Question: how come when we are afraid of something (e.g spiders and bugs) does our body make us feel as if their is one crawling up our legs and arms?

Susan Cartwright answered on 19 Jun 2015:

This sensation is caused by the fact that the little muscles attached to your hair follicles are trying to make your body hair stand on end. This is a singularly useless thing to do, but it’s left over from the time when we had fur. Lots of furry animals make their hair stand on end when they’re scared, because it makes them look bigger and more intimidating – cats are an obvious example. Yes, I know that looking bigger to a spider isn’t very helpful, but your body doesn’t know what it is you’re scared of – it just knows that the chemicals released from various glands and circulating in your blood show that you’re scared of something, and it’s trying to do whatever it can to help. Looking bigger and fiercer might have helped if what you were scared of was a bigger and fiercer member of your own species, or a predator that might just decide that bigger and fiercer you wasn’t worth the risk of attacking.

As I say, this is all totally useless now because we no longer have any fur. But that’s evolution for you: things that were once useful but aren’t so anymore can persist for millions of years, providing they aren’t actually harmful. Making your non-existent fur stand on end when you’re scared doesn’t do any good, but it doesn’t do any harm either, so there has been no evolutionary pressure to stop it happening.