How long do lice grow? - briefly
Lice mature to their full length of 2–4 mm within roughly five to seven days after hatching, and they stop growing once they become adults. Their size remains constant throughout the adult stage.
How long do lice grow? - in detail
Lice develop through a defined series of stages, each with a measurable time span. The cycle begins with the egg, commonly called a nit, which is firmly attached to hair shafts. Under typical indoor temperatures (20‑25 °C) hatching occurs after 7–10 days. The emerging nymph measures about 1 mm in length and resembles a miniature adult but lacks full reproductive capacity.
Nymphs undergo three successive molts. Each molt requires approximately 2–3 days, resulting in a total nymphal period of 9–12 days. During this time the insect enlarges from roughly 1 mm to the adult size range of 2–4 mm, depending on species (head, body, or pubic louse). Growth is linear between molts, with an average increase of 0.3–0.5 mm per day.
After the final molt, the adult lice are fully formed and capable of reproduction. Adult lifespan averages 30 days on a host, though it may extend to 45 days in favorable conditions. Reproductive output peaks after the first week of adulthood, with a female laying 6–10 eggs per day for the remainder of her life.
Key temporal parameters:
- Egg incubation: 7–10 days
- Nymphal development (three molts): 9–12 days
- Total time to mature adult: 16–22 days
- Adult longevity on host: up to 45 days
Factors influencing these intervals include ambient temperature, humidity, and host grooming habits. Higher temperatures accelerate development, reducing egg incubation to as few as 5 days, while low humidity can increase mortality at the nymphal stage. Regular hair washing and mechanical removal of nits can interrupt the cycle, effectively shortening the population’s growth period.