I once got stung by an ant (don't laugh, it bloody hurt) as a kid at junior school, it must have crawled up my short pants at playtime and let me have it high on the back of my thigh, i thought i'd sat on a drawing pin at first. I hear they inject formic acid into the bite and i can well believe it. It never swelled up or anything but i still remember it after all these years.
As for Grylls, I've just read his autobiog ('Mud Blood and Tears') in an attempt to figger him out but he's got me beat!
Somebody asked him on TV once "Can anybody do daredevil stuff same as you?" to which he replied "Yeah if they're messed up".
He certainly seems hyper and semi-neurotic at times, but he's 38 now so hopefully he'll slow down before he has a nervous breakdown or gets himself killed and leaves his wife and 3 kids behind.
He's been rolling the dice for years and has been lucky so far..
PS- I like chuckling at his shows though for their sheer entertainment value as he runs around (never walks) for maximum dramatic effect as if he's being chased by unseen demons. At least he shows us that nature is nothing to be scared of and can be overcome by sheer brute force..