Formication is the medical term for a sensation that resembles that of insects crawling (tactile hallucination) on (or under) the skin. It is one specific form of a set of sensations known as paresthesia, which also include the more common prickling, tingling sensation of "pins and needles". Formication is a well documented symptom that has numerous possible causes.
The experience of formication may sometimes cause feelings of itchiness, tingling, pins and needles, burning, or even pain. When it is perceived as itchiness, it may trigger the scratch reflex and because of this, some people who are suffering from the sensation are at risk of causing skin damage through excessive scratching. More rarely, susceptible individuals who fixate on the sensation may develop delusional parasitosis, becoming convinced that this sensation is being caused by actual insects, despite repeated reassurances from physicians, pest control experts, and entomologists.
The term is derived from formica, the Latin word for ant.
Differential diagnosis 
Causes of formication include normal states such as onset of menopause (i.e. hormone withdrawal). Other causes are medical conditions such as pesticide exposure, mercury poisoning, diabetic neuropathy, skin cancer, syphilis, Lyme disease or herpes zoster.
Formication can also sometimes be experienced during high fevers. Itching, tingling and formication often occur when surfacing from a dive or during ascent to altitude (decompression sickness or "the bends").
It can be a side effect of Ritalin (methylphenidate), Adderall (dextro/levo-amphetamine) and Lunesta (eszopiclone) and other prescription drugs or of cocaine or amphetamines. (Accordingly, it goes by the slang term "coke bugs".) Formication can also be a withdrawal symptom of weaning oneself off cocaine, opioids, or amphetamine. It can also accompany alcohol withdrawal in alcoholics, along with delirium tremens, and can often be accompanied by visual hallucinations of insects.
Formication is etymologically derived from the Latin word formica, meaning "ant", precisely because of this similarity in sensation to that of crawling insects. The term has been in use for several hundred years. In the 1797 edition of the Encyclopædia Britannica, a description of the condition raphania includes the symptom:
...a formication, or sensation as of ants or other small insects creeping on the parts.—
Described again in an instructional text from 1890:
A variety of itching, often encountered in the eczema of elderly people, is formication; this is described as exactly like the crawling of myriads of animals over the skin. It is probably due to the successive irritation of nerve fibrils in the skin. At times patients who suffer from it will scarcely be persuaded that it is not due to insects. Yielding to the temptation to scratch invariably makes the disease worse.
See also 
- Hinkle, Nancy C (2000). "Delusory Parasitosis" (PDF). American Entomologist 46 (1): 17–25.
- Vijverberg, H.P., van den Bercken, J. Crit. Rev. Toxicol. (1990) Neurotoxicological effects and the mode of action of pyrethroid insecticides.
- Encyclopædia Britannica, 1797, page 260
- William Allan Jamieson, Diseases of the Skin: A Manual for Practitioners and Students, 1890