The Deceiver and the Deceived
In the heathlands and damp forests of southeastern Australia, the Large Tongue Orchid (Cryptostylis subulata) practices a sophisticated form of biological deception. This terrestrial orchid produces a flowering stem up to 80 cm tall, bearing between two and twenty flowers. Each flower has a prominent, downward-pointing labellum, or lip, that is reddish-brown and shaped to imitate the body of a female insect. This visual mimicry is part of the ruse.
The orchid's target is a specific insect: the male Orchid Dupe Wasp (Lissopimpla excelsa). This ichneumonid wasp is the sole pollinator for all five Australian species of Cryptostylis. The orchid produces no nectar or other reward for the wasp. Instead, it has evolved to exploit the male wasp's mating drive through a process of sexual deception, a phenomenon first described in detail by Australian naturalist Edith Coleman in 1928.
The orchid's labellum looks like a female wasp to other wasps, which perceive color in the green, blue, and ultraviolet spectra, and emits a chemical signature that perfectly mimics the female wasp's sex pheromones. This chemical trickery is so effective that male wasps find the orchid irresistible.
A Chemical Charade
The relationship between the orchid and the wasp is pseudocopulation. Lured by the powerful chemical signals, a male Lissopimpla excelsa will land on the orchid's labellum and attempt to mate with it. The wasp's copulation is vigorous, and research has confirmed that the wasps frequently ejaculate, leaving visible sperm deposits on the flower. During this futile act, the orchid's pollinia—waxy sacs of pollen—are glued to the wasp's abdomen.
When the duped wasp eventually disengages and flies off, it carries the orchid's pollen with it. Still driven by the deceptive pheromones, it will likely visit another Cryptostylis subulata flower and repeat the process, transferring the pollen to the new flower's stigma and ensuring cross-pollination. This entire interaction is a costly one for the wasp, which wastes time, energy and sperm that could have been used for actual reproduction. Studies have shown that once a wasp has been fooled, it is less likely to attempt copulation with subsequent orchids it encounters, suggesting a degree of learning.
Even though all five Australian Cryptostylis species are pollinated by the same wasp, they do not hybridize in the wild. This suggests a molecular or mechanical mechanism is in place that prevents cross-fertilization between the different orchid species, maintaining their distinct lineages despite sharing a common pollinator.