Brains of Tiny Spiders Extend Into Their Legs

Posted on December 15, 2011

Smithsonian researchers have discovered that the brains of tiny spiders are so big that they overflow into their legs. The researchers studied nine species of spiders of different sizes. The spiders included rainforest giants and tiny spiders smaller than the head of a pin. Nephila clavipes, a rainforest giant weighs 400,000 times more than the smallest spiders in the study, nymphs of spiders in the genus Mysmena.

William Wcislo, staff scientist at the Smithsonian Tropical Research Institute in Panama, says, "The smaller the animal, the more it has to invest in its brain, which means even very tiny spiders are able to weave a web and perform other fairly complex behaviors. We discovered that the central nervous systems of the smallest spiders fill up almost 80 percent of their total body cavity, including about 25 percent of their legs."

Some of the tiniest, immature spiderlings have deformed, bulging bodies. The researchers found that the bulge contains excess brain. Adults of the same species do not bulge.

The researchers say that brain cells can only be so small because most cells have a nucleus that contains all of the spider's genes, and that takes up space. The diameter of the nerve fibers or axons also can only be so small. If the nerve fibers are too thin, then the flow of ions that carries nerve signals will be disrupted and the signals will not be transferred properly.

Wcislo says, "We suspected that the spiderlings might be mostly brain because there is a general rule for all animals, called Haller's rule, that says that as body size goes down, the proportion of the body taken up by the brain increases. Human brains only represent about 2-3 percent of our body mass. Some of the tiniest ant brains that we've measured represent about 15 percent of their biomass, and some of these spiders are much smaller."

SPIDERS