Horseflies and blackflies leave nasty holes and you know immediately when you are being bit. I don't think Outwest knew he had been bitten until well after it happened.
The snake theory is also a bit farfetched. One tends to feel fangs even from a spider.
The only insects I know, although I am certain there are many others, in the U.S. that can bite leaving you unaware at first are mosquitoes and chiggers.