psychosis
A depiction of delusional parasitosis in Dave Kellett's webcomic Sheldon. DP, a fixed delusion in which one believes s/he is infested with bugs despite no evidence, was famously described in Philip K. Dick's A Scanner Darkly. Superblogger and psychologist Vaughan Bell wrote a nice article on the subject, check it out.
Thanks again, Dave, for permission to share your comic (with slightly modified layout to make it blog-friendly).