Every move you make, the spider’s watching you

Today, it’s spiders. Specifically, jumping spiders, scientific name Phidippus audax. They are commonly referred to as “daring”, or “bold”, jumping spiders; in fact, the Latin word audax means daring. As spiders go, these are large ones—but only about a centimetre long. They are black, but with orange spots on their abdomens and stripes on their legs. They are also known for their mouth parts (chelicerae), which are often an almost fluorescent turquoise. And yes, they jump. They jump to catch prey, or to evade predators. They jump sometimes as much as 50 times their own body length. If that doesn’t grab your attention, understand that a corresponding jump for you would be nearly as long as a football field—and now consider these little beasts with a dose of new respect.

But even with that spectacular capability, perhaps what’s most unusual about these spiders are their large eyes—relatively large, of course. They have four pairs of them. In the largest pair, scientists have counted about 7,000 photoreceptor cells in each eye. That’s a striking number in an organ that’s only about half a millimetre in diameter. This density of photoreceptors gives these spiders truly remarkable eyesight: their “spatial acuity”—in essence, the ability to detect the shape of objects—is estimated to be as good as a cat’s or a pigeon’s. They can see colour, sense depth and are known to follow prey with their eyes—until, presumably, that final great leap to catch them. Besides, the four pairs together give the spiders nearly 360° vision. They are also famous for using their eyes in courtship “dances”, which puts in mind a hundred Bollywood films in which lovey-dovey couples eye each other coyly.

In fact, given the relative sizes of its eyes, a jumping spider’s vision is probably almost as clear as a dog’s. Think of that for a moment: an animal half the size of a one-rupee coin, and she probably sees the world around her just as well as Kumari, the stray dog that roams your street. All of which means that a potential prey loitering in the vicinity of a P. audax, I would think, should be seriously worried for its life.

At least this much about jumping spiders has been known for a while now. But in 2019, they were the subject of a most interesting scientific paper (Growing tiny eyes: How juvenile jumping spiders retain high visual performance in the face of size limitations and developmental constraints, John T. Goté, Nathan I. Morehouse and team, Vision Research, July 2019. Broadly, the theme of the paper goes like this: If adults of the species are half the size of a one-rupee coin, babies are a hundred times smaller—and yet, these biologists found that spider babies probably have vision that’s every bit as good as their parents’. Why?

But before we come to that … the paper fascinated me in many ways, trivial and not so trivial. One trivial note first: it tells us the precise latitude and longitude where the scientists collected spiders: 40° 44¢ 44.4² N, 80° 09¢ 49.0² W, the Kretschmann Family Organic Farm in rural Pennsylvania. Locating it on a map, I realize that I drove right past this spot—within a kilometre—several times many years ago. Had I known, I might have stopped and acquainted myself with jumping arachnids.

Call that a personal aside. The spiders gathered from the Kretschmann Farm “were fed twice weekly with cricket nymphs” —each time, cricket nymphs amounting to twice the weight of the spider being fed. I have to wonder: Even if I eat only twice a week, would I be able to work my way, at each meal, through a pile of food amounting to twice my weight? And then ready myself to leap across a football field?

To document how the spiders’ eyes and bodies changed as they grew to adulthood, the scientists needed to regularly photograph them. This is not as easy a task as you might imagine. Which is why, each time, the spiders “were lightly anaesthetized using carbon dioxide prior to imaging to help reduce movement.” Not a bad thing to do, given these are animals prone to jumping suddenly. Though even the anaesthetic wasn’t enough to guarantee they stayed immobile for the shoots. Which is why, each time, they mounted the semi-senseless spider on a metal rod, with beeswax.

Using an instrument much like the opthalmoscope your eye-doctor uses to check your eyes, only smaller, the scientists examined one of the four pairs of eyes on the juvenile jumping spiders. The surprise was that they found about 7,000 photoreceptors in each eye, as many as the adults have.

Also, they examined seven of these babies again after four months —enough time for them to, in turn, become adults themselves, with all the attendant body changes. And what did they find in these adult eyes? No new photoreceptors.

In other words, jumping spiders don’t grow to their acute sight. Human babies take up to five years to match the visual ability their parents possess. P. audax young, by contrast, are born with their acute vision essentially in place. In effect, their ability to see so well is an evolutionary compensation for being so tiny—the researchers describe it as “a means for ameliorating the impact of small size on light capture and other visual functions.”

Still, the only area in which those baby eyes are lacking is their sensitivity to light—perhaps exactly because the photoreceptors in their eyes are so tiny at that stage in their lives. As the researchers write, “young jumping spiders have eyes already equipped for high acuity vision, but these young spiders may struggle to perform visually demanding behaviours in low-light environments.” In any case, even with this lower sensitivity, the question that arises is: “How do juvenile jumping spiders seemingly retain visual functionality … despite extreme space limitations?”

One reason this is possible is that the juveniles’ eyes are proportionately larger, given their respective body sizes, than the adults’. This is hardly unusual—as the paper points out, the young of various animals—like fish, giraffes, geckos, primates and sharks—all “tend to exhibit disproportionately large” eye sizes. And is there a dog lover who hasn’t noticed and reacted to the large soulful eyes on the puppies of their favourite breed?

In fact, this is a more common phenomenon than you might imagine. If we run into uncommonly large eyes in our own species, we “associate [them] with youthfulness”. Chew on that for a moment. There’s also a cuteness factor, of course—with those colours and those large eyes, jumping spiders are enormously photogenic. As are young kids with large sensitive eyes.

And finally, the paper points out that we even favour large eyes as a symbol of youth in “fictional animals”. Reading that, I thought of manga-style animations. The paper referenced teddy bears.

A scientific paper that manages to link jumping spiders to teddy bears? A winner, I say.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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