I give you: a giant aggregation of Daddy Long Legs, also known as Opiliones. The coolest thing about this is that no one really knows WHY they form these aggregations. Is it predator defense? Thermoregulation? Maximizing their smelly gland effects? A plot to totally creep us all out?
I smell a dissertation in this!
EDITED TO ADD: I don’t think I was clear enough that these actually aren’t spiders–they are in a separate group. They do, however, have a lot of very long legs, and provoke the sorts of reactions spiders do. They don’t have venom or silk glands, and they don’t make webs. They will eat just about anything they can grab.