Darwin and the Christmas orchid

Mystery parcel

On a chilly morning in January 1862, an unsuspecting postman trudged up the drive to the house of Charles Darwin with a strange parcel.

Already in the early throes of orchid addiction, Darwin unwrapped a bundle of orchid specimens from Robert Bateman, which he’d requested for his ongoing research into insect pollination.

One plant was particularly intriguing: the Angraecum sesquipedale Thouars, a large Madagascar orchid with star-like flowers of ‘snow-white wax … and whip-like nectary of astonishing length’. Almost a foot, give or take an inch or two, as he discovered, when he set out to measure them. Darwin was hooked.

Darwin had first caught the bug in 1860. By July1861, he took his wife and daughter Henrietta to Torquay while he diverted himself considering the many species of wild orchids to be on the shore.

‘The orchids have been
a splendid sport’

On his return home, he was verging on orchid mania as he searched near Down and struggled to concentrate on his other work. According to a recent article, he described them as ‘wonderful creatures’,and found perfection in their form. They became his ‘splendid sport’, his ‘hobby horse’ and he was ‘sillily & very idly interested in them’, saying ‘this subject is a passion with me’.

He knew this group of orchids, also called Star of Bethlehem, was pollinated by moths, and noted: ‘In Madagascar there must be moths with proboscises capable of extension to a length of between ten and eleven inches!’ He pondered this postulated pollinator, and speculated that the Sphingidae was the most likely family. This was arguably the first predicted creature in the field of evolution.

Greenhouse effects

Darwin thought the long nectary was an adaptation to lure moths to the flower for the purposes of pollination. To test his hypothesis, he used a cylinder to imitate a moth’s feeding process and inserted it into the nectary.

On its removal, he saw the orchid’s pollinia had adhered to its base. He then reinserted the cylinder into the nectary and had some success in causing the pollinia to be pushed onto the stigma.

On further experiments in his greenhouse (pictured), he decided that the relationship was so specific, that if the moth were to become extinct on Madagascar, so too would the orchid.

By May of that year, he’d published what is now considered to be a classic text on evolution and orchids, The Various Contrivances by which Orchids are Fertilised by Insects.

British readers fell upon the book, undeterred by the reputation of Darwin’s other, more infamous tome of 1859, On the Origin of Species, which caused something of a stir.

Moth man’s prophecy

They accepted his explanation of how natural selection could be responsible for the adaptations seen in the orchids – but slightly suspicious of the missing and mysterious Madagascan moth – some, including the influential Duke of Argyll, seeing as clear proof of God’s work.

However, Darwin’s colleague, Alfred Russel Wallace, an explorer and naturalist himself, defended Darwin’s corner, came up with a credible scenario, starting with the time when the nectary was only six inches, and was chiefly fertilised by a species of moth which appeared at the time of the plant’s flowering, and whose proboscis was of the same length. If orchids had a variation in nectary length, those with the shorter nectaries

were not pollinated because the moth did not have to struggle to get all of the nectar and therefore did not cause the pollinia to be transferred. Conversely, flowers with the longest nectaries would be pollinated most often. And so, moths’ proboscises evolved to greater lengths.

Naturally, Darwin hoped someone would find the wretched moth. Wallace agreed, urging other naturalists to keep their eyes open and rulers handy. After a challenge in the press, Hermann Muller came forward with a close contender: his brother had found a moth in Brazil with a ten-inch proboscis. The hunt continued apace.

Years went by, and Darwin experienced a pang of doubt – and resentment. In the second edition of his orchid book (1877), he insisted on his prediction, adding: ‘This belief of mine has been ridiculed by some entomologists.’

Darwin died in 1882, but it took almost 20 years for the world to catch up with him. In 1903, Rothschild and Jordan found a giant moth which they named Xanthopan morgani praedicta. The had wingspans of about 150mm and proboscises of about 300mm, and had never been observed pollinating the orchid, because they are active at night and are apparently quite rare.

Web of intrigue

It is curious to note that just this past month, a unique night-flowering orchid has only just been discovered in New Britain. Experts believed the orchid adapted to the scarcity of the moth by remaining open and attractive for weeks.

In later years, scientists have pondered the co-dependence between the flower and the moth, by which the flower is guaranteed pollination and the moth is guaranteed nectar.

Another theory suggests that the long proboscises are an adaptation developed by the moths to avoid predation by spiders that hide on flowers and ambush pollinators. By having a long proboscis, the moth is able to drink nectar from a farther distance and keep itself out of reach of predatory spiders. The moth encourages the flower to elongate its spur to make certain the moth gets close enough to the flower to successfully pollinate it. Lepidopterist and fellow blogger Jacqueline Rae comments that here the moth is actually making the first evolutionary move and the flower is just changing to keep up with its pollinator.

Finally, just to prove that orchids really are bonkers, another Madagascan orchid, Angraecum longicalcar Bosser, has been found with an even longer nectary – nearly 40cm or 16 inches long. Currently, such specimens are propagated by growers keen to preserve the island’s native plants, but is there, somewhere in Madagascar, a gigantic moth with a proboscis even longer than Darwin’s Madagascan hawk moth?

By Pamela Kelt

Captions:

Angraecum eburneum var longicalcar, right, http://www.larsen-twins.dk
Others: Wikimedia