Silverfish

The Silverfish is a new mob added in Beta 1.8. Silverfish hide in stone bricks found only in strongholds, and pop out to attack when the blocks are mined. In regular gameplay, Silverfish blocks occur rarely. If left alone, silverfish can re-enter some blocks, including cobblestone. When attacked, Silverfish may awaken nearby Silverfish blocks. This can lead to a terrifying swarm if there are enough in the vicinity, and in which an uncontrolled swarm can quickly destroy dozens of stone bricks within a Stronghold. They scurry away after the player moves a short distance. They have the same sounds as spiders, which is a placeholder.

History
The image, silverfish.png, was added to the mobs folder in the Beta 1.7 update. The silverfish mob was later announced for 1.8.

In the Beta 1.8 update leak, Silverfish can be found in Stone Bricks in Strongholds, although they can also appear elsewhere.

In Beta 1.9, Silverfish Spawners may be found in certain rooms in Strongholds.

Trivia

 * In real life, silverfish are small, wingless insects that are often found in dark areas in buildings. They are infamous for damaging paper-based products.
 * There are also small ray-finned fish called Silverfish. These were proven unrelated after a tweet from Jeb.
 * The block they spawn from when destroyed is ID 97, when placed it will have the texture of regular stone.
 * Block 97 is mined faster with hands than with tools.
 * As of 1.8 They spawn additional Silverfish from nearby Silverfish blocks (Block 97) and blocks containing escaped Silverfish when attacked. Blocks are destroyed in the process of spawning.
 * Silverfish make the same noises as Spiders in 1.8. This is a placeholder and new sounds were made, but not yet implemented.
 * Silverfish will suffocate and can die if they crawl through a Portal block.
 * Silverfish do drown in water when forced under by a block above or something similar.
 * Silverfish can be beaten naturally by using fire without it calling in more of its allies, so long as the player does not hit it directly.
 * Silverfish can ride in minecarts like all mobs but only seem to get into them when jumping, they push them around when just crawling into them.
 * Silverfish can be killed by explosions without more spawning.
 * The stone-like block that produces silverfish when broken is often used in some servers as a form of griefing, in which the griefer places the stone-like blocks in players' houses, and the player gets attacked by silverfish as soon as they break it. It is also possible to create stairs, bridges or flying platforms out of silverfish blocks. When a player breaks a block (to open a chest, for example), a silverfish will spawn. Inexperienced players will attack the silverfish on sight, resulting in more spawning, which will cause the death or stranding of the player by falling or silverfish attacks.
 * Silverfish blocks can be found by almost breaking blocks in stronghold (though actually breaking them is not a good idea) with your hands,as a silverfish blocks breaks like wool with your hand.
 * In SMP, the silverfish seems to have difficulty when trying to harm a player, and typically results in pushing the player around.
 * Silverfish Blocks can be pushed and pulled with sticky pistons without breaking. This allows for traps with Silverfish blocks exactly where you want them in a completely legit, un-modded, un-hacked game. This is very easy but it takes a long time. To do this you simply place a piston facing a silverfish block (Silverfish Blocks can be found by trying to mine blocks with your hands) and then operating the piston with a power source (for example a redstone torch or a lever), repeating until the block is where you want it.
 * In SMP, even when mobs are turned off, Silverfish will still spawn.