The central nervous system — basically the brain and two nerve cords — of some small spiders can take up about 80 percent of their bodies, overflowing from their body and bulging out into their legs. This leaves only about 20 percent for all the other parts of the spider, including the eyes, stomach, throat and legs. By comparison, humans' brains take up about 2 or 3 percent of their mass, but if humans were built like these spiders, the brain and spinal cord would take up more body mass than the muscles and bones combined.
More facts about spiders and brains:
- Spiders aren't the only ones with proportionally huge brains. Some ants have brains that take up 15 percent of their bodies.
- The relationship between brain size and body size is known as Haller's Rule, which states that the smaller an animal is, the larger proportion of its body the brain will occupy. Haller's Rule also applies to the size of an animal's eyes.
- Spiders have smell-related receptors in some of the hairs on their legs.