The juvenile and adult bugs, which are about 2.5 millimeters long at their largest stage, live for about 40 days after they are hatched—but only on the scalp. They need the heat of the scalp and they need to bite the scalp and suck some blood, or they'll starve to death. They also can't fly or jump.

About eight days after hatching, female lice begin to lay nits, little pearly things less than a millimeter in length that are glued to the hair shaft quite close to the scalp. Since scalp hair grows about 0.4 millimeters a day for most people, the distance from the scalp to the outermost nit will give you a rough index to how long the infestation has been present: 4 millimeters, about 10 days; 8 millimeters, about three weeks; 1 inch, maybe two months. The nits are really tough—markedly more resistant to environmental stress and lice shampoos than the adults. The eggs hatch somewhere between seven and 11 days after they are laid, renewing the cycle.