Cave Spider

The Cave Spider is a hostile mob in low light levels and a neutral mob in light levels 10 or above, using the same model as the spider, but with a blue skin color and smaller size. They are more dangerous than the regular Spiders, but they in turn are much rarer than them. Cave Spiders do not spawn naturally; they only spawn from monster spawners found in abandoned mine shafts. Their small size lets them slip through small openings, and their poison can rapidly weaken players.

Behavior
Aside from their small size and venom (see below), Cave Spiders inherit most of their behavior from regular Spiders:
 * They are pacified (made neutral) by bright light—daylight or light levels of 9 or higher. Likewise, subjecting them to darkness will render them permanently hostile.
 * They can climb walls, jump, and track the player through solid blocks.
 * They are not affected by cobwebs or splash potions of Poison.
 * Cave Spiders will climb or leap into an area three blocks high and wait for the player to get close enough to attack.
 * A cave spider can hold its breath underwater for about 16 seconds (320 world ticks/160 redstone ticks). It takes about 6 seconds (120 world ticks/60 redstone ticks) to suffocate a cave spider to death.

They have two big differences from regular Spiders: One is the much smaller size of the Cave Spider: They take up only a fraction of a block space, specifically 0.7 x 0.7 x 0.5 of a block. Accordingly, they can fit through spaces one block wide and just half a block tall. This specifically includes those gaps created by slabs, atop fence posts, and by two stair blocks facing each other. Note that if both cave and regular spiders are trying to reach the player through the same 1x1x1 size opening, the regular spiders tend to crowd the opening (which they can't pass), and block the path of the cave spiders.

Venom
The cave spider's other special feature is its poisonous bite. When bitten by a Cave Spider on any difficulty except Easy, the player's health bar turns yellow-green and gradually decreases. The poison inflicts half of a heart of damage about every second and a half, and lasts for 7 seconds on Normal difficulty, or 15 seconds on Hard. Armor does not protect from venom or reduce the damage received from it. Milk has the ability to cure the player of the poison. Regardless of the difficulty, poison cannot kill the player, but it can drop the player's health to. If they are suffering from the Weakness potion effect, or if the difficulty is set to Easy, then they will not do any damage or inflict poisoning. They can see through blocks and track you down.

Tips for fighting
Cave spiders are extremely dangerous: a single attack and poison together can drop you from full health to half a heart if you are on hard with no armor. In addition, they can get through almost any hole (certain fence gaps may be an exception), making it quite hard to fight them on your own terms. Finally, unlike players, cave spiders are not slowed by cobwebs.

Like all spawners, cave spider spawners can be used as experience/drop farms, but due to the spiders' agility and lethality, only certain grinder types are safe. Destroying the spawner is the safest option, but provides little experience. To destroy or tame the spawner, the player will need to get past the spiders themselves and the many cobwebs that will surround the spawner. Some tips:


 * If the spawner is completely within a mineshaft corridor, the simplest and safest way to make use of a cave spider spawner is to make tactical use of the corridor and the fencing "supports" within. To arrange blocks so that you can attack it, but it can't attack you, create a blockade: Place any solid block two-high in front of the opening of one of the mineshaft supports, like so:


 * Notice that the blockade blocks are not in line with the arch. This is important, if it was in line with the arch the fence posts would attach to the block and form a solid barrier that you can't attack through. Having the block offset by one creates a small diagonal gap through which you can still hit the cave spider, but 'if you stand far enough back, the spider can not bite you. Do not use fence posts instead of the blocks, as this will create a diagonal gap wide enough for one cave spider to shove another cave spider through.
 * You can now attack the cave spiders without them being able to return the favor, allowing you to kill them faster than the spawner can breed them.Once you've finished off all the cave spiders on the other side of the blockade, move up to it and place two more blocks on the far side of the archway in mirror fashion to the current set up. You can now remove the first set of blocks as the second set will keep the spiders from you. Dart back and kill the few that will have spawned while you did this, then move forward again and block off the next archway leading to the spawner. Progress in this manner until you are able to reach the spawner and destroy it.

