In the 1930s, an amateur naturalist solved an evolutionary riddle that had flummoxed even Charles Darwin: how and why do orchids lure male wasps? Danielle Clode and Sue Double tell the story of Edith Coleman.
It is early January of 1927 and Mrs Edith Coleman is sitting at her desk, admiring a vase of native orchids by the window.
The pink labellum of the small tongue orchid curves upwards, studded with two rows of shiny dark spots, and surrounded by five thin leg-like spikes—the sepals and petals. The orchid hardly looks like a flower at all.
But the orchids attract more than just Coleman’s attention. Through the window comes a stream of insect visitors: small black ichneumon wasps. They bustle enthusiastically around the orchids, each anxious to be the first to enter the flower, not head-first as most nectar foragers do, but backwards.
After a time, individual wasps depart, each carrying a packet of pollen on the tip of its abdomen, ready, no doubt, to deposit their parcels into the next orchid that attracts their fervent attention.
What was it that attracted the wasps? Orchids produce no nectar or edible material. Wasps are normally predators, hunting caterpillars and other larvae. And all the visitors were male—they weren’t laying any eggs.
Coleman could draw only one conclusion. ‘They are answering,’ she wrote, ‘to an irresistible sex-instinct.’
The behaviour Coleman observed pseudocopulation, attracting pollinating insects through perceived sexual favours.
Coleman wasn’t the first to observe this; in an earlier century, Charles Darwin had been puzzled by the frequent ‘attacks’ made upon orchids by their pollinators. ‘What this means I cannot conjecture,’ he declared.
Coleman, however, was the first to explain the strange approach of the pollinating wasps.
Her experiments proved beyond any doubt that the wasps were pollinating the orchids, and that the orchids were mimicking female wasps—in effect, persuading the insect to mate with flowers.
Coleman was not a professional scientist, but rather an avid amateur naturalist and prolific nature writer. Her early work was published in the pages of The Victorian Naturalist—the journal of the Field Naturalist’s Club of Victoria, which she joined at the age of 48.
Over the course of 27 years, Coleman contributed 135 articles and notes to that journal, and had her scientific work published internationally. In addition, Coleman wrote regularly for The Argus, The Age and The Australian Women’s Mirror. In 1949, all these contributions were recognised when Coleman became the first woman to be awarded the Australian Natural History Medallion.
But it was for her work on pseudocopulation that Coleman became best known. Her publications attracted international attention, and earned her the respect and admiration of eminent orchidologists worldwide
Dr R.S. Rogers presented her work in his address to the Australian and New Zealand Science Congress of 1932. ‘Perhaps I lingered longer over your discovery than on the other papers as it brought so prominently before the world one of the strangest and most weird devices in the history of pollination,’ he noted.
‘It will prove a worthy supplement to Darwin’s classic study on the same subject,’ he added later.
Another eminent orchidologist, H.R. Rupp, put it even more directly. ‘Your name ought to be Darwin,’ he wrote.
Coleman hypothesised that the orchid’s mimicry worked on several levels. To the human eye, the pink tongue of the orchid, with its shiny spots and long, spiky legs does look a little wasp-like. But in wasp-vision, the orchid is uncanny: it mimics the precise dimensions and shape of a female wasp’s body.
Wasps are sensitive to UV reflection, and the orchids’ reflection mimics the wings and abdomen of a female wasp. This is not to say that the orchids neglect the wasp’s other senses—they even use tactile features to stimulate the male wasps into mating or guide their bodies into the correct position.
Coleman knew that there had to be more than visual and tactile mimicry at work, however. The wasp’s attraction operated even at considerable distance—wasps rapidly locate flowers even when they are inside a house. She suspected scent—but to our nose, the orchid gives off no discernible scent at all.
It would be three quarters of a century before the mystery Coleman identified was finally solved using a combination of two new technologies: gas chromatography, which identifies individual compounds in a chemical, and electroantennal detection, which measures the electrical activity present in an insect’s antennae when exposed to a chemical compound.
In 2004, Florian Schiestl and his team at the Australian National University put these two technologies together and discovered that the odour produced by the orchid exactly mimics the sex pheromone of the female wasp. Even more astonishingly, different species of orchids have reproduced the identical single compound present in the pheromone of its specific pollinator.
These compounds are so attractive that a drop placed on the head of a pin is enough to stimulate copulation. So persuasive is the orchid’s mimicry that the wasps will even discard a real female wasp in favour of the alluring flowers.
Pseudocopulation is a relatively rare phenomenon amongst Australian orchids. Of our 1,300 named orchid species, only 14 are known to practice sexual mimicry to effect pollination. It is not only wasps that are deceived into providing this service, but also ants, gnats, bees and flies.
The orchid’s sexual deceit and the strategies it uses to ensure the transfer of its genes is so detailed and complex that it is impossible to conceive the evolutionary process that might have given rise to it. It seemed a great mystery that such a process, so beneficial to the orchid, so exploitative to the wasp, could possibly have evolved.
Coleman noted as much, remarking that male wasps often emerge before the females, and it is these early-season enthusiasts that are most vulnerable to the lure of the orchids. Orchids are relatively rare, so the momentary distraction of a few male wasps is unlikely to have any long-term impact on the wasp population. In any case, male wasps can mate multiple times and females are self-fertile. Like the orchids themselves, which can reproduce vegetatively, the wasps have many ways of ensuring the continuation of their species.
Modern scientists, like Coleman’s contemporaries, continue to acknowledge the importance of her work. Her studies are still cited in scientific papers today, not just for her contributions to the study of orchids and pollination, but also for her research on birds, spiders, phasmids and echidnas, as well as the impact of mistletoe.
Coleman herself had no anxieties about stepping into the territory of professional biologists. ‘We nature lovers,’ she told The Age in 1950, ‘may open our windows on all aspects of nature, even though we may sometimes abut on the preserves of the specialist.’
Her influence on others has been broad and indirect. She kept up a voluminous correspondence with other scientists around the world, sending them an abundance of samples with which to continue their work, and she supported other up-and-coming students of nature, particularly women.
Amongst her protégés were Rica Ericson, who wrote: ‘[Coleman] helped me in many ways, yet always treated me as an equal, and not as the learner I was. We loved the same things, and that was what mattered to her. I shall always remember her keen interest in all living things, and her enjoyment of beauty.’
Lisa Galbraith, another student, related the story of a walk with Coleman. After seeking out orchids, Coleman stopped at the fence of a bush garden and watched honeyeaters dance among the salvia flowers.
‘Sometimes,’ Coleman said to Lisa, ‘when I see a garden like that, I find out who it belongs to and post them some roots or a packet of seeds.
‘They don’t know who sends them, but I like to think of their surprise, and of my seeds growing in so many different gardens.’
And the seeds Coleman planted are almost certainly growing still in more gardens than she could ever have imagined.