Act of moving about in a large mass (of insects, people, etc.)

ETYM Latin formicatio, from formicare to creep like an ant, to feel as if ants were crawling on one's self, from formica ant: cf. French formication.
Hallucinated sensation that insects or snakes are crawling over the skin; a common side-effect of extensive use of cocaine or amphetamines.
Sensation like ants crawling on the skin.