A Whole New Meaning for Thinking on Your Feet

In a recent study by Smithsonian researchers, it was discovered that, for many tiny spiders, their brains are literally too big to fit inside their heads. In fact, they're even too big to fit inside their bodies. In an ongoing study on miniaturization and its effects, researchers examined nine species of spiders of varying size and determined that, while spiders get smaller their brains stay about the same size, and wind up overflowing into their body cavities.

William Wcislo, one of the staff scientists for the research, explained that even the tinnier spiders must maintain the amount of brain space needed todo complex behavior like weaving webs. Therefore, "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." The smaller the spider, the more troublesome this can be for the animal. Tiny spiderlings often will have deformities and bulges in their bodies due to the excess amount of brain.

There is a biological basis for why this excess of brain must be the case. The diameter of nerve fibers cannot be smaller without becoming too thin, disrupting the flow of ions that carry the nerve signals. If the nervous system cannot be made smaller, the only alternative is to accommodate it with more room. By Haller's rule, a general rule for all animals, "as body size goes down, the proportion of the body taken up by the brain increases." We see this in all animals, even humans.

Seemingly, these large brains are not cumbersome. Much of what small spiders eat can easily be converted into energy to fee the brain. Larger spiders, such as the Nephila clavipes, weighing 400,000 times more than the smallest spiders in the study, clearly needs more food and energy to support its substantial body mass.

Source: http://www.eurekalert.org/pub_releases/2011-12/stri-awn121211.php