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An orchid that smells like roadkill

An orchid from southern South Africa has evolved a novel way to attract carrion flies and use them as pollinators, by smelling like rotting meat.

A stinker of an orchid in South Africa fools certain types of carrion flies into acting as pollinators. Exuding the faint scent of rotting flesh, the flower entices a flesh-fly, drawing it deeper inside the bloom with more tempting smelly scents until the fly reaches just the right location to pick up some pollen. Flesh-flies are better at finding carrion than other flies, and this orchid, Satyrium pumilum, has evolved a certain shape and size – and smell – just to attract these flies.

This orchid is found in a region of South Africa known as the Cape Floral Kingdom, a place with a very rich diversity of plant life. Timotheüs van der Niet at the University of KwaZulu-Natal was curious to find out how the plant attracted flies. He and his colleagues observed the orchids in the wild to see what types of flies were attracted to it, and compared those flies with ones found on decaying dead animals.

Fly with pollen entering an unpollinated flower of the orchid Satyrium pumilum. Photo Credit: Dennis Hansen.

Said van der Niet, in a press release,

We didn’t kill creatures to entice the flies. Instead we used dassies (rock hyraxes). They’re small animals and they look a little like a guinea pig. You can find them almost anywhere in South Africa, and that means you can also find them as roadkill. So we examined the flies visiting the dead dassies, and compared them to the flies visiting the orchids.

Because of the high density of orchids we didn’t see many flies visiting the flowers, but on the nearby dassie carcass we caught a lot of flies carrying orchid pollen, providing ample “smoking gun” evidence of how common this interaction was. However, we found that not every species of carrion fly at the dassie carcass had orchid pollen on it. The ones that were carrying the pollen were flesh-flies, mostly females.

Fly with pollen on a flower of the orchid Satyrium pumilum. Photo Credit: Dennis Hansen.

Van der Niet added,

The flowers of the orchids are incredibly specialised. Not only do they have to entice flies in, but they have to get flies of the right size into the right position to pick up the pollen. We’ve found that scent plays a hugely important role in pulling in the flies, and even inside the flower different scents attract the flies into the right location to pick up the pollen. The combination of smell and sight is irresistible to some flies. The level of carrion mimicry is amazing; we even saw a female fly leave larvae in a flower because it thought it was carrion.

What we’ve done is show for the first time that carrion-mimicking flowers are highly sophisticated tools for orchids. It’s not just any fly that the orchid is after. For Satyrium pumilum we can now see exactly how successful mimicry is for pollination. It also disproves a cliché. You don’t always catch more flies with honey.

The orchid Satyrium pumilum. Photo Credit: Dennis Hansen.

Flesh-flies in southern South Africa sometimes get cheated of a meal and a place to lay their eggs by an orchid posing as roadkill. These flies are better than other carrion flies at finding dead animals. Perhaps that’s why the orchid Satyrium pumilum evolved to specifically attract flesh-flies, by releasing faint rotting-meat odors, attracting the flies to the flower, and manipulating them into the flower to pick up or drop off some pollen.

The habitat of the orchid Satyrium pumilum at the village of Leliefontein in the Namaqualand Kamiesberg, South Africa. Photo Credit: Dennis Hansen.

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