The human head louse has a ghostly quality. It tends to glimmer in and out of view, leaving only subtle signs and omens of its presence. Is that oblong speck an egg sac or a flake of dandruff? Was that a prickle on your scalp? Is it normal that your son is scratching just behind his ear? Maybe you have lice and he has lice, and you’ve all had lice for weeks. The possibility is frightening. The uncertainty leads to madness. The louse evolved to be intrepid and a sneak. Its behavioral imperative