Grubs gnaw roots, maggots munch fruits and caterpillars chew leaves. In textbook food chains, animals eat plants, not the other way round.

But there are plant species that break this rule – at least 600 species of them on the last count. These are the carnivorous plants, and they routinely feast on insects, spiders, worms –even potentially small mammals.

Life for a carnivorous plant is challenging. They cannot very well march across the landscape in search of a meal. Dinner has to come to them. The plants have evolved sticky leaves, water pots and the like to catch animals, but how – if at all – do they lure their prey into these traps?

A study published in February 2016 shows for the first time that some carnivorous plants use smells to secure meals – validating an idea that Charles Darwin suggested 140 years ago.

Darwin worked on the sundews, a type of predatory plant with leaves covered in tentacles, each tentacle having a drop of sticky fluid at its tip. Darwin described the sticky leaves as“temporary stomachs” with which the plants catch live prey, break it down with acids, and “feed like animals”.

Surprised by the large numbers of insects caught in sundew traps, Darwin suspected that the plants release odours that attract insect prey to their sticky leaves – although he did not test the idea.

In fact, nobody thought to explore Darwin’s hypothesis for over a century.

“It’s common to analyse plant volatiles, so it’s quite amazing that nobody has tested Darwin’s hypothesis,” says chemical ecologist Ashraf El-Sayed at the New Zealand Institute for Plant & Food Research Limited.

Studying sundews in New Zealand, El-Sayed’s team found that one species, Drosera auriculata, uses smells to lure prey. Their leaves emit volatiles that beckon gnats, midges and mosquitoes.

Carnivorous plants face a more profound problem: sex

“I was working on lure-and-kill tactics in pest management when I realised that wow, carnivorous plants have been at it for a very long time,” says El-Sayed.

Carnivory evolved independently at least six times across the plant kingdom. Carnivorous plants live in places like bogs and rocky slopes where the soil – if there is any – is so nutrient-poor that few plants can survive. Carnivorous plants eke out a living here because they converged on the same solution to the nutrient problem: animals are nutritious, so eat them.

But the path to meat-eating is costly. As plants transform their leaves into traps that can trick, bind, drown, and digest prey, they gradually become less effective for harnessing sunlight to produce energy. Therefore, most carnivorous plants grow slowly and stay small.

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