Silverfish

Silverfish are small, bug-like monsters that hide in special blocks found in strongholds. These blocks will look identical to stone bricks, cobblestone, or regular stone but take slightly longer to mine. When the block is mined, the silverfish pops out and attacks. In regular gameplay, Silverfish blocks occur rarely. If left alone, silverfish can re-enter some blocks, namely cobblestone, regular stone, and stone bricks. Silverfish don't damage the player directly, but they will scurry towards the player and push them around, which can be aggravating and sometimes dangerous.

When attacked, Silverfish may awaken other Silverfish blocks within a 21x11x21 block area around them. This can lead to a terrifying swarm if there are enough in the vicinity, and an uncontrolled swarm can quickly destroy dozens of stone bricks within a Stronghold. They scurry away after the player moves a short distance.

In addition to the special blocks, monster spawners that spawn silverfish can also be found in strongholds.

Spawning
Silverfish spawn only from silverfish blocks and monster spawners. In the latter case, they can only spawn in light level 11 or lower, except on top of stone blocks, 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.

History
The texture, silverfish.png, was added in the /mobs folder in the Beta 1.7 update, though the mob itself was not yet present. This was due to Beta 1.7 being most of Jeb's new content at that point for the first part of the Adventure Update, and Silverfish had apparently been completed prior to the release of Beta 1.7 by Jeb.

In Beta 1.8, Silverfish were properly implemented. They could only be found by mining certain blocks with the appearance of Stone Bricks in Strongholds.

As of Beta 1.9 Pre-Release 3, a Silverfish Spawner can be found in the End Portal room 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, which can appear as stone, cobblestone or stone brick.
 * Block 97 is mined at the same speed regardless of the tool used.
 * 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 made the same noises as Spiders in 1.8. This was a placeholder and new sounds were made, but not yet implemented at the time.
 * Silverfish do drown in water when forced under by a block above or something similar.
 * 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.
 * 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.
 * 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 break like wool with your hand.
 * Silverfish Blocks can be pushed and pulled with sticky pistons without breaking. This allows for traps with Silverfish blocks exactly where you want them completely legitimately. 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.
 * When the difficulty is set at peaceful, breaking a block 97 will make a puff of smoke as it would when spawning a silverfish, but the silverfish despawns instantly, similar to how mob spawners act during peaceful difficulty.
 * Silverfish do not stop and look around like most other land mobs; they constantly scurry around even when not in chase of anything (Tested in Minecraft Beta RC2)
 * If you attack a Silverfish with a Diamond Sword that is enchanted with Sharpness I, it will kill the Silverfish in one hit and it won't call any reinforcements.
 * Silverfish are the only mob which have their own sounds for walking. They make a scuttling noise, regardless of what kind of block they are stood upon.
 * A silverfish block can be mined much faster than a normal stone block, wich can be used to tell the differance
 * The silverfish, and the spider are the only real hostile animals. (However spiders are smaller)

Bugs

 * Silverfish will suffocate and can die if they crawl through a Portal block.
 * In SMP, even when mobs are turned off, Silverfish will still spawn.
 * In SMP, when a silverfish breaks out of a block, the dust effect is not visible.
 * In SMP, sometimes when a silverfish spawns, the silverfish appears to be completely black