Silverfish

Silverfish are small, bug-like hostile mobs found in strongholds and the extreme hills.

Drops
Silverfish have no drops other than 5 experience when killed by a player or tamed wolf.

Infested blocks
Silverfish spawn from broken infested blocks, which generate in strongholds, igloos with basements, woodland mansion fake portal rooms and mountains biomes. Silverfish will not spawn if the block is broken with Silk Touch.

Monster spawners
When spawning from monster spawners, they can only spawn in light level 11 or lower, except on top of stone and its variants, where they can spawn at any light level. They also cannot spawn within a 5 block distance of any player. Silverfish spawners naturally generate in end portal rooms in strongholds.

Behavior
Silverfish are the third smallest mobs in Minecraft, after the baby turtle and the endermite; their height is less than that of a slab. Because of this, they take suffocation damage from standing on soul sand.

Silverfish have the ability to "call" other silverfish in the area upon being hit. These silverfish have the ability to see through walls, and will pathfind until they reach the player.

When idle, silverfish will enter a nearby stone, cobblestone, stone bricks, mossy stone bricks, cracked stone bricks, or chiseled stone bricks block, transforming it into the respective infested block.

When they suffer poison damage or damage inflicted by the player, and survive, they may cause other silverfish within a 21×11×21 area to break out of their monster eggs. The player can therefore prevent other silverfish from appearing by killing them in one shot.

Silverfish, being arthropods, are weak against weapons with the Bane of Arthropods enchantment.

Data values
Silverfish have entity data associated with them that contain various properties of the mob.

Trivia

 * 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.
 * Unlike most Minecraft mobs, they do not have a plush toy.
 * They appeared in the April Fools 2.0 update as the Redstone Bug.
 * In Bedrock Edition, letting a silverfish enter cobblestone or any variants of stone bricks will turn these blocks into infested stone instead of the corresponding infested block.
 * Because of this, infested stone can also be found outside mountains biomes in this version.