Cave Spider

The Cave Spider is a hostile mob using the same model as the spider, but with a blue skin color and smaller size. They are more dangerous than the regular spider, but they in turn are much rarer than regular Spiders. Cave Spiders do not spawn naturally; they only spawn from monster spawners found in abandoned mine shafts. Their small size (0.7 x 0.7 x 0.5) allows them to fit through spaces one block wide and just half a block tall including those created by slabs, fence posts and two stair blocks facing each other. Cobwebs do not affect Cave Spiders.

Behavior
Aside from their small size and venom (see below), Cave Spiders inherit most of their behavior from regular Spiders. Cave Spiders behave just like normal Spiders in daylight or light levels of 9 or higher. Cave Spiders, like regular spiders, are pacified (made neutral) by bright light. 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. If both cave and regular spiders are attempting to get to the player through a 1x1x1 size opening, the regular spiders crowd the opening and block the path of the smaller cave spider, greatly reducing its chance of getting through the opening. 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.

Venom
The most notable feature of the cave spider is its venomous bite. When bitten by a Cave Spider on any difficulty except Easy, the player's health bar turns yellow-green and gradually decreases. The venom 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. Since 1.9 Pre-release 4 Jeb gave milk the ability to cure the player of the venom. Regardless of the difficulty, venom cannot kill the player, but it can drop the player's health to dangerously low levels. If they are suffering from the Weakness potion effect, then they will not do any damage or inflict poisoning.

Tips for fighting
While dungeon spawners can be tamed and used as experience/drop farms, doing this with cave spiders is excessively dangerous -- besides their venom, 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. Rather than merely lighting a cave spider spawner for later farming, it's better to take a pickaxe to it and collect some experience up front. The main challenge here is getting past not only spiders, but cobwebs, to destroy the spawner (especially since cave spiders are not slowed by the webs). Some tips:

Top View:   Side View:    Front View:    Key: W           W            WWW            W = Wood BW          BF            FBF            F = Fence W          BF            FBF            B = A Block
 * The simplest and safest way to get to a cave spider spawner to destroy it is to use the tactical advantage provided to you by the mineshaft its self. Never forget that the name of this game is MineCraft.  While any enemy can be defeated by getting a really big sword and strong armor, they can also be defeated far more easily - and safely - by crafting your environment to work for you and against them. In the case of the cave spider, while it is very small and capable of wiggling through any gap you can, and some you can't, it is still possible to arrange blocks such that you can attack it, but it can't attack you.  To do so, place any solid block in in front of the opening to one of the wooden archways in the mineshaft containing the spawner to form a blockade, 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 the spider - If You Stand Far Back Enough - can not bite you.  Do Not Use Fence Posts, 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
 * 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 sweep away a lot of the web.
 * Digging up from below can let you break the spawner more quickly.
 * You can always just seal the area up and avoid them.
 * Building an Iron Golem in there can get you some breathing room to reach the spawner.
 * Play on peaceful.
 * When in doubt -- C4! Yes, you can use TNT 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)

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.

Bugs

 * Because of the natural tendency of cave spider spawners being created with cobwebs and ceilings, but without walls such as the Moss Stone walls in dungeons, cave spiders can be spawned in the middle of a cavern or a ravine.

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 as cave spiders do not share an entity ID with spiders, like wither skeletons do with skeletons.