Pitcher Plant Uses Raindrops to Trap Insects

By Chelsea Whyte on June 14, 2012 7:18 PM EDT

pitcher plant with ant
(A) N. gracilis pitcher plant with visiting Polyrhachis pruinosa ant, showing the wax crystal surfaces on the inner pitcher wall and on the underside of the pitcher lid. (B) The horizontal orientation directly above the pitcher opening puts the lower lid surface in an ideal position for prey capture. (Photo: Creative Commons: Bauer et. al)

It's raining, it's pouring, and the pitcher plant is eating. With a little trick, the pitcher plant - a stove-pipe-shaped plant with tubular leaves that trap insects in a pool of digestive fluid - lures its prey with the help of precipitation.

In humid climates, the pitcher plant's leaves and rim are covered in water, creating slippery surfaces that are hard for insects to grasp onto and resulting in a hydroplaning effect that dumps unsuspecting ants and beetles into the plant's belly.

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Researchers from the University of Cambridge have now discovered a new way for the pitcher plant to get some grub.

Pitcher plants, or Nepenthes gracilis, use a highly-attractive nectar to attract insects to the underside of its leaves, which are covered in wax crystals that are sticky enough for a bug to walk on when dry, but make it difficult for insects to cling to when wet. These structures also stop trapped prey from climbing up the walls of the tube and out of the flesh-eating plant's catch.

And if an ant happens to be hiding out on the underside of the plant's lid during a storm, the force of the raindrops hitting the top of the lid can hurtle the insect into the cup of the plant below.

"It all started with the observation of a beetle seeking shelter under a N. gracilis lid during a tropical rainstorm. Instead of finding a safe - and dry - place to rest, the beetle ended up in the pitcher fluid, captured by the plant," said study author Ulrike Bauer from the University of Cambridge's Department of Plant Sciences.

"We had observed ants crawling under the lid without difficulty many times before, so we assumed that 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," Bauer said.

"It's amazing to see things like this in the field," Bauer said, according to The Daily Mail. "The pitcher plants have very unique mechanisms, but this is perhaps the most amazing."

So they took to the lab to recreate the effect, using a hospital drip as a stand in for raindrops and a colony of ants feeding on the nectar under the plant's lid. They found the ants had little trouble walking across the underside of the lid before and after the simulated rainstorms, but 40 percent of the ants who visited the plant were dropped into the cup below when drops were hitting the lid.

Looking at the makeup of the plant's lid under a microscope, the researchers found that instead of the flaky crystal structure found on the inner walls of the pitcher, which are difficult for insects to grab hold of, the underside of the lid has a crystal structure that forms columns.

By providing a normal foothold, the lid ensures that insects are well positioned over the fluid below, before the impact of falling rain dislodges them, according to Discover Magazine. This might explain why the slender pitchers contain more insect corpses after rainy days.

"Scientists have tried to unravel the mysteries of these plants since the days of Charles Darwin," Bauer said. These findings, published in the journal PLoS One, add to the surprising nature of these "amazing plants," she added.

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