I was on a fast down hill about three weeks ago and felt something hit my right forearm real hard. Glancing down, I saw a yellow jacket clinging to my arm hairs. I was really flying on a somewhat busy, narrow, two lane road and really did not feel comfortable removing one hand to knock the little critter away.
After a second or two, I could feel the dreaded biting sensation and finally was able to flick the damn thing off without crashing. Too late of course.
My arm swelled so badly it looked like a marble had been implanted under my skin, and didn't go down for over a week. It still itches in a 2 inch area around the sting mark.
About those helmet vents--I've always wanted to glue some screen inside my helmet at the vent openings to keep those gnarly little guys out. But like most other things--I never get around to it.