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.
"Spiders are very smart, that's why we're studying them," says Ronald Hoy, a professor of neurobiology and behavior at Cornell University. "They use visual cues to steer by, and the kind of mazes that they can solve is considered to be pretty impressive for an invertebrate."
Jumping spider attacking in mid air. Credit: Gil Menda, Paul Shamble, Tsevi Beatus.
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.
Perhaps as a solution to space limits, some small spiders have brains that spill out all the way into their legs. Scientists have discovered that 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. In general, spiders have a two-part body in which the head and the thorax are fused into one big segment called the cephalothorax.
Insects, on the other hand, have a separate head, a separate thorax and a separate abdomen found in the front end of the combined cephalothorax. This means that in spiders, there's much more fusion of the nervous system than in most insects.
In terms of wiring, however, spiders follow the same sorts of rules found in both vertebrates and invertebrates.
"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."
A male Phidippus audax, a common jumping spider in North America. Credit: Gil Menda
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.
Hoy is part of one of the first teams to record the activity of neurons in a spider brain, a monumental feat because the insides of spider bodies are under pressure, like air in a balloon, and even the smallest incisions could make everything squirt out, leaving the critter to die. Using a very fine electrode that made a fast-healing hole in the spider's head, Hoy's team successfully punctured the tiny brain of a jumping spider and recorded neuron responses associated with visual cues, such as flies, their natural prey. This gave the team an unprecedented look into the microscopic brain that processes information from the jumping spider's eight eyes.
Researchers can now utilize this method to learn more about the complex noodle of these tiny critters. In over 250 years of spider research, scientists have identified more than 44,500 species of spiders, but estimate there are at least as many yet to be discovered.
"There are very few studies on spider brains," Hoy says. "We think it's really exciting that now it's possible to record spider brains and that others will follow up and really start to study it in more detail."
Now, let's all watch a peacock spider dance: