It's an evolutionary trait, anything that looks even remotely like a spider trigers an ancient mechanism in the mammalian brain that wants you to get the fuck away from it, because most spiders were poisonous and wouldn't mind taking a tasy bite out of some monky ass
(, Thu 4 Sep 2008, 20:28, Reply)
However it has been disputed that back when we'd have evolved this trait there were far more deadly predators/threats to worry about which would have taken priority and spiders were not as prominent in the areas where we evolved as other nasty wee critters.
(, Fri 5 Sep 2008, 1:11, Reply)