When an opponent casts a creature spell with flying, if Hidden Spider is an enchantment, Hidden Spider becomes a 3/5 Spider creature with reach. (It can block creatures with flying.)
10/4/2004 When it turns into a creature, it is no longer an enchantment.
4/1/2008 This card now uses the Reach keyword ability to enable the blocking of flying creatures. This works because a creature with flying can only be blocked by creatures with flying or reach.
5/1/2008 The ability only triggers if the creature spell has flying, regardless of whether an effect would cause the creature to have flying once it entered the battlefield.
8/1/2008 A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of his or her most recent turn. It doesn’t matter how long the permanent has been a creature.