The head-louse life cycle (egg to adult) requires about 21-27 days on the human host. Each adult female deposits anywhere from 50-150 eggs (nits) during her lifetime. Nits hatch in 5-9 days. Nymphs (immatures) molt three times before becoming sexually mature adults. Young and adult head lice feed on blood several times a day. The adult stage can live approximately one month (30-40 days).
Fig.1. Head louse life cycle.
Head lice are completely dependent on their human host for nourishment and warmth. In fact, head lice cannot complete their life cycle if they become separated from their host. Adults and nymphs can survive off their host for only 2-3 days, whereas nits can live for up to 14 days.