Today I want to share with you one of my favourite stories of co-evolution, and how sometimes the right answer can seem the most unlikely. For those of you who don’t know, Charles Darwin spent a lot of time researching orchids, to the point where he even published a book called Fertilisation of Orchids (or its full name, On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects, and On the Good Effects of Intercrossing). At the time, 19th century Europe just caught wind of orchids, and it was all the rage to collect them. Wealthy collectors payed hefty sums to purchase them from explorers who brought them back from tropical regions of the world. As they did not yet understand their reproductive cycle, which we now know requires fungi to grow from seed, the plants had to be taken and kept alive for long, often unfavorably cold voyages. Early terrariums, dubbed “Wardian cases,” were often used to protect the plants during these times. They dubbed this Victorian orchid-collecting phenomena “orchidelirium“.
Darwin himself was said to be fascinated by the huge variety in orchid flowers. From his observations, he concluded that they were meant to be pollinated by insects or other animals rather than self-pollination or other methods. This would allow cross-pollination, which increases genetic diversity and provides greater fitness. In terms of orchid anatomy, I’ve mentioned it before in a past post, but orchids have a tissue called the anther cap, which forms somewhat of a physical barrier to reduce the odds of self-fertilization. While the majority of orchids are technically self-compatible despite not preferring it, there are some that are self-incompatible and will not germinate or abort self-pollinated fruit, while others only produce unisexual flowers. But going back to Darwin, the interesting colors, shapes, sizes, and scents act to promote cross-pollination as they all attract specific pollinators.
One of the orchids Darwin observed was Angraecum sesquipedale, or what is now known as Darwin’s star orchid. Endemic to Madagascar, this orchid has a uniquely long nectar spur, or the part of the flower that is a hollow tube sometimes filled with nectar. This is often where you see butterflies and stick their proboscis and hummingbirds their beaks.
The nectar spur of A. sesquipedale is a whopping 10.6–16.9 in long. In his own words, in a letter to J.D. Hooker, “Good Heavens what insect can suck it.” As there is often a lot of co-evolution seen between flower design and their pollinator, Darwin made the guess that there was probably a moth with a similarly long tongue adapted to feed from it. At the time, critics laughed this off a bit, notably George Campbell, 8th Duke of Argyll who claimed that a supernatural being must be behind it instead. A few years later naturalist Alfred Russel Wallace mentioned that the African sphinx moth of southern Africa had a long proboscis, and it was possible that species was on Madagascar.
Years later in 1903, that very moth, Xanthopan morganii, was discovered there. Comparatively, there are some differences between those on Madagascar and those of southern Africa, likely caused from the geographical separation causing divergence, but nevertheless, the predictions were true. I find this tale so fascinating. Imagining such a creature back then must have been absurd, but to be right truly solidifies the power in co-evolution and the various factors that lead to the species morphologies we see today.
Happy reading,
-Beppa