Read: What it feels like to get bitten by a black widow spider
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.