A female zebra jumping spiderKaldari / Wikipedia

Last Wednesday, a spider fell onto Jamie Lomax’s laptop. Two days later, it happened again. Soon enough, several spiders were crawling across the ceiling of her office. “It was a little unnerving,” says Lomax, who’s an astronomer at the University of Washington. “I’m not scared of spiders but if someone else wants to take care of the spider in a room, I’ll gladly let them do it over me. And I don’t really want them raining down on my head.”

Lomax identified the abseiling arachnids as zebra jumping spiders, and tweeted about her experiences with the hashtag #ItIsRainingSpidersNotMen. And after considering options including “nukes and fire,” she settled for notifying her university. They sent over an exterminator, who failed to find any lingering spiders within the ceiling. He figured that a nest had probably hatched, and the newborn spiders had scattered. “But a couple of hours later, there were still spiders everywhere,” she tells me. “As of yesterday, there still were.”  

Meanwhile, fellow astronomer Alex Parker had read Lomax’s tweets. “Have you tried lasers?” he replied. “Seriously though, some jumping spiders will chase laser pointers like cats do.”

There are, indeed, many Youtube videos of them doing exactly that. But Emily Levesque—Lomax’s colleague, with an office two doors down—wanted to see it for herself. “She has a laser pointer and she happens to be the only other person with spiders in her office,” says Lomax. “She ran down to me and said: You have to see this.”

Being scientists, Lomax and Levesque tested laser pointers of different colors. They found that the zebra spiders seemed to be mildly interested in a red dot, but completely transfixed by a green one. They even tried using both lasers at the same time—and the spiders seemed to prefer green over red.

“Do all zebra spiders react more to green vs red laser pointers?” Levesque tweeted. “We need some kind of ‘science Twitter bat signal that we can put up when different fields need input from one another.”

But on Twitter, no such signal is necessary. Last night, spider researcher Catherine Scott saw the thread and looped in her friend Nate Morehouse, who studies spider vision at the University of Cincinnati. Morehouse was up late watching the Stanley Cup final and was distraught to see his team, the Pittsburgh Penguins, losing to the Nashville Predators. “I was all bummed out, and I decided to check Twitter before I went to bed,” he says. “I had like 150 notifications.”

“We can explain all of this!” he wrote to Levesque. Jumping spiders are visual hunters, which track their prey with the large pair of eyes at the front of their heads. The retinas of those eyes contain two types of light-detecting cells—one that’s sensitive to ultraviolet light and another that’s sensitive to green light. The latter cells aren’t only sensitive to green light; they react to red too, just less strongly. So jumping spiders can see red light, but it would just appear as a dimmer form of green to them.

No one has really studied the eyes of the zebra jumping spider, says Morehouse, but based on what we know from other species, it should react to laser pointers in exactly the way that Levesque and Lomax found. “It’s not a controlled experiment and the green laser might just be brighter or larger than the red one,” Morehouse tells me. “But if you had two equal laser pointers, one red and one green, we’d expect that the jumping spider should track the red one less enthusiastically.”

But there’s another reason why jumping spiders should fascinate astronomers, besides their occasional penchant for raining from the ceiling or chasing lasers. As Morehouse told Lomax and Levesque, their eyes “are built like… wait for it… Galilean telescopes.” These telescopes, which Galileo started using in 1609, are basically tubes with a lens at each end. Only three groups of animals have similar eyes: falcons, chameleons, and jumping spiders.

In the spider’s case, each of the two main eyes is topped with a large lens that’s fixed to the rest of the spider’s body. Beneath that is a long tube, filled with a clear gel. And at the bottom of the tube, the gel changes in a way that we still don’t understand, but that causes light to bend. It effectively acts like a second lens, even though there’s no distinct physical structure that you can dissect out.

The two lenses work in tandem: The top one collects and focuses light, while the bottom one spreads it out. This arrangement enlarges images before they hit the spider’s retina, which allow it to resolve a huge amount of detail for its size. A jumping spider can see objects as clearly as a pigeon or a small dog, even though its eye tube is less than a millimeter long, and its whole body gets no bigger than 5 millimeters.

Scientists know all of this because they can peer straight into a jumping spider’s eyes and study the retinas below. Those retinas have muscles and can swivel around like the back of a telescope, so the spider can change where it’s looking without moving its head. By watching them do this, and measuring their anatomy, people like Morehouse can work out how light travels through their eyes. And by extension, they can also calculate what sorts of things the spiders can see.

And as Morehouse told Lomax, Levesque, and the other gathered astronomers on Twitter, “They can definitely resolve the moon in the night sky.”

The astronomers did some quick calculations and worked out that the spiders can indeed see the moon, but not planets like Jupiter or Mars. They have enough spatial acuity for seeing Andromeda—our nearest galaxy—but they probably can’t make out light that dim. “The unfortunate thing about this crazy telescope-like eye is that it’s not as good for capturing light as something designed for night vision,” says Morehouse.

The moon, however, is almost certainly bright enough for them. Its light would hit between five and six light-detecting cells on their retinas, which might just be enough for them to make out the subtle darkness of the moon’s craters. “They could, in principle, see variation across that surface,” says Morehouse. “I’d have to look at luminance differences across the moon, but it’s possible.”

There’s something rather magical about all of this. Twitter has been variously described as an echo chamber where people go to have their views confirmed, or a cesspit where harassment and abuse go unchecked. But occasionally, it is also a place where astronomers with a sudden interest in spider vision can meet spider scientists with a sudden interest in the stars, and geek out over laser-chasing arachnids with telescopes for eyes.
