthopan morganii (then known as Macrosila morganii) had a proboscis almost long enough to reach the bottom of the spur. In a footnote to this article Wallace wrote "That such a moth exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune,--and they will be equally successful!"
It was only in 1903 that a population of Xanthopan morganii (commonly called Morgan's sphinx moth) with an especially long proboscis was discovered in Madagascar, and it was named subspecies praedicta by Rothschild & Jordan in honor of Wallace's (not Darwin's) prediction (Darwin's prediction was not even mentioned in their paper: Rothschild, L. W. & Jordan, K. 1903. A revision of the Lepidopterous family Sphingidae. Novitates Zoologicae Supplement 9: 1-972). Since Wallace predicted that the mystery pollinator would turn out to be a hawkmoth, rather than simply a large moth as Darwin had suggested in 1903, such a moth was discovered in Madagascar. It was described as a sub-species of the African hawk moth and named Xanthopan morganii praedicta, though the subspecies was later determined to be invalid (it is identical to the mainland form of the species). The moth approaches the flower to ascertain by scent whether or not it is the correct orchid species. Then the moth backs up over a foot and unrolls its proboscis, then flies forward, inserting it into the orchid's spur.
- Kritsky, Gene (1991). "Darwin's Madagascan Hawk Moth prediction" (PDF). American Entomologist 37. pp. 206–9. Retrieved 2009-02-09.
- "Darwin Correspondence Project - Letter 3357 — Bateman, Robert to Darwin, C. R., (1862)". Retrieved 2009-02-09.
- "Darwin Correspondence Project - Letter 3356 — Bateman, James to Darwin, C. R., (1862)". Retrieved 2009-02-09.
- "Darwin Correspondence Project - Letter 3411 — Darwin, C. R. to Hooker, J. D., 25 Jan (1862)". Retrieved 2009-02-09.
- Darwin 1862, pp. 197–203
- Afromoths[dead link]
- Darwin, Charles (1862). "On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing". London: John Murray. Retrieved 2009-02-07.
Media related to Xanthopan morganii at Wikimedia Commons