Silverfish

Silverfish are small, bug-like hostile mobs.

Natural generation
Silverfish spawn from broken monster eggs, which generate in strongholds and Extreme Hills biomes.

Monster spawners
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.

Behavior
Silverfish are one of the smallest mobs in Minecraft; their height is less than that of a slab.

When idle, silverfish will enter a nearby stone block, transforming it into a monster egg.

When they are attacked or take poison damage, they may cause other silverfish within a 21x11x21 area to break out of their monster eggs (unless they are killed in one shot).

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.
 * Due to their tiny hitboxes, silverfish will suffocate on top of Soul Sand and End Portal frames.