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 very dark brown 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 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 1.8 update, there is a new, superior, blue variety called Cave Spiders which spawn in Abandoned Mine Shafts and are able to poison their targets.

Uses
Spiders are the main sources 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 so long as the light level immediately around them is 9 or less and enter neutral mode. In neutral mode, they will only attack if a player attacks it, like wolves. 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 well-lit areas and will only attack if the player hits it first. If there is light, a spider's aggro will be reset if it sustains another source of damage during pursuit, such as from falling.

Moreover, because spiders can draw line of sight through a solid wall, they will enter pursuit mode even when the player is completely cut off from them, so long as they are within the standard aggressive mob's 16 block search radius. The spider will then approach as possible and ambush the player even if it subsequently becomes light. Spiders on the surface can also track a player in a cave or tunnel if there is a clear path above the player they can follow and vice versa.

Spiders can climb up over walls and other obstacles as if all blocks had ladders on them. However spiders are careless climbers and will often fall off other blocks, (taking fall damage,) while in pursuit of the player. Unknown if spiders can climb vines.

Combat
Since they can pounce and move almost as fast as the player can when hunting at night or aggravated, spiders should never be underestimated. As the only naturally occurring source of string apart from dungeon chests and cobwebs in abandoned mine shafts, preparation and effort can pay off when hunting them. Wearing armor and carrying a stone or iron sword or bow is recommended. A bow is the preferred weapon, as arrows can damage a spider at a safe distance and spiders present a large and visible target due to their size and 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 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. Players should also keep in mind that trench warfare leaves them vulnerable to narrower mobs (skeletons, zombies, and especially creepers) in the vicinity.

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.The last defensive method is to avoid "incomplete threes"(threes that might be grown from a raw wood block randomely placed) from your building and stay away of it's leave grown radius.

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 Indev, 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 blocks 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 did not naturally occur at the time, but they were fully functional if hacked into the game. As of Beta 1.8, cobwebs are generated in Abandoned Mine Shafts.

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 require an area at least 3 blocks wide by 3 blocks long and two blocks tall for spawning. The top block in this arrangement however can be filled with a transparent block such as glass.
 * They appear to be based off of real life jumping spiders due to their eye pattern, the fact that they pounce to attack, and that they become neutral in daylight.