Silverfish

Silverfish are small, bug-like hostile mobs which have silver skin and black eyes.

When attacked, silverfish may awaken other silverfish within a 21x11x21 area. This can lead to a terrifying swarm if there are enough in the vicinity, and an uncontrolled swarm can quickly surround and kill the player. If left alone, silverfish will enter a nearby block, transforming it into a monster egg. Non-direct attacks will not awaken other silverfish. Killing a silverfish in one hit will not awaken other silverfish.

Spawning
Silverfish don't spawn naturally, but appear from Monster Eggs which are only found in strongholds and randomly underground in extreme hills biomes. The blocks look identical to normal blocks, but when the block is mined a silverfish pops out and attacks.

When spawning from monster spawners, they can only spawn in light level 11 or lower, except on top of stone, where they can spawn at any light level. They also cannot spawn within a 5 block distance of any player. These restrictions only apply when spawning from a monster spawner, and not from a Silverfish block.

Trivia

 * Silverfish were the smallest mob in Minecraft before Endermites were added in 1.8.
 * In real life, silverfish are small, relatively harmless, wingless arthropods that are often found in dark areas in buildings and the wooden pilings of seaside piers and trees. They are infamous for damaging paper-based products. During cold temperatures they are known to enter a sort of "hibernation" by expelling most of the water in their body. During this state they greatly resemble small stones or gravel, likely inspiring their Minecraft counterparts.
 * Silverfish can be killed with fire, lava or explosions without the Silverfish turning hostile, preventing them from calling in more allies, so long as the player does not hit them directly.
 * When the difficulty is set at peaceful, breaking a monster egg will spawn a silverfish, but the silverfish would despawn instantly.
 * Mining a Silverfish Block with a silk touch pickaxe will not spawn a silverfish but will instead drop the normal version of the monster egg as an item.
 * A block containing a Silverfish takes longer to mine with a pickaxe, and quicker to mine with your hand. These are good indications when you are mining and want to avoid any silverfish.