Either small arachnides or Giant beasts. Small spiders lived in the Shire and were unimportant. But Large Spiders were evil beasts who inhabited Mirkwood. Many lived there, eating Elves, Men, Dwarves, and, in Bilbo Baggins's case, Hobbits. The Spiders almost ate Bilbo, but he escaped. He then saved his fellow dwarves of Thorin and Company. Shelob of mordor tried to eat Frodo Baggins, but she failed and went back into her cave. Spiders were never mentioned again.