Spider

The spider is a mob that is neutral in sufficient light levels and hostile towards the player at night and in dark, shadowy areas. A spider measures 2 x 2 x 1 blocks, with black coloring 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. 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 comparable to 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.

Uses
Spiders are the sole source of string, which is a critical ingredient of bows and fishing rods. Each spider drops 0 - 2 pieces of string upon death.

Dungeons that contain a spider monster spawner can be fitted with cactus blocks so the player can harvest spider string. Because spiders are two blocks wide, they cannot pass through a one-block gap between two cactus blocks (or a cactus and a wall) and will press themselves to death against a cactus trying to chase the player.

Behavior
Spiders are aggressive and will chase and attack the player during the night and in dimly lit areas. Once they begin their pursuit, they will continue to chase them even if they are exposed to daytime or other well-lit locations. Conversely, a spider will be neutral around a player when roaming in daylight, but if it wanders into a moderately shaded area (i.e. under a big tree or a rocky overhang) it will turn hostile and remain that way. Otherwise, spiders are neutral in very well-lit areas and will only attack if the player hits it first.

Because spiders can see the player through a solid wall, they will move up against it and ambush the player by climbing over when the player decides to go outside or near the wall (unless the spider despawns or the player moves out of sight). Spiders can also keep track of a player underground in a cave or tunnel if there is a clear path above the player they can follow.

Combat
During the day, spiders will generally not attack unless first attacked. They pose a larger threat at night, when they are actively hunting, and since they can pounce and move almost as fast as the player can, they should never be underestimated. Because of their agility, spiders are difficult to kill, but as the only source of string, the effort can pay off. Proper equipment for hunting spiders includes a weapon, like a stone/iron sword or bow, and wearing armor is also recommended. A bow is the preferred weapon, as arrows can damage a spider at a safer distance and the spider presents a large target due to its size and visibility at night (from its glowing eyes). When attacking a group of spiders, pick off the ones that aren't too close to others in order to avoid aggravating several at once.

Alternatively, the player can dig a narrow, 1-block trench (with a depth of 2 blocks) in order to be able to attack spiders safely from underneath, trapping them above the gap. This strategy has mixed results, as some players have reported spiders waiting for them to exit the trench before pouncing.

Defensive Measures
Like other hostile mobs, spiders are vulnerable to fire, fall damage, lava, and are injured by contact with cacti blocks. They can be set on fire with a flint and steel in the day time, taking damage but remaining neutral towards the player. Surface spider populations can be controlled if the player sleeps regularly in a bed at dusk, since spiders will spawn during the night if given the chance to do so. When traveling at night, the player is advised to watch for active spiders' glowing red eyes in the darkness.

Construction Safeguards
Spiders can climb any vertical solid surface. One way to deal with this is to make sure the roof is accessible through only a 1x1 hole (or a 1x2 high doorway with or without a door, although other hostile mobs will be able to fit through this gap) that spiders cannot crawl into. Alternatively, a lip or overhang on a wall will stop them from breaching it. The player can alternate an overhang every other block to conserve materials, as spiders are 2 blocks wide and cannot climb walls upside-down. A third deterrent to spiders is to place ladders on a wall 4 blocks above the ground level, as the edges of the ladders will block the spiders' climbing.

For more tips on spider-proofing, please consult this guide.

History
The spider was the second mob added to Minecraft in Survival Test. Spiders were originally brown in early development. They did not jump as far (compared to their range in the Beta update) and killing them gave the player 105 points. In Alpha, spiders began to drop 0 - 2 pieces of string upon death.

In the Beta 1.2 update, Spiders were given the ability to climb vertical walls made of any block and could "see" the player through solid to track them down (creepers used to have this ability before it was transferred to the spiders). As of Beta 1.4, spiders will not trample crops, nor will they make step sounds (if they do, they will be much quieter).

As of Beta 1.5, a cobweb block has been added to the game files. Cobwebs do not naturally occur yet, but they are fully functional if hacked into the game. Notch is planning to have spiders produce these cobwebs.

Trivia

 * If a player throws an egg at a spider, the spider will take damage but stay neutral.
 * Sometimes spiders can be found inside blocks. Once the block is removed or destroyed the spider will attack regardless if it was hit in the process or not (maybe a glitch).
 * Causing a spider to take fall damage or damage from a cactus after attacking it during the day causes it to become neutral again.
 * Sometimes after attacking a spider in sunlight, the spider will cease attacking the player; this is presumably the high light levels turning the spider neutral.
 * Spiders slow down when climbing vertical surfaces, making it easier for a player to outrun them around steep mountains and cliffs.
 * Because the code controlling the spider's facing and jumping are not linked, it is possible for a spider to jump sideways or backwards if it was previously attacking the player, halted because it could not find a way to them, started looking in other directions, and then became capable of attacking the player again.
 * Spiders will not drop onto fences. If a spider climbs over the fence and on top of a house is surrounded by fences, the spider will be trapped on the roof.
 * Spiders can climb over wolves if the wolf is 1 block underneath it.