I think it would be cool if spiders were able to poison you. It would not happen every time a spider hits you though just randomly. But if you are poisoned a purple mist appears on your screen for a second before going away. then it will slowly drain your HP for a while but not kill you, probably about twice the damage the normal spider attack does. To prevent getting to much damage from poison you could craft potions. Potions to heal you, poison antidote, potions that make you stronger for a short time, and defense potions that make you take half as much damage as usual for a short time. And back to the poison spiders, when you kill them they have a chance to drop a poison fang. You can put the poison fang on top of an arrow to make it a poisonous arrow. when you shoot it at an enemy it has a chance of poisoning an enemy and when an enemy is poisoned it will have purple steam coming from it and will slowly die. The more poison arrows you shot at it the faster it will die from it. It would make spiders a much more important enemy to find instead of finding one whenever you rarely need string. It would make them a lot more threatening...however, some might not like poisonous spiders but remember they don't poison you every time you get hit by them..The poison spiders could also look different then normal spiders so you would know if you need to be more careful about fighting them. I think it would be cool and add this and it would add a better reason to find spiders.
Poison spiders and craftable antidotes.