Pitcher plants catapult prey

Pitcher plants have developed a novel mechanism for capturing their prey – they use heavy rain to catapult insects into their fluid-filled pitcher.

Nepenthes is able to colonise nutrient-poor habitats because they capture insect prey in specialised pitcher-shaped leaves with slippery surfaces on the upper rim and inner wall. Once caught, the insect drowns in the digestive fluid at the bottom.

In humid conditions, the pitcher rim is covered by a thin film of water – when an insect walks on the wet surface its feet can’t make contact with the surface and they slip into the water. But in wet conditions, the lid of the Nepenthes gracilis acts like a springboard flicking insects into the digestive juices.

“It all started with the observation of a beetle seeking shelter under a N. gracilis lid during a tropical rainstorm,” said Dr Ulrike Bauer from the University of Cambridge. “Instead of finding a safe – and dry – place to rest the beetle ended up in the pitcher fluid, captured by the plant.”

“We had observed ants crawling under the lid without difficulty many times before, so we assumed the rain played a role, maybe causing the lid to vibrate and ‘catapulting’ the beetle into the trap, similar to the springboard at a swimming pool.”

To test this, researchers from the Department of Plant Sciences simulated rain with a hospital drip and recorded its effect on a captive colony of ants searching for nectar under the lid. They counted how many ants fell in relation to the total number of visitors. Ants were safe directly before and after the ‘rain’ but when it ‘rained’, 40% of ants got trapped.

Further investigation revealed the lower surface of the N. gracilis pitcher is covered with highly specialised wax crystals which provide the right level of slipperiness to enable insects to walk on the surface under calm conditions, but to lose their footing when the lid is disturbed. The pitcher was also discovered to secrete larger amounts of attractive nectar than other pitcher plants.

“Scientists have tried to unravel the mysteries of these plants since the days of Charles Darwin,” said Bauer, lead author of the paper published in PLoS One. “The fact we keep discovering new trapping mechanisms in the 21^st Century makes me curious what other surprises these amazing plants might still have in store.”

• With a flick of the lid: a novel trapping mechanism in Nepenthes gracilis Pitcher Plants