Well as you already know one's skin can't really crawl! But people will use this phrase in order to express disgust, or a jittery feeling as well as other feelings. Because idioms say one thing but mean another.
Skin Crawl - 2007 V is rated/received certificates of: USA:R
Millipedes do not crawl into skin. Millipedes are more likely not to even bite. You might be thinking of centipedes but either will not crawl under skin. Centipedes though are poisonous to certain levels and are able to bite.
bugs crawl on people because the salt on your skin they eat it or they think your skin is like the ground
Skin and bones?
The idiom, 'jump out of your skin,' was first seen in England in the 1800s. It refers to a person being so scared that they 'jump out of their skin,' by dying and becoming a ghost.
see here http://www.answers.com/article/1188007/14-abandoned-places-that-will-make-your-skin-crawl
Reptiles have scaly skin
That depends on the species, but most can bite hard enough for you to feel and many can penetrate your skin.
save someone's life
A long time.
Don't know the answer?
I think you heard the idiom wrong. It should be "by the skin of her teeth," meaning that she barely succeeded.