Here's what you might not know about these banes of childhood: Though other species of lice infest other mammals and even birds, Pediculus humanus capitis lives on the human scalp and nowhere else. A louse spends the first week or so of its life as an egg, or nit, before hatching into a nymph. Over the course of about seven days, the nymph will molt three times, growing bigger each time, until it reaches the adult stage.