What one is said to feel crawling on or under the skin during a drug-induced hallucination. ("Crank" is a slang name for methamphetamine.) I would guess that crank bugs are the reason his arms are all scratched up.
crank bugs
n. a drug-induced hallucination that insects are crawling under one’s skin. (Drugs.) There’s no such thing as crank bugs, so stop scratching them.