Perhaps "Colonies" isn't the best name, but it'll do for now.
Spider Colonies would be structures found deep in Mirkwood, filled to the brim with Mirkwood Spiders. The structure would consist of larger Mirk-oak trees connected to each other by bridges of webs that the Spiders can walk across. You can find 2-block-tall "Web Bundle" blocks that work like Remains strung up on these bridges, and breaking them will provide "loot" that includes Rotten Flesh, Man-Flesh, Bones, Elf Bones, Orc Bones, and Skulls.
Some of these bundles might not be blocks, however! You can rarely find "Wriggling Bundles" which are actually entities that will wriggle about while hanging. After 2 or 3 hits they will drop like anvils before breaking on the next block, releasing the caught prisoner inside. These will usually be Woodmen (when those are added) or Wood-elves, (as I don't believe any Dwarves would have been in Mirkwood during the War of the Ring), but rarely a Dol Guldur Orc will pop out and begin attacking you! After being freed the caught prisoner will quickly thank you for freeing them (unless they're an Orc) and give you a reward such as coins or food before running off into the woods and despawning.
I believe that this would help flesh out Mirkwood a bit more, and give a reason to actually wander off the path, no matter how much the resident skinchanger warns against it!