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Formication is the medical term for a sensation that exactly resembles that of small insects crawling on (or under) the skin. It is one specific form of a set of sensations known as paresthesias, which also include the more common prickling, tingling sensation known as "pins and needles". Formication is a well documented symptom, which has numerous possible causes. The word is derived from formica, the Latin word for ant.
Formication may sometimes be experienced as feelings of itchiness, tingling, pins and needles, burning, or even pain. When formication 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.
In some instances, static electricity can attract particulates to the skin and can also cause body hair to move, giving a sensation like insects crawling over the skin, However, in many cases no external trigger creates the sensation.
In rare cases, individuals become convinced that the sensation they are suffering is due to the presence of real insects on or under the skin. In these cases, patients have what is known as delusional parasitosis. They believe that their skin is inhabited by, or under attack by, small insects or similar parasites, despite repeated reassurances from physicians, pest control experts, and entomologists.
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 (shingles). Formication can also be a result of stimulant intoxication (e.g. methamphetamines, cocaine) or alcohol withdrawal in alcoholics (i.e. delirium tremens), and is often 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.
- Potter, Mike. "INVISIBLE ITCHES: Insect and Non-Insect Causes". ENTFACT-58. University of Kentucky. Retrieved 2013-12-13.
- 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.
- Rusyniak, Daniel E. (2011). "Neurologic manifestations of chronic methamphetamine abuse". Neurologic Clinics 29 (3): 641–655.
- Encyclopædia Britannica, 1797, page 260
- William Allan Jamieson, Diseases of the Skin: A Manual for Practitioners and Students, 1890
- DiagnoseMe, Season 1 - Episode 8 Covered in Bugs, beautifully follows a patient suffering skin bugs sensation...formication...due to old amalgam dental fillings