Cave Spider

The Cave Spider is a hostile mob re-textured from the Spider with a blue 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 1.9 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.

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 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 1.8) to easy mode (as of 1.8).

Trivia

 * Cobwebs don't affect Cave Spiders.
 * Cave Spiders are faster than normal spiders.
 * Cave Spider poison does affect Skeletons, but will not kill them because the venom only brings entities down to half a heart.
 * 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.
 * 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.
 * Cave Spiders can track you through walls.
 * 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.
 * Cave Spiders will leap into an area three blocks high and wait for the player to get close enough to be attacked

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.