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Anonymous
Those are opiliones. They're not spiders as they have a more ticklike body; rather than having a head and a cephalothorax, they only have a single body segment.
They're called Daddy-Long-Legs spiders, however this is incorrect, as DLL also stands for common cellar spiders, which are members of the Pholcid group. Pholcids are true spiders as they have spinnerets, venom glands, book lungs and a head and cephalothorax. A better common name for the Opiliones would be a Harvestman.
They cluster together, I gather, for the same reason that arthropods tend to gather together. Either, like ladybeetles (coccinellidae) they huddle together for warmth when it is cold, or they cluster together to mate, like cerapogonid midges, or maybe they take a flock or school like approach when resting. If you're alone, and a predator successfully attacks, there's a 100% chance that you will be killed. However, if you're in a crowd, and a predator attacks, there's at best a 1% chance it's you.
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