Spider

he spider is an arachnid-like mob that is neutral in sufficient light levels and hostile towards the player at night and in dark, shadowy areas. They are able to see the player through solid blocks. A spider measures 2 x 2 x 1 blocks, with brownish-gray skin and red eyes that glow eerily in the dark. Because of their dimensions, spiders cannot enter 1-block wide spaces that zombies, skeletons, and creepers can, but it can crawl into 1-block high gaps (as long as they are 2 or more blocks wide). Spiders' distinctive hissing noises can alert a player to their presence - creepers make a similar, much shorter noise when damaged. Spiders are easier to hit with arrows due to their size, but can quickly catch up to the player with their leaping (they can cover 2 - 3 blocks in a bound) and wall-climbing abilities. Spiders will often jump around when attacking, making them a moving target and harder to hit. When hunting, spiders move at a pace slightly slower than the player's walking speed. Unlike zombies and skeletons, spiders will not catch fire in daylight.
There is a 1% chance that a spider will spawn with a skeleton on its back, forming the dreaded Spider Jockey. The skeleton's ability to fire arrows combined with the speed, leaping, and wall-climbing of the spider makes this a very dangerous opponent.
In the Beta 1.8 update, there was added a new, superior, blue variety called Cave Spiders which spawn in Abandoned Mine Shafts and are able to poison their targets.