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A new study shows that the lid of a pitcher plant not only keeps rain from flooding the plant’s pitcher but it doubles as an insect catching device.

Polyrhachis pruinosa ant visiting a pitcher

2012 Bauer et al

A new and cunning way of trapping insects has been discovered in the pitcher plant Nepenthes gracilis. Pitcher plants’ nectar attracts insects to their pitcher-shaped leaves, which are partially filled with digestive fluid. Insects typically tumble into the fluid and get digested when they lose their footing on the wet and slippery upper rim of the pitcher, or on the inner pitcher walls. A new study shows that in N. gracilis the lid, which keeps rain from flooding the plant’s pitcher, doubles as an insect catching device.

One of the authors noticed that a beetle, seeking shelter from the rain under the pitcher’s lid, was flicked down into the pitcher when a raindrop jolted the lid from above. University of Cambridge biologist Ulrike Bauer and colleagues decided to investigate further. She videotaped ants foraging on N. gracilis pitchers in the lab and subjected the plants to “rain” from a hospital drip. The scientists analyzed the videos and compared the number of ants fallen from the different trapping surfaces before, during, and after the simulated rain.

Under dry conditions, only a few ants got trapped and almost all of them fell from the slippery inner pitcher wall. After the “rain,” most victims fell from the pitcher’s rim, which is safe to walk on when it’s dry but gets slick when wet. But while drops fell, the majority of trapped insects fell from beneath the drop-shaken lid, whose underside is coated in specially shaped wax crystals. The wax crystals provide a slightly rough surface and the insects can walk under the lid in calm conditions, says Bauer, but they can’t hold onto it very strongly.

Coating the underside of the lid with a non-slippery polymer film reduced the plants’ capture success in the wild, which suggests that the lid-flicking trick plays an important role for N. gracilis and helps it to capture more prey. (PLoS ONE)