"If you must know, when I was three, Fred turned my - my teddy bear into a great big filthy spider because I broke his toy broomstick... You wouldn't like it either if you'd been holding your bear and suddenly it had too many legs and... "
Spiders are small (with the exception of the Acromantula) eight-legged and eight-eyed creatures. They are very common and come in many species, varying from region to region; all are carnivorous and some are venomous, but for the most part they are relatively harmless and can sometimes be used as a potion ingredient. The largest species of spider is the Acromantula, which also has the ability to speak. Spiders are known for spinning webs. The webs of the Acromantula are large and dome-shaped. Spiders of all sizes fear the Basilisk above all other things, which caused those at Hogwarts to flee into the Forbidden Forest in the 1992–1993 school year. Ron Weasley is arachnophobic, meaning that he is deathly afraid of spiders, though he does not mind them if they are dead. The Tarantula is a family of spiders, examples of which can be found for sale in Knockturn Alley. The spell Arania Exumai knocks spiders back. Barty Crouch Jr, disguised as Alastor Moody, taught his fourth year Defence Against the Dark Arts class the use of the Unforgivable Curses, performing one of each of the three curses on one of each of three spiders. Spiders like living in dark places like the cupboard under the stairs, Broomsheds, and even the Forbidden Forest. In 1995 they find spiders as big as saucers in a dresser in the dining room of 12 Grimmauld Place (Ron left the room to make a cup of tea and didn't return until they were gone). These creatures can also be found in the Muggle universe. They eat the dead bodies once they die.
At one point of the film adaptation, Ron wakes up from a nightmare in which spiders presumably tried to make him tap dance. Harry, who was pouring over the Marauders Map at the time, told Ron to, "tell those spiders," that he didn't want to dance, and he promptly fell back asleep.