Darwin’s Orchid

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Darwin’s orchid (Angraecum sesquipedale) with its long spur.

I just saw, for the first time in my life, Darwin’s orchid (Angraecum sesquipedale). That was at the annual show of my local orchid society (Les Orchidophiles de Québec), held on April 8 and 9, 2017. It was labelled, but it didn’t need to be. I’ve known about this orchid since I was a teen and avidly read everything I could find about evolution. I recognized it immediately. And how could you fail to notice it? It’s has large white flowers… but more importantly, it’s unique in that it has a long downward-hanging green spur that projects behind the flower.

It’s certainly attractive enough, but its history is even more fascinating.

A Surprising Discovery

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Louis-Marie Aubert of Petit-Thouars. Illustration: Wikimedia Commons

In 1798, the French botanist Louis-Marie Aubert of Petit-Thouars discovered in Madagascar an orchid with large white flowers bearing a very long spur. He only named the plant 25 years later, in 1822, calling it Aeranthes sesquipedalis, later changed to Angraecum sesquipedale. Angraecum comes from the Malay word for orchid, “angrek”, while sesquipedale refers to the very long spur and means “a foot and a half” (45 cm). Indeed, the spur ranges from 10.6–16.9 in (27 to 43 cm), just short of 1 ½ feet (45 cm). If you include the distance from the tip of the spur to the tip of the flower’s lip from which it extends, that would indeed make “a foot and a half”. It it by far the longest spur found on any flower.

Darwin Makes A Prediction

Charles Darwin in 1881

Charles Darwin studied and wrote about

In 1862, a British orchid enthusiast James Bateman sent Charles Darwin, the famous scientist and father of modern biology, blooming specimens of this orchid to Charles Darwin… and he was immediately fascinated.

You see, in flowers, such a spur, also called a nectary or a nectar spur, is a projection of the flower that contains a sugary liquid called nectar… and a flower only produces nectar to attract a pollinator. Thus an insect or other pollinator needs to have some means of reaching into the spur to suck up the nectar and, in so doing, picks up pollen which, one hopes, it will then deposit when it visits another flower of the same species.

But only insects with a long proboscis, a bird with a long beak or an animal with a long tongue have access to flower spurs. That way the plant ensures a certain fidelity in its pollination: it pre-selects pollinators, eliminating those less likely to ensure its fecundation.

But the extremely long spur of Angraecum sesquipedale only contains nectar in the last few centimeters of its extremity. Why was nectar placed so far from the flower’s anthers and stigma, where pollination takes place?

Darwin guessed almost immediately. He quickly began dissecting flowers and testing the mechanism necessary to reach the nectar and release the pollen. He soon announced that it was now certain that in Madagascar there had to be a moth with a proboscis ten to eleven inches (25 to 28 cm) in length and which would then be able to reach the nectar at the base of the spur. If he predicted it would be a moth, that’s because the flower was white and gives off an intense scent only at night, a combination of traits only moths find appealing.

The prediction is greeted with much skepticism by the scientific community of the time. But it turns out Darwin was correct.

The Prediction Comes True

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Darwin’s sphinx moth (Xanthopan morgani praedicta). Photo: Esculapio, Wikimedia Commons

In 1903, 40 years later, and well after the death of Darwin in 1882, such a moth was indeed identified. Darwin’s sphinx moth (Xanthopan morgani praedicta, now Xanthopan morgani) has a long proboscis that it carries coiled. The sphinx first approaches the flower, attracted by its odor, then once it has inspected it, backs up about a foot (30 cm), extends its proboscis, then inserts it into the spur as far as it will go. It’s a bit of a struggle, so it pushes hard against the flower, thus inadvertently picks up pollinia (masses of pollen) that it will later carry to another Darwin orchid, ensuring its fecundation.

Interestingly, even in 1903, the relationship could only be postulated: no one had yet actually seen a Darwin’s sphinx moth pollinate an Angraecum sesquipedale flower. That didn’t happen until… 1992, nearly a century later!

Darwin’s sphinx moth is the only pollinator of what is now called Darwin’s orchid. The two are believed to have co-evolved, each adapting to the shape of the other to ensure a perfect symbiosis.20170412A

Isn’t nature marvelous?20170412A

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