Spilagmite

"It is infested."

- Wendy

A Spilagmite is a different kind of Spider Den found only in Caves. It spawns Cave Spiders and Spitters.

Spawning and Regeneration
Spiders will regularly spawn from a Spilagmite at a rate of one approximately every 4 minutes, unless something makes them spawn differently (e.g. something walks on the webbing or an earthquake occurs).

A Spilagmite may contain up to 2 or 3 spiders in total (the maximum may vary between Spilagmites). If the number of spiders drops below the maximum, a new spider will be generated (inside the Spilagmite) in approximately 2 minutes.

Every time a spider is spawned from a Spilagmite, there is a 33% chance that it will be a Spitter; the rest of the time it will be a Cave Spider. Unlike Spider Dens on the surface, the type of spider spawned is always based on this probability and doesn't change due to circumstances (e.g. attacking the Spilagmite).

Webbing
Like Spider Dens, Spilagmites are surrounded by webbing, which will alert the Cave Spiders and Spitters inside if a creature touches it. When the webbing is stepped on, two spiders will come out at a time (provided at least 2 are inside the Spilagmite at the time). If any Cave Spider or Spitter is attacked within range of the Spilagmite, or the Spilagmite itself is attacked, then all remaining spiders in the Spilagmite will emerge to engage the threat.

If the spiders that emerged are not killed and lose interest in their foe, they will stay outside their Spilagmite and roam the area.

Earthquakes
If an earthquake occurs, all Spitters and Cave Spiders that are currently outside and that still have an active Spilagmite (as their home) will return to it.

Destroying Spilagmites
A Spilagmite takes four hits from a Pickaxe or Opulent Pickaxe to destroy, after which it'll disappear and drop 2 Silk and 2 Rocks.

Attacking an active Spilagmite will spawn all remaining spiders still contained within.

Bugs
After leaving the ruins, there will be new webs, which will disapear after a second, around already destroyed Spilagmites.