Larva

Larva is a little dangerous hostile creature that spawn in cocoons in the underground of all worlds (not in biomes).

In multiplayer games, larvas may also spaw in great number through the monsters' portals underground, among other creatures.

Behaviour
The larva will remain dormant until a dwarf approaches its cocoon. Once the larva wakes up it will start moving and breaking blocks, including stones and ores.

The larva is the the only creature that will intentionally try and will break natural blocks (not placed by dwarfs).

Despite its description, no larva will ever grow into a giant worm. And while larvas are present in all worlds, giant worms only spawn in the Snow and the Desert Worlds.

Mobility
It can only climb in foreground walls and never in background walls.

It can pass through open doors and hatches. It won't hit closed doors and hatches; it will walk away.