Like many wealthy people of the Victorian Era, Charles Darwin was a passionate orchid collector. He was amazed by their beautifully complex shapes, patterns and structures, and he began amassing a collection of rare specimens from around the world. Even before he wrote On the Origin of Species, Darwin knew that every unique orchid flower must be a result of some advantage that was bestowed upon that species in its particular habitat.
But some orchid shapes were so extreme, so elaborate, that they seemed to conflict with Darwin’s theory of natural selection. The purpose of the orchid’s flowers, he knew, was to coax insects to visit them, enabling them to inadvertently transfer pollen from one individual to another. Darwin studied the lives of many orchids and dissected them, looking at the seemingly endless ways that the plants guided bees or moths to their flowers to interact with their reproductive structures.
Upon completing On the Origin of Species, Darwin began work on another book, which explored in detail the evolutionary interactions between insects and the plants they fertilised. In early 1862, during the midst of his research, a new orchid puzzle landed on his desk, quite literally. It was a package from renowned horticulturist James Bateman, containing a very striking orchid from Madagascar called Angraecum sesquipedale.
Upon studying this plant, Darwin could hardly believe his eyes. The orchid’s nectaries hang down beneath its petals and are unusually long, up to 30 cm in length. Darwin could tell that the orchid reproduced sexually and would therefore need an insect to pollinate it, but at the time no insect was known to possess an implement long enough to reach the bottom of the orchid’s nectary to claim its prize of sweet nectar. Darwin theorised that a Madagascan insect not yet discovered, probably a moth, must exist with a proboscis at least 25 cm long, if not longer.
When Darwin’s new book, Fertilisation of Orchids, was published a few months later, he included in it his theory about Angraecum sesquipedale and its undiscovered pollinator. Many people ridiculed him for his suggestion. No tongue could possibly be that long, they claimed. The Church stated that if a moth with such a tongue did exist, then it must be proof of God, since evolution could not have conceivably created such an intimate relationship between a plant and its pollinator.
Alfred Russel Wallace, the British naturalist who independently conceived the theory of evolution at around the same time that Charles Darwin did, was quick to back Darwin up. He explained how a moth-orchid relationship could evolve with natural selection guiding the process, highlighting the fact that a large hawk moth with a very long tongue, Morgan’s Sphinx moth, had recently been discovered in Africa. Wallace, like Darwin, was extremely confident that a similar moth with an exceptionally long proboscis would eventually be discovered in Madagascar.
And indeed it was. In 1903, twenty-one years after Darwin had died, and 40 years after his initial prediction, the moth in question was finally found. The team who discovered it named it Xanthopan morganii praedicta – the predicted subspecies of Morgan’s Sphinx moth. Just as Darwin had speculated, the moth had a proboscis that was long enough to reach the bottom of Angraecum’s nectary – around 28 cm in length. When not in use, the moth usually keeps its tongue curled up beneath its head.
At first, it might seem paradoxical that a plant would develop longer and longer flowers, for surely this would make it increasingly difficult for insects to visit and pollinate it. Likewise, it would seem as though the moth was merely wasting energy by developing a longer (and rather unwieldy) tongue if there is a plentiful supply of shorter flowers available elsewhere. But by evolving side by side, both moth and orchid win. The moth’s gain is exclusive access to Angraecum’s nectar, since no other insect has a tongue long enough to compete with it. The orchid’s benefit is that it has an exclusive pollinator, for the moth’s mouthpart is now so specialised that its owner finds it difficult, if not impossible, to use on any other flower. The moth, in visiting only one species of plant, does not waste precious pollen by smearing it onto another species immediately afterwards. Plant and insect have become so dependent upon one another that if one were to disappear, the other would very likely starve or remain sterile.
From extreme length to extreme miniaturisation – in the next Madagascar Month post, we’ll be looking at an Animal World Record: the smallest reptile on the planet.