In the 16th and 17th centuries, wealthy people wore perfumes to mask their nasty body odours. In nature too, some organisms produce smells to disguise their true nature and deceive others. When it comes to lying with scent, orchids are the world champions.
Across Europe, Australia, Africa and South America, orchids have independently evolved ways to manufacture irresistible bouquets. Many of these smells mimic the sex pheromones – otherwise known as aphrodisiacs – of insects.
Orchids release a perfume that mimics a fertile female insect, luring in male insects. Overcome by the sexy female scent, the males are duped into attempting to mate with the flower. During their futile attempts at copulation, they pick up a gobbet of pollen, so that when they visit another flower they will pollinate it.
And what do the male insects get in exchange for their labours? Absolutely nothing.
The sexy scent of spider orchids
The most beautiful of the sexually deceptive orchids, suggests Ryan Phillips of the Australian National University in Canberra, are the spider orchids. Their large, often colourful flowers don't look like insects. Instead, they rely on perfume chemistry alone.
Not all males are stupid
Spider orchids mimic the pheromones of solitary wasps. Their mimicry is so effective that male wasps zig-zag their way to the spider orchids, following wafts of scent on the wind.
But the insects do sometimes figure it out. "Not all males are stupid," says Rod Peakall of the Australian National University. His team, including Phillips, has tracked individual insects by dabbing tiny dots of gaudy nail polish onto their bodies or back legs.
Some males "either get tricked more thoroughly or are more desperate," says Phillips. These guys make repeated futile visits to what smells like a luscious female. Other males "tend to learn to avoid that specific flower".
Regardless of these occasional eureka moments, the system persists. The orchids are mainly pollinated by newly-emerged males, which are naïve to their tricks.
Hammer orchid flips you over
Australia's endangered hammer orchids go a step further, and use visual lures as well as smell. Each flower has a landing platform called a labellum ("lip"), which is actually a modified petal. It is shaped like a hammer, hence the name.
The only insects that can pollinate these strange-looking orchids are thynnine wasps. Peakall and his colleagues showed in 2012 that hammer orchids produce chemicals called pyrazines to lure the male wasps. It's a powerful scent, says Peakall. Using it, "we can get perfectly strong attempted matings with little black beads."
But the hammer orchid doesn't stop there. Its hammer-shaped labellum resembles a female wasp, so it is an additional lure. Its shape also manipulates the male's mating practices.
Female thynnine wasps are flightless, so when a female releases her pheromone she first crawls up a stick. "The male flies down, picks her up, and they start to mate in flight as he carries her off to a food source," says Phillips. So when a male gets lured by a hammer orchid, he grasps the labellum and attempts to simultaneously copulate with it and fly off with it.
This doesn't really work. The labellum is mounted on a hinge, so the male's momentum makes it swing over, flipping him upside-down. He lands neatly on the stigma, ensuring that his pollen load ends up exactly where the orchid needs it.
To catch a fungus gnat
Some orchids go even further than the hammer orchids. They entrap their insect victims, to ensure that they transport the maximum amount of pollen.
The fungus gnat now tries to squeeze his way out
The banded greenhood orchid Pterostylis sanguinea does just this to luckless fungus gnats of the genus Mycomya. It's one of the most sophisticated deceptive systems thus far described for orchids, says Phillips.
Attracted from far away by the orchid's female-mimicking smell, the male gnat lands on the flower and attempts to copulate with the labellum. But he is in for a shock. His movements cause the labellum to swing up like a drawbridge, shutting him inside a chamber.
The fungus gnat now tries to squeeze his way out. On its way, the gnat smears its load of pollen onto the orchid's stigma, and then picks up a new parcel of pollen. To ensure that the orchid doesn't get pollinated with its own pollen, the gnat's prison is precisely shaped and lined with angled bristles, so the insect can only move in one direction.