Cave Spider

The Cave Spider is a hostile mob that was introduced in Minecraft Beta 1.8. It is a Spider with a blue re-textured skin and is more dangerous than the regular spider, but in turn they are much more rare than regular Spiders. Cave Spiders do not spawn naturally, they only come from Spawners found in Abandoned Mine Shafts. Cave Spiders are also smaller than regular spiders, presumably so that they can fit through the mine shafts that they spawn in, including 1 x 1 x 1 holes, and can pass through passages only half a block high. Their exact size is 0.7 x 0.7 x 0.5.

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 (or 1.5 seconds), and lasts for 5 seconds. Armor does not affect venom; neither the conditions under which you will receive venom, nor the damage received from it. Since Pre-Release 4 Jeb gave milk the ability to cure the player of the venom. Regardless of the difficulty, venom cannot kill you, but it can drop your health to dangerously low levels.

Trivia

 * Cobwebs don't affect Cave Spiders.
 * Cave Spider poison does affect Skeletons, but will not kill them because the venom only brings entities down to half a heart.
 * In SMP, Cave Spider spawners are the only spawners that don't have Pigs in them when they are currently spawning a mob.
 * Cave Spider spawners are surrounded by Cobwebs, making them difficult to get. Since as mentioned above, Cave Spiders are not slowed by Cobwebs, players can only take extreme caution when passing these areas, unless playing on Easy or lower.
 * Cave Spiders behave just like normal Spiders in daylight or light levels of 10 or higher.
 * Because of their small dimensions, building a mob farm from a cave spider spawner would be considerably difficult (unless the player is on the Peaceful setting) and somewhat inefficient to build.

Bugs

 * Cave spiders spawn in peaceful mode on SMP.

Trivia
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.