The insects being specifically ants. This can be caused by any number of reasons, including substance abuse, high fever, menopause, and diabetic neuropathy, among others. The term was borrowed around 1705 from Latin, like all good medical jargon, specifically from formīcātiōn, from formicāre, to have such a sensation (which seems to have been coined by Pliny), from formīca, ant.
Geoffrey, I've told you before -- please do not confuse formication with fornication. They are very different sensations.