There is nothing obviously bee-like about the sombre bee orchid, Ophrys fusca. It is a simple little orchid, green on top with a two-tone lilac and purple tongue-like labellum. Quite unlike the yellow bee orchid (Ophrys lutea), with its brown beetle labellum, or the bizarrely patterned mirror orchid (Ophrys speculum), which gives every impression of already having a shiny purple insect with long red hairs in residence. But Godfery knew that this little bee entering an orchid backwards was something noteworthy because he had read the papers by Correvon and Pouyanne.
Pouyanne spent years watching mirror orchids in Algiers, waiting for the perfect conditions: fine, still weather with an abundance of flowers growing close to the burrows from which the female bee-wasps (Dasyscolia ciliata) emerge. Here, too, waited the male bee-wasps, born a month earlier.
‘Time hangs heavy with them and they have every leisure to indulge in endless exploration above the hallowed mound,’ explained Pouyanne. ‘To shorten their wait O. speculum give themselves over to entertaining them with certain distractions which are much appreciated by these gentlemen.’
Pouyanne had no doubts about what was happening.
‘The entire insect enters into a kind of paroxysm,’ he said. ‘Its movements and position seem exactly like those of insects engaged in attempting copulation.’
He suspected mimicry but was uncertain about its nature. The orchid’s iridescent labellum resembles the female insect’s shiny blue wings. The orchids share their hairiness with the bee-wasps. But the orchids still attracted the males even when they were hidden, beneath newspaper or in a cardboard box.
‘It would be interesting to ascertain whether the female of the visiting insect has a similar smell,’ commented Godfery.
In 1928, Edith was back on the trail of her mysterious wasps and orchids, setting out to replace Godfery’s and Pouyanne’s ‘conjectures’ into certainties. By this point she was convinced that the wasps must be ‘answering to an irresistible sex-instinct’. But could she prove it?
Edith was soon convinced that she was indeed observing copulation. She noticed that the male insects used their modified mating claspers to grip the orchid’s labellum, that their palps, used to deliver spermatozoa to the female, were ‘extruded’ when leaving the flower. And she’d seen a droplet of liquid being expelled from the insects. Mr Arthur Lea, the government entomologist for South Australia, suggested that she smear the droplets on a glass slide and try to draw them. The images she drew were the same as those drawn by Tarlton Rayment when he dissected the male insects and extracted their sperm.
So, while most flowers offer nectar or pollen in exchange for pollination, these orchids offer sex.
The notion that plants might lure their pollinators with sex rather than food was certainly unconventional and unexpected and, for many, perhaps a little bit disturbing. Edith must have known she would have her work cut out persuading others of her conclusions. As the French mathematician Laplace had once said, ‘The weight of evidence for an extraordinary claim must be proportional to its strangeness.’ And Edith’s findings were very strange indeed.
The strength of Edith’s work lies not simply in her discovery that wasps pollinate orchids by copulating with them, but in the systematic way she investigated and ruled out any possible alternative explanations, amassing a persuasive and convincing body of evidence in support of her theory. Like Darwin’s theory of natural selection, it was the weight of her evidence, not her extraordinary claim, that established her scientific achievement.
Edith found that the entire structure of the orchid has adapted to accommodate the wasps’ preferences. When the wasp lands on the orchid and clasps the labellum to its underbelly, the upper surface of its body rests on a sticky disc attached to the rostellum, a cap which separates the female stigma from the male anther, preventing self-pollination. Thus glued into place, the insect has to struggle free, a movement which causes the anther cells to open and release their pollen, without coming into contact with the flower’s own stigma. Instead the pollen is transferred to the wasp and transported to the next flower, to be deposited onto the stigma of a different plant.
An orchid dupe wasp ‘visiting’ a small tongue orchid
‘A glance at the strange labellum, modified out of all proportion to the thin, almost thread-like petals and sepals, with its double row of dark glistening glands that gleam in the hot sunshine loved by the wasp, is perhaps sufficient to justify the theory of an attraction based on the resemblance of the flower to a female wasp,’ concluded Edith with some caution. ‘Even to our eyes, the likeness is apparent. To the inferior eyesight of the insect, the resemblance may be still more convincing.’
The insects could locate a single flower with remarkable swiftness and determination. Having placed some of her flowers in a room, Edith closed the windows so that they would not be pollinated in her absence. On returning a few moments later, she found three wasps on the windowpane and one inside, having squeezed through a tiny crack.
‘I have, this season, seen an insect “tacking” swiftly once or twice over a flower before entering. I concluded it was tracking the orchid by its scent,’ she wrote. It would be decades before technology could investigate the molecular nature of the orchid’s spectral, tactile and pheromonal attraction, revealing an unimagined level of sophistication and specificity.
‘We now know that the glistening magenta bumps on the labellum of the tongue orchids reflect UV exactly like the wings of the female L. excelsa wasps,’ summarised Susan Double in 2016, reviewing the latest research in the field. The modern versions of Edith’s work require technology she could hardly have imagined: not just spectrometry, but electroantennal detection and gas chromatography. ‘In some species the orchids have reproduced an identical single compound present in the pheromone of its specific pollinator . . . Each species of orchid which practices sexual mimicry replicates exactly the correct pheromone of its insect pollinator, whether bee, wasp, fly or ant.’
The most important aspect of Edith’s
