Pitcher plants use raindrop impacts to fling insects into their trap

Raindrops trigger a predatory diving board that cannonballs insects into a pool of digestive fluids

Life 3 August 2022

Five green slender pitcher plants are suspended on vines

A leafy catapult helps pitcher plants (Nepenthes gracilis) capture insects

GFC Collection/Alamy

The slender pitcher plant of South-East Asia has a leafy lid that acts like a springboard, launching prey into a deep cavity filled with digestive juices. Now, scientists have figured out how this macabre machinery works.

Carnivorous pitcher plants have specialised leaves shaped like elongated sacks that hold digestive fluid. The vessels are baited with nectar to lure unsuspecting insects and lined on the inside with slippery wax that sends the critters tumbling towards death.

Typically, insects just slip in, unable to cling to the waxy inner lining. But the slender pitcher plant (Nepenthes gracilis) plays a more active role. It baits ant troops to the underside of the leafy lid covering the pitcher. Then, when raindrops pluck the lid, it triggers fast twitches that catapult prey into the gastric pool below – a bit like a lethal version of the children’s trampoline game Crack The Egg.

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To find the elastic components behind the trap, Anne-Kristin Lenz at the University of Bristol in the UK and her colleagues used high-powered x-rays to see inside the plants as they recoiled during rainfall.

Initially, the researchers expected to find the “spring” in the neck. However, they discovered that pitchers deformed well into their hollow bodies. “The most interesting thing was to get proven wrong,” says Lenz.

When rain strikes the lid, it flexes down, channelling energy through the narrow neck connecting it to the pitcher and compressing a springy region several centimetres down the body of the pitcher. Then, the plant releases stored elastic energy and the lid springs upwards. The jerking motion whiplashes bugs into the trap.

The plant’s geometry constrains lid movement on the upswing so that it doesn’t lift far beyond its resting position. “It’s much easier to push down than up,” says co-author Ulrike Bauer, also at the University of Bristol.

That dampens the diving board’s oscillations by halting it on the way up and quickly resets the trap to catch the next round of raindrops.

“This is the only known carnivorous plant that uses a really fast, completely externally powered movement,” says Bauer.

“It’s remarkable that such a thing has evolved,” says Simon Poppinga at the Technical University of Darmstadt in Germany who was not involved with the study.

The discovery of the botanical catapult’s mechanics could lend itself to technology, Poppinga says, inspiring the design of devices like switches, locks or rain energy harvesters.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2022.0106

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