Formication

Formication is the sensation of insects crawling on or within the skin, when there are no bugs found. Skin crawling sensations may also be part of paresthesia, which is caused by disrupted nerve sensations in the body.

Delusions
Formication can occur with delusions of parasitosis, which is also known as parasitophobia. This is the false beliefs that bugs are crawling in or under the skin, and typically occurs in people with schizophrenia or obsessive-compulsive disorder, who may go to extreme lengths to try to remove or kill the presumed bugs.

Nerves
Crawling sensations affecting the skin may also be caused by disrupted nerve signalling.

Malnutrition
Formication can be caused by beri-beri, a disease caused by inadequate levels of vitamin B.