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The paper
S. Scherzer et al., “Venus flytrap trigger hairs are micronewton mechano-sensors that can detect small insect prey,” Nat Plants, 5:670–75, 2019.  

The “mouth” of a Venus flytrap (Dionaea muscipula) bears several trigger hairs, multicellular spikes that send electrical impulses across the lobes of the trap when bent by contact with an object. Sönke Scherzer, who studies the plants at the University of Wuerzburg in Germany, says he’ll often gift Venus flytraps to his students and instruct them to feed the plants. Initially, the trap will close on a bit of cheese or a dead insect, but, to the frustration of the students, it will reopen after a few hours, indifferent to the gift. That’s because the initial stimulus doesn’t fully seal the trap and launch the digestive process; complete closure requires sustained wiggling for...

BEWARE THE HAIR: The Venus flytrap’s trap has several mechanosensitive trigger hairs that propagate action potentials across the trap when bent with a particular force, velocity, and angle. Closure is a two-step process, in which the initial snap is caused by two action potentials (1 and 4). Subsequent contacts with trigger hairs (2) signal the plant to seal the trap and start the digestive process (3). Recent experiments found that the hairs are sensitive enough to respond to ants walking across the trap, but that smaller traps are more sensitive than larger ones (5), giving small prey the opportunity to escape from large traps (6) that might otherwise waste digestive energy on tiny meals. WEB | PDF
Kelly Finan

Just how the plants can tell dinner from debris was the question Scherzer’s group recently sought to answer by observing Venus flytraps in the lab. Using a tiny force meter in combination with electrophysiological recordings to capture action potentials, the researchers measured the trigger hairs’ responses to ants walking across the trap leaves. They reported in Nature Plants last year that the force applied to the trigger hairs didn’t matter so much as how far and how quickly they were bent. The plants responded to stimuli that were fast, like those from a wriggling insect. Too slow, and they ignored the movement. 

“This mechanism would ensure that it is something living that is inside the leaves, rather than something like a little piece of stick or other things that they are not interested in investing in digesting,” says Naomi Nakayama, who studies plant biomechanics at Imperial College London and was not involved in the project.

Venus flytraps have an additional method of selecting the right meals, Scherzer’s team found. Smaller traps were more sensitive to stimuli than larger traps, responding to smaller forces. Scherzer speculates that this could allow big traps to avoid wasting resources digesting tiny prey, an idea that is backed up by his observations that small insects can escape the initial closure of large traps before they fully seal. “The point is, there are so many prevention mechanisms” to avoid wasting digestion efforts, he says.

It’s possible that Venus flytraps also have a means of detecting slow-moving prey—say, larvae. In 2019, Ueli Grossniklaus and his colleagues at the University of Zurich reported in a preprint on bioRxiv that, contrary to the commonly held belief that two trigger hair deflections are needed to spring the initial closure of the trap, one very slow push can also cause two action potentials and snap the jaws of the plant shut (DOI:10.1101/697797). “Maybe snails or slow-moving prey could get caught,” Grossniklaus says.

Kerry Grens is a senior editor and the news director at The Scientist. Email her at kgrens@the-scientist.com.

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