How the slender pitcher plant springboards its prey to their doom

The slender pitcher plant, Nepenthes gracilis, is a particularly clever carnivorous plant.

Like other pitcher plants, it uses a long, slippery funnel to capture prey. Once an insect has fallen into the plant’s tube, it’s not going to make its way out again.

But gracilis also comes with a lid. An insect is lured to the underside of the leaf at the top of the funnel. Then, when a falling raindrop – very common in the plant’s native Southeast Asian jungles – lands on the lid, the weight causes it to snap shut. The sudden movement topples the insect down the pitcher plant’s deadly funnel.

This springboard mechanism, using external energy as it does, is an absolute boon to the plant. A study published in Biology Letters explains just how smart it is.

The Slender Pitcher Plant using raindrops to capture prey. Credit: Anne-Kristin Lenz

The researchers, who are based at the University of Bristol, UK, examined 3D-scans of the slender pitcher plant capturing its prey. They watched the way the plant deformed as the lid snapped shut, and gently reopened.

Two pitcher plants, they are long green tubes connected to vines at their base and little lid-like leaves at the top
The Slender Pitcher Plant, Nepenthes gracilis. Credit: Dr Ulrike Bauer

To their surprise, they found the secret for the lid’s springboard sat far down the pitcher’s tube.

“If you look at the pitcher shape you would assume that the deformation happens at the smallest cross section, which is the transition point from lid to pitcher tube, but in fact it also deforms further down at the back of the pitcher tube,” says lead author Anne-Kristin Lenz, a PhD candidate at Bristol’s School of Biological Sciences.

This gives it a couple of advantages. It makes the spring direction-dependent: moving down quickly, but up slowly. It also stops the lid from twisting or wobbling, which means that more energy can be used to snap the lid shut.

The researchers examined a similar pitcher plant, Nepenthes rafflesiana – but found it didn’t have these mechanisms.

Nepenthes gracilis uses small changes in the trap shape to transmit impact energy with astounding efficiency,” says Lenz.

Photo of pitcher plant, closeup with ants underneath leaf, next to two 3d images of pitcher plant with lid up and down highlighted
Imaging the movements of the pitcher plant. Credit: Anne-Kristin Lenz

“We can learn from these plants how to optimize structures geometrically, which could help to save material and weight, while still having a functional spring.

“The springboard trapping mechanism might even provide inspiration for designing new mechanical devices for harvesting energy from rain or hail.”

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