Progression Down Mineshaft (Top View): W   W    W        W = Wooden Arches 1W2 3W4  5W6 S   1-6 = Successive Blockades W   W    W        S = Spawner Diamond swords are useful and can kill a cave spider in a quick amount of time. Also Iron golems are useful too and their strong impact can kill them.
 * Shears are obviously helpful -- flint and steel provides both light and damage for the spiders, while burning away cobwebs. In general, you want to light up the area as you clear your way to the spawner.
 * While milk can cure poison, it isn't stackable, and drinking it during a battle is slow; the best time to use the milk is between rounds of fighting, or afterwards.
 * Pouring lava on the spawner won't destroy it, but the light will suppress spawning, and the lava will deal with any spiders it does manage to produce.
 * Pouring water will remove all of the webs the water touches.
 * Digging up from below can let you break the spawner more quickly.
 * Building an Iron Golem in there can get you some breathing room to reach the spawner.
 * TNT can be used to blow up the mob spawner and kill most of the spiders in the area. Cave spiders only have twelve hitpoints so they die easily when there is an explosion. In the process, you'll also deal with a lot of cobwebs. However, you can get yourself killed if you're not careful. Try to get above the spawner, dig a hole in the ceiling, remove any cobwebs in the way, drop the TNT in the space, and light it with your Flint and Steel.

Mob farming

 * As noted above, the venom and small size of cave spiders makes farming them quite dangerous. If you must, consider these points:
 * Ensure the roof is greater than 3 blocks high on the player’s side of the wall, or they will leap to the ceiling and get stuck.
 * Cave Spiders do not drop more than one spider eye even after being killed by a looting-enchanted weapon, but the chance of one is increased.
 * Because cave spider spawners are so common, they are quite commonly generated near each other. This can provide you with an extremely efficient XP farm, as two or more spawners can work simultaneously (subject to limits of spawning behavior).
 * The most plausible way to contain them is between a door and a fence pillar, leaving a gap that they can't pass, but which you can attack through. (See the picture in the gallery.) The block arrangement discussed above will also work. Try to suffocate them with blocks that fall such as sand or gravel. Lava is good maybe water to drown them. Redstone traps are useful too. Pressure plates with TNT Near is useful so it will explode. However, the trap is single-use and therefore less efficient.

History
The Cave Spider was first introduced in the Beta 1.8 Pre-release.

SMP servers upgraded from older versions running in peaceful mode may begin spawning cave spiders, though no other monster appears in the world. This is because in older versions setting the Server.properties key  to   would place the server in peaceful mode. As of Beta 1.8, a separate key  must be set to zero to select peaceful difficulty; setting   to   prevents monsters from spawning randomly in dark areas but does not disable spawners. Because the  key defaults to 1 (easy), servers without the key configured at all changed from running in peaceful mode (prior to Beta 1.8) to easy mode (as of Beta 1.8).

Before the 1.0 update, Cave Spiders had health.

As of 1.2 both spiders and cave spiders have gained the ability to climb Ice blocks.

Cave spiders were added to the Xbox 360 Edition in the 1.2 (Beta 1.8.2) update.

Trivia

 * The Cave Spider's texture appears to be at a higher-resolution than its normal counterpart - this is because the textures occupy a smaller space.
 * They have the same size shadow as a normal spider.
 * Many speculations have been made about the possibility of a cave-spider-jockey. This however is impossible without third party tools, as cave spiders do not share an entity ID with spiders, like wither skeletons do with skeletons.
 * Cave spiders are one of the four mobs that give a debuff to the player. The other three being the Wither, the Wither skeleton and the Witch.
 * If cave spiders are allowed to spawn via hacks, they will only spawn if the light level is 7 or below.

Gallery
Picture of cave spider going through a 1 x 1 x 0.5 block.