Silverfish

Silverfish are small, bug-like hostile mobs.

Natural generation
Silverfish spawn from broken monster eggs, which appear 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; their size is less than a slab.

When attacked (or on each poison damage tick), silverfish may awaken other silverfish (turn monster eggs into silverfish) within a 21x11x21 area, except if attacked non-directly or killed in one shot.

If left alone, silverfish will enter a nearby block, transforming it into a monster egg.

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.