Jumping spiders look like most any other spider, but don’t act much like their araneid cousins. They don’t use sticky webs or traps to catch meals, or ambush prey that strays too close to a hiding place. Instead, they hunt by sight, actively stalking insects and then attacking with a precise leap onto their victims. It’s behavior that researchers say is “more vertebrate, or even mammalian, than spider-like.” They can hunt this way thanks to eyesight that’s sharper than most other arthropods and rivals that of big cats like lions, and an arrangement of eight eyes that gives them a near 360-degree field of view.

The spiders’ amazing vision is processed by a brain that’s about the size of a poppy seed and has long been a black box to researchers. The problem with studying the spiders’ neurobiology is that their internal fluids are highly pressurized. This allows them to power some of their movements, including their jumps, with hydraulic pressure, but it also means that cutting them open to access their brains results in “catastrophic fluid loss.” The spiders “bleed out,” sometimes bursting, and die.

Ron Hoy and the researchers in his neurobiology lab at Cornell University wanted a better look at how the spiders’ visual system is structured and functions by making a recording of their brain activity. To get around the exploding spider issue, one of the scientists, Gil Menda, suggested that a very tiny incision, just big enough to fit a hair-width recording electrode, would allow a spider’s body fluid to clot and the cut would heal. Making such a small hole and inserting the electrode would require a delicate touch and a subject that held very still, though, and a spider probably wouldn’t oblige the researchers with sitting calmly while they poked a hole in its head. The keep their research subjects in place, the team designed a tiny harness, made the parts with a 3D printer, and then sealed the spiders in it with sticky dental wax.

With the spiders immobilized and the electrodes inserted, the team was able to record the spiders’ brain activity as they were shown images of flies and other spiders on a video screen. When pictures of flies, the spiders’ natural prey, popped up, the researchers saw a burst of activity from certain neurons, revealing the cells associated with the spiders’ visual system.

To see how the spiders’ eight eyes split the work load, the researchers also made little blinders with the 3D printer and showed the spiders the fly images while blocking some eyes and leaving the others unobstructed. They found that the two pairs of forward-facing eyes handled different tasks. The large primary eyes are responsible for high-acuity vision, while the smaller secondary eyes take care of motion detection. When either pair was covered, the spider’s neural response to the images wasn’t as strong. Because different information comes from different sets of eyes, the spiders need all eyes on the prize to detect their prey.

In this video from the lab, you can watch Menda secure a spider in its harness and see how their brains react to spotting a fly.

And if you ever find yourself needing a spider harness of your own, the researchers’ design is available here.

[h/t ‏@realavivahr]