Coalescipede

Edit:

These shadowy spiders are found within the Shaded Citadel. Small spiders are afraid of light and will flee from the slugcat when carrying a light source. However, large numbers of spiders can aggregate, making them less fearful of light and consequently more hostile. Smaller clumps of spiders are typically not dangerous, but will kill and snatch lanternmice from slugcat. Medium sized clumps can be dangerous to slugcat ( need to look into this more ). Large aggregations of spiders form what looks to be a "Spider-pede", which will attack the slugcat irrespective of a light source. Spider clumps can be broken up by throwing items at them (eg. spears or rocks), causing them to disperse back to individual, small spiders and thus reducing their hostility. The light generated from throwing the purple glow balls (needs a proper name) will kill spiders and can be useful when dealing with large numbers of spider-pedes. One purple glow ball will typically kill all spiders in a single room.

Even though lanternmice generate their own light, individual lanternmice are unable to fend off even small spider clumps.

Original:

In most cases, the smaller spiders are afraid of you and will run away from the slugcat. Within Shaded Citadel, nests of spiders are dangerous. Spider behaviour changes when there are large numbers of them, causing them to attach to each other, become hostile, and attack slugcat. Sometimes, spiders can be encountered outside their nest in a centipede-like formation. However, they only do this when they feel threatened, or if slugcat comes too close. Avoiding them can decrease the chances of a spider-pede formation. The spider-pede can be broken apart using spears or rocks as well as scared away killed by throwing the purple balls seen growing in Shaded Citadel. Currently, the effectiveness of firecracker plants is unknown. They appear to fear light, such as the light emanating from lightstones glowstones. Lightstones Glowstones (for name consistency) can be traded from scavenger merchants or stolen.

Trivia

 * The number of spiders needed to kill you depends on their size: 3 when large, 6 to 7 when medium.
 * Tiny spiders aren't harmful.
 * Scavengers in the Shaded Citadel have lightstones with them. This appears to indicate that they are aware that spiders fear bright light.
 * A spider-pede formation will snatch lanternmice from slugcat.