Silverfish

The Silverfish is a new mob that is planned for Beta 1.8. Its texture can currently be found in the Beta 1.7.3 minecraft.jar. In the Beta 1.8 pre-release, Silverfish will pop out of an unamed block that holds the same texture as Stone found in caves. If the player mines these blocks, Silverfish may pop out and then attack them like so. Silverfish will also spawn near others of their kind, which can cause a swarm if not controlled. They scurry away after the player moves a short distance.

History
The image, silverfish.png, was added to the mobs folder in the Beta 1.7 update. The silverfish mob was later announced for 1.8.

Jeb confirmed that the silverfish won't be a water mob, ruling out the possibility of the Silverfish being an actual fish.

In the Beta 1.8 update leak, Silverfish can be found in Stone Bricks in Strongholds.

spawning
as an example find simon and lewis's video on the 1.8 pre update part 6 (on a channel called bluexhepos)(i think) and skip to about halfway in.

here is what i have gathered from what ive seen:

in strongholds some areas have blocks that have a chance to spawn silverfish. once a silverfish is spawned other silverfish blocks will break releasing their silverfish. the speed of the blocks breaking grows exponantialy.

at first here and there some blocks acationaly break

then more and more keep coming faster and faster.

at a sertain point it will simply be a flood of silverfish rushing towards you.

at this stage these things are relentless popping up all over the place in rapid succsesion. from the walls the floor the cieling.

they will be everywere in MASS amounts.

Trivia

 * In real life, silverfish are small, wingless insects that are often found in dark areas in buildings. They are infamous for damaging wood and paper-based products.
 * The block they spawn from when destroyed is ID 97, when placed it will have the texture of regular stone but with a faster mining rate.
 * Silverfish make the same noises as Spiders in the pre-release version of 1.8. This may be a placeholder.
 * Silverfish will suffocate and can die if they crawl through a Portal block.
 * Silverfish do not seem to drown in water, despite swimming underneath the surface when submerged.