Meth can cause you to hallucinate bugs crawling around under your skin, which people then will sometimes pick at in an effort to make the sensations stop.
Formication is the word for the sensation of bugs under the skin. It's a normal part of the sensorium (maybe built in as a way of representing, at a perceptual level, real insects in the same way as itching and tickling are) but is triggered excessively by methamphetamine in the same way as itching is by opiates.
I notice weird paraesthesias like excessive itching, chills, very mild formication and ticklishness after being awake for more than 24 hours even when I'm sober (though I would usually have drank coffee if that was the case).
Since meth binges and sleep deprivation as well as sleep deprivation on its own cause psychosis, delusion and paraesthesias, it's hardly surprising that one on a multi day meth binge might hallucinate insects crawling on themself as a perceptual gap-fill for the actual sensations. Especially since people perpetuate the idea. (like with MDMA and glasses)
I've actually never had this side effect, for me, I kinda got to a point to where I know what comes with a couple days being up and high("bugs", shadow forms/people, air/mist/wisps of smoke in the room but no one is smoking<--does anybody else see this?), and even though you might see it, you know its not real..its kinda like welcoming back old friends whenever it happens, and I just smile and go back to what I was doing.
I've heard the 'bugs' thing before, but never seen it first-hand.
Similar: I let this chick try it once and we ended up doing like 1.5g total in about 8-12 hours time, she got so spun she passed out, woke up (still mega-fucked) and thought she'd been raped and people had written on her skin w/ blacklight ink, and left notes on a ton of used napkins and toilet paper in the house. i couldn't calm her down and she tried to cut the "words" off her with a razorblade.
Not eating or sleeping for days at a time will do a lot of crazy things to the human mind and body, believe me. I'm sure many have experienced this with meth, those that have been up for days and days an days know what I'm talking about.
I've never had the bug things, but I've had the "bugs crawling on the ground", "shadows", "the people following us", "the car that's always parked outside our door", etc etc.