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 sneaky. Its behavioral imperative is simple and relentless: 'They are naturally negatively geotropic,' Ian Burgess, a medical entomologist who runs a company that tests insect-control products, told me. 'They will always climb upwards towards the head.' He recalled a day when one must have fallen on his shoe during a comb-out in his lab. He was driving home that afternoon and noticed that the bug was sitting on his knee, apparently confused. 'It had climbed to the highest point it could get, and it didn't know where to go from...
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