Spider

Spiders are common neutral mobs that have the unique ability to climb walls.

Spawning
Spiders spawn in the Overworld in 3×3×2 space on solid blocks (centered on the middle block) at a light level of 7 or less, in groups of 4. The top blocks can be transparent, but not solid.

In Pocket Edition, leaves can spawn spiders, provided that there are the same conditions as above.

Monster spawners
Spiders spawn from spawners found in dungeons or Spiders spawn from monster spawners in abandoned mine shafts.

Drops
Spiders drop 0-2 string upon death. If killed by a player or tamed wolf, it also has a $1/3$ chance to drop a spider eye. They drop 5 experience when killed by a player or tamed wolf.

Behavior
A spider measures 1.5×1×1.5 blocks of size.

Spiders are hostile to the player and to iron golems as long as the light level immediately around them is 9 or less; otherwise they won't attack unless a player attacks first. Hostile spiders will continue to chase the player even if they are exposed to well-lit locations. If a spider sustains damage from a source other than the player, such as falling, its hostility will be reset.

Spiders can climb up over walls and other obstacles. If a spider cannot find an ideal path to the player (when a player goes behind or on top of a wall), it will get as close as it can to the player's position from the outside or below, and proceed to climb the wall vertically until it gets to the top, even if it loses its aggression towards the player. 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.

If shot from distance with an arrow when outside detection range, spiders will now turn and run in the direction the arrow was fired, essentially towards the player. If the player moves away, the spider will still follow the same path unless the player enters detection range, then the spider will change direction and attack.

Like most mobs, Spiders seem to be oblivious to cactus damage.

While not poisonous themselves, spiders are unaffected by poison status effect.

Spiders will often jump around when attacking. At close range, spiders occasionally pounce their attackers instead of being knocked back.

In the Pocket Edition, spiders move at a pace slightly slower than the player's walking speed.

Variants
There is a 1% chance that a spider will spawn with a skeleton on its back, forming a spider jockey.

On hard difficulty, spiders have up to a 10% chance (depending on regional difficulty) of spawning with a single beneficial status effect. This effect can be speed (40%), strength (20%), regeneration (20%) or invisibility (20%), and has an essentially infinite duration.

Trivia

 * You can still see a spider's eyes when it is hit with a potion of invisibility.
 * If the water a spider is swimming in is only 1 block deep, they may pounce nearby enemies upon touching the submerged floor.
 * Spiders do a flip and land on their backs when they die, unlike most mobs (which land on their sides).