The World Shifts When a Black Widow Squats
By changing its posture on its web, the arachnid can tune vibration in its legs to different frequencies.
A spider’s web is more than a trap or a home. It is also an extension of the spider’s senses. By paying attention to vibrations traveling through the silken threads, the arachnid can learn about its surroundings. Certain vibrations might mean ensnared prey. A different frequency might reveal a nearby mate. And since spiders extrude their webs from their bodies, they can also change the stiffness, tension, and other properties of the silk to bring certain details into focus.
A spider, in other words, can actively tune its web to channel specific kinds of vibrations, just as a musician might tune an instrument.
But as Natasha Mhatre from the University of Western Ontario has found, a spider can also tune itself. Simply by changing its stance, the infamous black widow can make its sense organs more receptive to particular frequencies of vibration. It’s like a postural squint, which allows the spider to focus its attention on certain sources of information.
When Mhatre started studying black widows, as part of Andrew Mason’s team at the University of Toronto, she initially focused on how vibrations move through the silk. But she soon realized that “there was another problem, which was staring us in the face and which no one had considered.” Which is: How do those vibrations move through the spider itself?
Together with Senthurran Sivalinghem, Mhatre allowed captive black widows to build webs on square arenas, with a pillar in each corner. These webs aren’t the elegant, vertical, circular constructions that most people might picture. Instead, they’re a chaotic mess of strands, surrounding and supporting a loose, horizontal mesh, almost like an acrobat’s safety net, from which the spider hangs upside down.
When the webs were finished, the team placed a tiny magnet on them. By holding a powerful electromagnet nearby, they could move each web and then, by bathing the animal in lasers and analyzing the reflected beams, measure how the vibrations affected different parts of the suspended spider. Through the process, the widows were remarkably chill. Despite their infamous venom, “they’re very docile,” Mhatre says.
As with most spiders, the black widow’s entire body acts as a sensor. It’s dotted with thousands of organs called slit sensilla, which appear as tiny cracks in the exoskeleton. As vibrations pass through the animal, the cracks narrow and widen, and those minuscule movements are picked up by sensitive cells inside the slits. These slits are everywhere, but they’re especially concentrated in the joints of the legs.
Scientists have been studying slit sensilla for decades, and most experiments have shown that they respond to a wide range of frequencies, without much in the way of tuning. But that’s only true if you study the sensilla in isolation, as most researchers have. Mhatre showed that in an actual spider, hanging from its web, different joints are indeed tuned to different frequencies. “While the sensors themselves aren’t particularly tuned, the body gives the joints tuning,” she says.
When the spider changes its posture, it also retunes its joints. Typically, it sits in a neutral stance with its body horizontal and its legs outstretched. But it can also “crouch” by drawing all its legs in. In this pose, almost all of its joints become more sensitive to higher frequencies. By taking up a kind of predatory power-pose, the widow alters its senses.
“Hearing organs, in animals that use vibrations, are usually thought of as passive devices,” says Damian Elias from the University of California at Berkeley, who studies spider communication. That’s especially true for the slit sensilla, “as they’re just strain gauges sitting on joints, without any obvious way to modulate their sensitivity.” But Mhatre’s study shows that there is a way—and a very simple one.
She suspects that the crouched posture allows the widow to pay closer attention to higher frequencies, such as those produced by small prey insects. Alternatively, it could be trying to ignore low frequencies, such as those produced by wind. Both explanations make sense, since widows usually crouch when they’re hungry or when their webs have been significantly disturbed. In this position, they could better detect the movements of meals. And if a spider needs to get back in touch with low-frequency vibrations, all she has to do is extend a leg.
The widow’s abilities are part of a concept called “embodied cognition,” which argues that a creature’s ability to sense and think involves its entire body, not just its brain and sense organs. Octopus arms, for example, can grab and manipulate food without ever calling on the central brain. Female crickets can start turning toward the sound of a male using only the ears and neurons in their legs, well before their central nervous system even has a chance to process the noise. In the case of the black widow, the information provided by the sense organs in the legs depends on the position of the entire animal.
Earlier, I described this as a postural squint. That’s close, but the analogy isn’t quite right, since squinting helps us focus on particular parts of space. Here, the spider is focusing on different parts of information space. It’s as if a human could focus on red colors by squatting, or single out high-pitched sounds by going into downward dog (or downward spider).
The ability to sense vibrations that move through solid surfaces, as distinct from sounds that travel through air, is “an often overlooked aspect of animal communication,” says Beth Mortimer from the University of Oxford, who studies it in creatures from elephants to spiders. It’s likely, then, that the widow’s ability to control perception through posture “almost certainly [exists in] other spiders and web types, too, and other arthropods, including insects, that detect vibrations along surfaces through their legs.” Scientists just need to tune in.