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How Much Brain Can You Pack Into a Spider Head?

MalePeacockSpider.jpg

Thirty years ago, state-of-the-art "minicomputers" (as they were then called) were refrigerator-size contraptions loaded with large, flat circuit boards, heavy cables, and noisy fans. Disk drives the size of washing machines spun stacks of large metal plates for a grand total of 300 megabytes of storage. If a minicomputer bore unquestionable evidence of advanced intelligent design just a few years ago, how much more so a modern smartphone (now a smartwatch) with more processing power, and one hundred times the memory, in a thousandth of the space?

Well, now consider the lowly spider. Its entire brain is about the size of a pinhead. Yet this creature can dance! Here is a video of Maratus volans, the "peacock spider" of Australia -- this little guy is a beauty:

No wonder the folks at Brain Decoder call this a "great mystery in a tiny head." Sara Goudarzi writes:

The spider may be itsy bitsy but its brain is nothing short of amazing. With just a poppy-seed-sized noggin, these arthropods employ sophisticated hunting methods, can find their way out of complicated labyrinths, and some have mating dance moves that take the breath away from their fellow spiders. [Emphasis added.]

The elaborate decorations on the male's abdomens come in bright fluorescent colors, with patterns that look like human faces and other artistic designs -- yet the whole spider is only about five millimeters long! Still more amazing is the central nervous system that allows the male to raise and shake its colorful banner on command, and raise its third legs, in a complex mating display that may last almost an hour.

Mating is just one behavior controlled by a spider's brain. There are many others: engaging all its senses, knowing how to hunt and find food, knowing how to operate its eight legs in a coordinated way -- in fact, many of the behaviors larger animals need to survive and reproduce. Goudarzi focuses on one brain power in particular:

Another amazing feature of some spiders is their sophisticated visual systems. Jumping spiders, for example, have eight eyes, giving them a nearly 360 degree panoramic view, with two front-facing eyes that are as acute as human eyes. The visual combo allows these hunters to pursue and pounce on prey, much like cats do. But an interesting question for scientists is how the spider brain actually processes the visual information.

"As acute as human eyes"... the tiny camera lenses in today's smartphones, allowing them to take pictures almost as sharp as SLRs, come to mind.

We joke about teachers or parents with eyes in the back of their heads, but imagine actually having to operate eight eyes without getting confused. That's a lot of processing power in a poppy-seed brain, especially when you consider how fast these creatures can move. While dancing, the male is alert to the female's advances, and can retreat backwards as fast as her forward attack, if she doesn't like the dance. These motions are all controlled by fast-firing neurons in the spider's brain.

To be able to do such fine mental tasks, spiders need an elaborate nervous system. But they don't have much space in their tiny bodies and there's a limit to how small neurons can get and still function.

Part of the solution is that the nervous system extends into the legs: "the central nervous systems of the smallest spiders fill up almost 80 percent of their total body cavity, including about a quarter of the space inside their legs."

Adding to the wonder, think about their webs. Orb-spinning spiders construct beautiful geometric webs out of one of the most perfect materials in nature -- a substance that is the envy of engineers for its strength and flexibility. Spiders can manipulate the strength of each strand, and make them sticky or plain. They maneuver deftly about their webs without getting stuck themselves, and use their legs to wrap their prey quickly while simultaneously secreting a specialized web material for that purpose.

Goudarzi writes about new techniques to get inside a spider's brain. It's delicate work, because the tissue is so tightly packed it tends to explode when punctured. Cornell biologist Ronald Hoy is one of the first to get a look inside a live spider's head with a tiny electrode. He finds that the organization of the spider brain is similar to any other brain:

"If you look at a section of spider brain you'll find that there are clusters of cell bodies with a cabling of the axons going from one part to another part and that's true of insects and that's true of us too," Hoy says. "Things are just more compact in a spider's brain because you're packing a normal head brain into the thoracic ganglion."

There are more than 44,500 identified species of spiders, Goudarzi says, and probably "at least as many yet to be discovered." National Geographic assures us that the vast majority are not venomous. The variety is staggering: orb weavers, jumpers, wolf spiders, tarantulas, trap-door spiders, water spiders at the bottom of the sea, and spiders that are hairy, smooth, black, white, or multicolored. Some can change color to match their backgrounds (PhysOrg). Not long ago, the biggest web weaver of all was discovered in Madagascar, able to build orb webs eighty feet wide that can span rivers (Daily Mail). (Shelob, call your office.)

Most spiders, however, are small enough to fit on a finger. The thought of a spider on your finger will creep some people out, but it's time to teach children to respect these small wonders and learn more about them. Before you step on that spider running across the floor, think about the high-tech brain you're squashing. You wouldn't do that to an Apple Watch, would you?

Oh, and check it out: a whole series of J�rgen Otto's entertaining music videos is on YouTube.

Image: Maratus volans, by Jurgen Otto [CC BY-SA 2.0], via Wikimedia Commons.