Spiders aren't generally thought of as particularly smart creatures, but some spiders' brains are literally too big for their heads. It seems that no matter how big or small a spider's body is, its brain is always the same size.
No matter their size, all spider species have to perform the same basic set of tasks. Some of those behaviors are fairly complex, and that means a sizable part of the body must be given over to brain cells. There's a lower limit on how small brain cells can be and still function properly, which means miniature spider species have to resort to some pretty unusual biological solutions to fit all their brains in. The spiderling young of some of the smallest species actually have bulging bodies just to contain all their brains, though they grow out of the more extreme deformities.
Smithsonian Tropical Research Institute scientist William Wcislo explains:
"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 behavior. 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.
"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% of our body mass. Some of the tiniest ant brains that we've measured represent about 15% of their biomass, and some of these spiders are much smaller."
It's maybe the most extreme mismatch we've ever seen of an organism's size and its brainpower requirements, and part of it is because of the enormous variation in spider size. The largest rainforest spiders weigh about 400,000 times more than their tiniest counterparts.