It can be muscle twitches or it can be something else such as peripheral neuropathy that gives a tingling, twitching sensation sometimes. It can feel really as something is crawling under your skin.
Some medications can give you a sense that something is crawling under your skin. Sometimes it can be intense, all over the body at different times, to the point of driving people crazy. It can also happen without any apparent reason.
I'm not saying it can't be parasites. There are parasites such as scabies mites and some worms that actually burrow under the skin.