Q: Do all spiders bite, and are they poisonous?
A: There are two problems with this question: a technicality, and a set of false assumptions.
First, the technicality. “Poisonous” and “venomous” are two different things. No spider is poisonous — harmful to eat, breathe, or touch. Mushrooms are sometimes poisonous, but spiders are not. Spiders are venomous; their toxins are proteins which only work when injected.
Second, all spiders do bite, but most local spiders are harmless because they are not aggressive and will not bite indiscriminately, or their fangs are simply too small to nip through our comparatively thick skin. Just because they are venomous does not mean they are
dangerous to people.
Spider venom does not exist to harm creatures which are too large for spiders to eat, like humans. The purpose of spider venom is to subdue the spider’s prey, almost always insects. In brief, it’s an insecticide.
Nevertheless, all larger spiders with a body length of a half inch or more should be treated with caution. Avoid flicking them away from your body. People allergic to bee stings may react more strongly to the bite of a spider than an ordinary person.
Bees and wasps kill more people in the United States in one year than spiders and snakes combined kill in ten years, and dogs and cats kill or injure more people each year than bees and wasps. Yet most people like dogs and cats and fear spiders and snakes.
For More Information: http://www.iflscience.com/brain/why-are-we-afraid-spiders
By Dick Brewer