Plant's smelly trick on flies

The arum (with cutaway showing chamber) is irresistible to flies

Its name describes it perfectly - the dead horse arum is a flower that smells like rotting meat.

Flowers have learnt to use these irresistible odours

Prof Bill Hansson

It uses the stench to lure female blowflies looking for the perfect place to lay their eggs.

The insects find no flesh when they land, of course, but as they race from plant to plant searching for the stinking meat, they pollinate the arum.

It is a very clever chemical trick, which scientists can now describe in some detail.

All the same

Researchers from the Swedish University of Agricultural Sciences and Italy's University of Cagliari did a comparison of the smell-producing compounds produced by rotting meat and the flower (Helicodiceros muscivorus).

They used gas chromatography and flame ionisation to get a precise signature of both and were astonished to find that the original chemical and its copy were virtually identical - a bouquet of three oligosulphides, a byproduct of protein decomposition in decaying meat.

"And as far as the fly is concerned, there is no difference," Professor Bill Hansson, from the Swedish University of Agricultural Sciences, said.

"From its perspective, the odours are identical," he told BBC News Online.

In the dark

The arum, which is found on small islands off the coasts of Sardinia, Corsica and Spain's Balearic islands in the western Mediterranean, is not naturally self-pollinating.

Its male parts mature after its female parts and the delay means the plant must take a fly prisoner for a short period if it wants to pass its pollen on to another flower.

This is how it works. An insect, laden with pollen from one arum, will crawl down into a chamber of a stinking plant where the female parts, or florets, are located.

Spines and filaments at the opening to the chamber then close, trapping the fly for a few hours - long enough for it to stumble around fertilising the florets.

Can't resist

"Put your ear to a plant at this time and it is amazing - it's buzzing like hell. There may be 20 to 40 flies trapped in the chamber," Professor Hansson said.

When the spines move back, the fly scrambles out of the chamber - but not before it has brushed past the newly-matured male parts and picked up fresh pollen to take to the next arum.

"All such systems in plants build on mimicking odours that are totally irresistible to the insects. With the pheromones, it is sex - and you cannot resist sex because if you do you cannot procreate.

"Here, the odours are for ovipositing (egg laying) and you cannot resist them because then you cannot lay your eggs, or you lay them in the wrong place. Either way, you are genetically dead. So the flowers have 'learnt' to use these irresistible odours."

Look and feel

The arum may reinforce the smell deception with visual and thermal cues, the researchers speculate.

The inside of the plant rises to 15 degrees above the ambient temperature - another draw to the flies.

"Also, the flower has the same colour as flesh - it is reddish-brown. And it has hairs like an animal's pelt and a dark hole where flies like to climb in."

The research team will do experiments to verify these theories.

Their latest work was published in the journal Nature.