I think mostly that our seemingly "irrational" fear of scorpions, snakes and spiders (for those that have that fear) is basically a self-preservation instinct in case they are poisonous. That's why people feel scared sometimes even if they know the creature is harmless. For instance, I don't mind spiders and even like them and I don't like to kill one by mistake... but I was brought up in a country that doesn't have poisonous spiders. If I'd grown up in a place with lots of poisonous spiders/insects, the chances are I'd have at least a healthy wariness of them.