Here’s a great Charles Darwin story you may not have heard before: In 1862, the famed naturalist foretold the discovery of an unusual animal, based on his observations of a species of orchid endemic to Madagascar. The creature was ultimately discovered in 1903—some 20 years after Darwin’s death.
Photo Credit: kqedquest | The Xanthopan morganii praedicta specimen in this photo was collected by staff entomologists at the California Academy of Sciences and is part of their collection | CC BY-NC 2.0
The species of orchid in question was Angraecum sesquipedale, a plant notable for the unusual depth of its nectar reservoir. The orchid’s “whip-like green nectary,” Darwin would write in his 1862 book on orchids and the insects that fertilize them, measured “eleven and a half inches long, with only the lower inch and a half filled with sweet nectar. What can be the use, it may be asked, of a nectary of such disproportional length?”
To Darwin, the orchid represented the first half of an evolutionary puzzle. Here was a most unusual flower, the form of which seemed to actively discourage fertilization; what animal would even bother to visit an orchid so possessive of its sugary prize? Such a strange plant, Darwin reasoned, necessitated a correspondingly unusual pollinator. But such a creature was theretofore unknown in Madagascar. As Darwin had written in a letter to his good friend, botanist J.D. Hooker, shortly after examining the orchid for the first time: “I have just received such a Box full from Mr [James Bateman, a well-known orchid grower,] with the astounding Angraecum sesquipedalia [sic] with a nectary a foot long. Good Heavens what insect can suck it.” [Photo credit: Thérèse Viard | CC BY-SA 3.0]
Based on a thorough inspection of the orchid, Darwin would later speculate not only on said insect’s existence, but on its coevolution with A. sesquipedale. He describes his observations and presents his hypotheses on pages 198–203 of his book on orchids and their fertilizing insects. I’m tempted to cite the whole damn thing (because it’s that good, and you should really read it for yourself), but I’ll settle for a lengthy citation (bolding mine):
We shall, I think, see that the fertilisation of the plant depends on this length, and on nectar being contained only within the lower and attenuated extremity. It is, however, surprising that any insect should be able to reach the nectar: our English sphinxes have probosces as long as their bodies; but in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches [25—28cm]!
...We can thus partially understand how the astonishing length of the nectary may have been acquired by successive modifications. As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life, either in the larval or mature state, or as the proboscis alone was lengthened to obtain honey from the Angræcum and other deep tubular flowers, those individual plants of the Angræcum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed, and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angræcum and the proboscis of certain moths; but the Angræcum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as far as possible in order to drain the last drop of nectar.
Darwin had made two bold predictions. The first pertained to the existence of an as-yet undiscovered species of long-proboscised moth, the second to the coevolutionary relationship between said moth and A. sesquipedale. The former hypothesis would bear out some decades afterward; direct support for the latter, however, would come much, much later.
Darwin and his predictions were criticized not just by entomologists, but anyone who took issue with his evolutionary theories at large. Among his supporters was Alfred Russel Wallace, the largely overlooked co-discoverer of natural selection. In 1867, in a response to criticisms leveled by one George Campbell (who contended that Darwin’s theories on A. sesquipedale and its pollinator had omitted “that function of power of Mind which we know as Purpose and Design”—an early articulation of the concept known today as “intelligent design”), Wallace doubled down on Darwin’s moth prediction, while sharpening its edge.
His correspondence included this illustration of the as-yet undiscovered moth, its proboscis unfurled, pollinating the orchid with which it was thought to have coevolved. Artist Thomas William Wood had based the illustration on Wallace’s descriptions of the predicted moth. These descriptions were, in turn, based on Wallace’s familiarity with another well-endowed insect. The African hawkmoth Xanthopan morganii (then Macrosila moranii), Wallace noted, had a proboscis seven and a half inches long. “A species having a proboscis two or three inches longer could reach the nectar in the largest flowers of Angraecum sesquipedale, whose nectaries vary in length from ten to fourteen inches,” he 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 astronomer searched for the planet Neptune,—and I venture to predict they will be equally successful!”
Darwin and Wallace were right. Gene Kritsky describes the discovery of the foretold moth in a 1991 issue of American Entomologist:
The quest for the giant moth was realized in 1903 when Rothschild and Jordan described a large Madagascan sphinx moth . The new moth was a subspecies of the same moth that Wallace had examined and was appropriately named Xanthopan morganii pradicta. As expected, the moths are large with wingspans of ab out 150 mm and proboscises of about 300 mm.
I cite Kritsky for two reasons. The first is to call attention to his own expert telling of this epic evolutionary yarn, which the entomologically inclined will no doubt enjoy. The second—and this, in my mind, is maybe the coolest thing about this story—is that when Kritsky wrote these words in 1991, the moth STILL had never been observed visiting or pollinating the orchid—or at least, these visits had never been documented. The moths, Kritsky explained, “are active at night and are apparently quite rare.”
In other words, Darwin’s larger prediction—that the moth not only existed, but had co-evolved with this plant, a plant to which it now served as ideal, pollinating counterpart—remained unproven. But it would not stay unproven for long.
In 1992, 130 years after Darwin’s initial prediction, a male X. morganii praedicta was captured bearing a viscidium of A. sesquipedale. A viscidium is a disc-shaped structure, found on orchids, that sticks to a visiting insect the way crumbs from a cookie might stick to your face. It wasn’t direct evidence, but it was close.
That same year, however, with the aid of night-vision equipment, researchers led by University of Erlangen biologist Lutz T. Wasserthal would capture the first ever photographic evidence of X. morganii praedicta visiting its orchid. In 2004, 143 years after Darwin’s predictions, University of New Orleans biologist Philip J. DeVries would capture videographic evidence of the long-proboscised moth feeding from and pollinating A. sesquipedale:
I already mentioned Kritsky’’s piece in American Entomologist. Also of note is this outstanding overview of Darwin’s theories regarding X. morganii praedicta and A. sesquipedale, his correspondence and publications on the subject, the moth’s history, and subsequent photographs, illustrations, and literature elaborating on these subjects. Lutz T. Wasserthal, the biologist whose team captured the first photographic evidence of the moth visiting the orchid, is a co-author. Read it here.
H/t Kara E. Rogers for putting this story on my radar!
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