Tiny spiders are tiny, but relative to their body size it turns out their brains are ginormous . In some cases, 80 percent of a spider's body cavity contains central nervous tissues. Other spiders store parts of their brains in their legs. In other words, step on a tiny spider, and most of the goo that comes out will be braaaaains. Or think of it like this: If dogs breeds had a similar relationship to brain size, chihuahuas would have brains all the way to their tails.