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 rarer than regular Spiders. Cave Spiders can only be found in Abandoned Mine Shafts, introduced in Beta 1.8. 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 holes. Their exact size is 0.7x0.7x0.5.

Venom
The most notable feature of the cave spider is its venomous bite. When bitten by a cave spider on any difficulty except easy, your 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 5 seconds. Armor does not affect venom; neither the conditions under which you will receive venom, nor the damage received from it. There is currently no known cure for the venom, but Jeb is considering using milk as the cure. Regardless of the difficulty, venom cannot kill you, but it can drop your health to dangerously low levels.

Trivia

 * It was originally going to be called a "tree spider" according to Jeb, but that wouldn't make any sense due to it spawning in mine shafts.
 * Cobwebs don't affect Cave Spiders.
 * Cave Spider Spawners can be found outside of dungeons.