All arthropods have relatively complex brains with well-developed sensory systems.
Spiders all have multiple eyes, but generally they do not have good vision, being nearsighted and able to detect primarily rapid motion or light. Some spiders, such a jumping spiders (Salticidae, below), wolf spiders (Lycosidae) and net-casting spiders (Deinopidae) do have amazing vision, including the ability to see in low light, in the ultraviolet, or in any direction.
Most spiders rely heavily on vibration, smell, touch, and taste instead of vision. Web-spinning spiders rely on tiny vibrations in their silk to signal that prey has contacted the web, or to warn of incoming predators. Webs are a way of expanding the sensory range of a spider. A spider doesn’t just locate items that she contacts directly but gets detailed information about anything that encounters the entire large web. Thus, a 8mm spider may get information about an area over 50 to 100 cubic millimeters. Vibrations in the web indicate the presence, size, and type of prey, any damage to the web, or even the presence of potential predators. Sticky webs are particularly good at holding prey for long enough that the spider can capture the prey.