Silverfish

Silverfish are hostile mobs that were added in Update 0.9.0. They spawn exclusively in Strongholds (via a spawner in the portal room and within the walls) and Extreme Hills Biomes inside of Monster Eggs.

Combat
Since Silverfish only have a melee attack, they can be easily killed by using a low tier Sword.

However, when attacked, all other Silverfish in the area will also attack the player, making them very dangerous, especially in Extreme Hill Biomes where Monster Eggs are found abundantly. The players can also use Fire or Lava to kill them, since they only drop experience.

Detecting Silverfish
Mining Monster Eggs with any item other than a pickaxe will take about 3 seconds. Mining a Monster Egg with a Pickaxe will take a little bit longer than with normal Stone.

Trivia

 * Silverfish are currently the smallest mob in Minecraft Pocket Edition.
 * Silverfish can be killed with fire, lava or explosions without the Silverfish turning hostile, preventing them from alerting more Silverfish, as long as the player does not hit them directly.
 * When the difficulty is set to peaceful, breaking a Monster Egg will spawn a Silverfish that will de-spawn instantly.
 * Silverfish will never stop moving unless forced into a one-block hole or till they enter a stone block.
 * If a Silverfish is killed instantly by the player, another Silverfish won't attack the player.
 * 1 Silverfish Spawner can be found in the Portal Room of a Stronghold.
 * Silverfish are so small that they suffocate when walking on Soul Sand.