Head lice are tiny, wingless insects that live on the human scalp and feed on blood. The life cycle begins with the egg stage, known as nits, which are the most common sign of an infestation. Adult female lice firmly attach these eggs to individual hair shafts. Understanding the timeline of when nits hatch is important for addressing an infestation, as hatching generally occurs seven to ten days after the egg is laid.
Identifying the Nits
Nits are the first stage of the head louse life cycle and are distinct from debris like dandruff. The eggs are tiny, oval-shaped structures, typically yellowish-white or tan, and are about the size of a knot in a strand of thread. They are cemented tightly to the hair shaft with a strong, glue-like substance, meaning they cannot be easily brushed off.
The location of the nit indicates whether it is viable or already hatched. Adult female lice lay eggs very close to the scalp (usually within a quarter-inch) to ensure the developing embryo receives warmth for incubation. Nits found further down the hair strand are typically empty casings from which a louse has already hatched, or they are non-viable. These empty shells remain attached until physically removed or the hair grows out.
The Critical Hatching Window
The development within the nit depends highly on the temperature and humidity provided by the human scalp. Under these optimal conditions, the eggs generally hatch seven to ten days after they are laid. This period is the primary hatching window for the infestation.
The female louse lays eggs close to the scalp to ensure the developing embryo receives consistent heat. Nits that are dislodged more than six millimeters (about a quarter-inch) from the scalp are less likely to survive and hatch due to the loss of warmth. This seven-to-ten-day window represents the time before the eggs transition into the next life stage.
Life After Hatching (The Nymph Stage)
Once the nit hatches, the louse emerges as an immature form called a nymph. Newly hatched nymphs are very small and must begin feeding on blood immediately to survive. They cannot survive for more than a few hours away from the warmth and blood source of the scalp.
The nymph undergoes growth through molting, shedding its exoskeleton in three distinct stages, known as instars. This growth process is rapid, lasting approximately seven to ten days before the louse reaches full maturity. After the final molt, the louse is considered an adult and is capable of reproduction.
The timing of this maturation is significant because the newly mature female louse can begin laying eggs just a day or two after reaching adulthood, restarting the entire cycle. This seven-to-ten-day period from hatching to sexual maturity explains why follow-up actions are necessary. If an initial treatment kills only the live lice and nymphs but not all the nits, the eggs that hatch in the subsequent week will mature and lay a new generation of eggs, perpetuating the infestation.