If you’re an organism with a hankering for flesh, a few things can come in handy. Fast feet, for one, to give chase, and maybe some claws and a sharp set of teeth. Good vision doesn’t hurt, either—especially when it’s teamed up with a keen nose and big, sensitive ears.
Unfortunately, the carnivorous Venus flytrap (Dionaea muscipula), which dines on bugs and spiders to compensate for the nutrient-poor soil of its natural wetland environment, has none of these lethal assets at its disposal.
But there’s no need to pity these predatory plants: An exquisite sense of touch is all it takes for Venus flytraps to snare their tiny prey. That’s all thanks to the sensory capabilities of a few stiff, sensitive hairs poking out from the inner surfaces of their mouth-like leaves—each tuned to discern the infinitesimally small shudders of an insect on the go. Now, scientists have finally put numbers to just how perceptive these trigger-happy hairs can be.
A study published today in the journal Nature Plants shows that Venus flytrap hairs are sensitive enough to detect critters that weigh as little as 3 milligrams—less than a typical sesame seed. While this threshold may exclude the most petite of prey species, having a size cutoff may actually help the plant avoid meals too small to be worth the trouble.
When vacant, a Venus flytrap leaf resembles an open pairs of jaws. In reality, each trap is an entire digestive tract collapsed into a single, bi-lobed pouch, rimmed with fang-like protrusions. Studding the inside of the trap are a handful of thin, rigid hairs, each attached to a flexible base that can register the movements of the rod above—a bit like a joystick.
This unusual setup allows the hairs to function like tripwires. When something bumps up against them, electrical signals spread through the trap’s leafy lobes. Digestion then proceeds in a series steps: First, the trap slams shut; then, it hermetically seals. Eventually, the now-airtight chamber will flood with flesh-melting fluid, transforming whatever’s inside into sticky sludge.
Once this process begins, it can take over a week for the trap to reopen—making mistakes very costly.
Luckily, the Venus flytrap has evolved a few safeguards to avoid responding to false alarms like breeze-borne debris, or the pitter-patter of intermittent raindrops. Three years ago, a team of researchers led by Rainer Hedrich, a plant physiologist at the University of Würzburg in Germany, discovered that traps can “count” the number of times their hairs are nudged. Each bump triggers its own electrical impulse—and the more times the hairs are touched in a row, the further along the path of consumption the plant will progress. Two touches, and the two halves of the leaf come together in a loose cage; a third zippers them tightly shut. A few more, brought on by the catch struggling within, and the whole contraption morphs into an acidic stomach, liquifying its contents.
Before any of this can happen, though, a trap’s hairs need to know when it’s appropriate to sound the alarm. “It was a simple question,” says study author Sönke Scherzer, who conducts research under Hedrich’s supervision. “But we just wanted to know how sensitive these hairs really are.”
To define the limits of flytrap touchiness, Scherzer and his colleagues hooked a series of plants up to a computer that monitored their electrical activity. They then used a small, motorized device to flick the trap’s hairs, all the while looking for the subtlest movements sufficient to trigger a response.
It didn’t take much. For a signal to be sent, a tilting hair had to meet three criteria: It needed to bend more than 2.9 degrees, move fast enough to traverse at least 3.4 degrees within the span of a second, and feel a force of at least 29 micronewtons (roughly equivalent to being stepped on by a three-milligram bug, like a small mosquito).
These numbers, Scherzer says, matched up nicely with what the hairs experienced when the researchers allowed them to be trampled by a colony of ants—a common meal for wild flytraps.
Though several of the team’s findings are pretty intuitive, their paper is the first to quantitatively assess how these hairs are triggered, says Barbara Pickard, an expert in plant sensory physiology at the Washington University in St. Louis who was not involved in the study. “Plant mechanosensing [response to detection of touch] is far more generalized and sensitive than people realize,” she says.
Not all traps are created equal, however. Each Venus flytrap hosts several sets of “jaws” of different sizes—and they don’t appear to behave in the same way. In the study, the researchers found that larger traps contained hairs that were both thicker at the base and less sensitive than those in smaller traps.
From a physical standpoint, this simply reflects the fact that bulkier hairs are more difficult to bend. But there may be an evolutionary impetus behind this adaptation as well, points out Laura Hamon, a Venus flytrap researcher at North Carolina State University who was not involved in the study.
While the teeniest traps can still get a nutritious meal out of insects and spiders weighing only a couple milligrams, it may be less advantageous for larger traps to invest in such puny portions. The higher threshold of big traps, then, might function as a precaution that prevents them from biting off less than they can chew, Hamon says.
Pickard supports the notion of size selection, highlighting the work of plant biologists Siegfried Hartmeyer and Stephen Williams, which indicates that Venus flytraps have an entire suite of traits to help them avoid unsatisfying meals. “Closing and reopening the trap takes a lot of the leaf’s available energy,” she says. “So catching small prey can be a net loss.”
According to Hartmeyer and Williams’ research, which is currently being submitted for peer-reviewed publication, the plants may use specialized scent glands to lure especially tiny prey, like certain types of ants, to the edges of their traps and away from the trigger hairs. Even after the leaf shuts, bugs small enough to slip out of the leaf’s gap-toothed grin are able to escape before the trap seals completely.
All this goes to say that Venus flytraps may be even more calculating killers than once thought. “Plants are a lot more complex than we give them credit for,” says Angela Schlegel, who studies mechanical signals in plants at Washington University in St. Louis, but was not involved in the study. “This paper is a highlight of how we can study that complexity.”