Spider

Spiders are common mobs that can be neutral or hostile depending on the light level. They are hostile in light levels of 7 or less, and neutral in light levels of over 7. They do not burn up in sunlight like Zombies and Skeletons. Spiders have the unique ability to climb most blocks vertically to reach the player if needed.

Appearance
A spider measures 1.5 x 1.5 x 1 blocks, with black skin and red eyes that glow eerily in the dark. They make distinctive hissing noises.

Spawn
Spiders can spawn on a 1 x 1 block space, if there are no blocks next to them. Water or glass blocks are considered a block in this regard, glass panes or iron bars are not. They also cannot spawn in a 1 block high space, if the ceiling is a solid block. They can spawn in a 1 block high space, if the ceiling is a transparent block (like glass).

Abilities
Spiders have the ability to climb vertical block stacks, run fast and can leap 3 - 4 blocks.

Because of their dimensions, Spiders cannot enter 1-block wide spaces that Zombies, Skeletons, and Creepers can, but they can crawl through 1-block high gaps (that are 1.5 blocks wide) whereas most other mobs cannot.

Variants
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.

On Hard difficulty and on a full moon, spiders have 10% of chance of spawning with a single beneficial status effect. This effect can be Swiftness, Strength, Regeneration or Invisibility, and has an essentially infinite duration.

Uses
Spiders are a good source of string, which is a critical ingredient of bows, fishing rods and leads and can also be used to make (white) wool. Each spider drops 0 - 2 pieces of string on death. If the player kills the spider (i.e. death is not caused by another mob or mob farm), spider eyes will drop.

Dungeons that contain a spider monster spawner can be fitted with cactus blocks so the player can harvest spider string. Because spiders are 1.5 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. Dungeon doorways can also be modified to trap spiders inside, allowing the player to kill them and obtain spider eyes and experience orbs, as well as the string.

Behavior
Spiders are hostile as long as the light level immediately around them is 9 or less. If they are found in an area with a higher light level, they won't attack unless a player attacks first. Hostile spiders will continue to chase the player even if they are exposed to daytime or other well-lit locations. Conversely, a spider won't become hostile around a player when roaming in daylight, but if it wanders into a moderately shaded area (e.g. under a tree or rocky overhang) it will turn hostile and remain that way. If a spider sustains damage from a source other than the player, such as falling, its hostility will be reset, so that if it is in a well-lit area, it will not be hostile towards the player.

Spiders can climb up over walls and other obstacles as if all blocks had ladders on them. Spiders can take advantage of their climbing ability through a unique AI system. Note that for this to work, the player must hit the spider first, causing it to see the player through blocks (this occurs with all mobs). If the spider is chasing the player, and there is a possible horizontal path that it can take to get to the player, they will immediately take it, regardless of the situation. However, if the spider cannot find an ideal path to the player in this condition (for example, if the player enters a house, or the spider notices the player while they are standing on a raised platform such as a roof), it will get as close as it can to the player's position from the outside (if in a house) or bottom (if on a cliff), and proceed to climb the wall vertically until it gets to the top, even if it loses its aggression towards the player. This behavior allows spiders to continue chasing players after they climb a ladder, block piller, or enter a shelter, especially with no roof. Additionally, when a spider loses its aggression on the player, it will continue moving forward blindly for about two seconds. This behavior causes the spider to climb up any walls in its path, and is especially useful if the player did not hit the spider at all while moving into cover, causing the spider to lose track of the player.

Spiders do not turn sideways when climbing up walls and cannot climb along the tops of ceilings. If there are blocks around the top of a wall, a spider will not be able to climb to the top, making this a good defense against the mob. However, if there are no blocks around the top of a building, the spider may climb on top of the roof of the building and ambush the player as they exit the building. Spiders seem to be oblivious to cactus damage as they may touch or even climb onto cacti, often being pricked to their death, as with most other mobs.

While not poisonous themselves, Spiders are unaffected by Splash Potions of Poison.

Spiders will often jump around when attacking, making them a moving target and harder to hit. In the Pocket Edition, spiders move at a pace slightly slower than the player's walking speed.

Farming
Farming spiders for experience orbs is more difficult than other monsters because of the spider's ability to climb and the tendency for spiders to constantly target the player up to 16 blocks away, making controlling their movement more complicated than a skeleton or zombie. It is still possible to control them by building certain safeguards to force the spiders into containment, and has the advantage that, unlike most other mobs, it is easy to build entrances and spaces where the player can go but the spider cannot.

Defensive Measures
Like other hostile mobs, spiders are vulnerable to fire, fall damage, lava, and are injured by contact with Cactus 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 the night gives more chances to spawn than the day time. 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 room 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 1.5 blocks wide and cannot climb walls sideways, however, due to skeleton AI, it is not unusual to find a couple of skeletons hiding from the sun under any eave you create along walls.

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

Trivia

 * You can still see a Spider's eyes when a Spider is hit with a Potion of Invisibility.
 * 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.
 * A spider can jump sideways or backwards.
 * They appear to be based on real life jumping spiders due to their eye pattern and the fact that they pounce to attack. However, most jumping spiders hunt in the day.
 * Spiders can travel through a space that is 1½ blocks wide. This can happen when glass panes are part of a wall composed mostly of solid blocks. If a glass pane on the end of a long window is broken, spiders can crawl through the gap that is created.
 * If you set a spider on fire, it burns with a huge flame.
 * Due to Spiders (And Endermen) having a separate texture for their eyes file, they can be seen as a bright-red "shadow" from within lava. (Endermen have the same effect).  Similarly, Spiders hidden by fog will appear pure white.
 * Spider's footstep noises get some special handling: They have a lower chance than other mobs to make a noise when they walk on a block, but they are wider, and so touch more blocks at once.  The result comes out about even with other mobs.
 * The looting enchantment does not affect number of Spider Eyes dropped, only the probability of a drop.
 * While the spider will always pounce to attack (unless swimming), it doesn't actually need to—their attack is by touch like most unarmed mobs. However, their jumping may make the spider harder to hit.
 * Curiously, in Pocket Edition, neutral spiders sometimes are found without their abdomen.
 * At close range, Spiders occasionally pounce their attackers instead of being knocked back.
 * If the water a Spider is swimming in is only 1 block deep, they may pounce nearby enemies upon touching the submerged floor.
 * A spider will actually follow you regardless of y axis. That means if you are above ground and a spider is underground, it will still 'see' you.
 * Spiders do not have a 'hurt' sound file; they simply play one of their 'idle' files.
 * Spiders, along with their cave cousins and silverfish, instead of falling on their side upon death, they do a flip and land on their backs.