Cave Spider

The Blue Spider is a hostile mob introduced in Minecraft Beta 1.8. It is a spider with a blue re-textured skin and is presumably stronger than its black brethren. Blue spiders are much more rare and dangerous than regular spiders. When bitten by a blue spider, your health gradually decreases and vomiting occurs. Blue spiders can only be found in Abandoned Mineshafts, which will also be introduced in 1.8.

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.