Head lice are parasitic insects that live on the human scalp and feed on blood. A persistent infestation begins with the egg stage, which are commonly called nits. Understanding the life cycle, particularly how and when these eggs hatch, is necessary for successfully stopping the spread of these tiny insects. The eggs represent the stage of the life cycle most resistant to many common treatments, making them the source of re-infestation if not properly managed.
Identifying Lice Eggs (Nits)
Nits are small, oval-shaped eggs, typically measuring only about 0.8 millimeters long by 0.3 millimeters wide. Their color can range from translucent to yellowish-white when newly laid, but they darken as the embryo inside develops.
A female louse cements each egg firmly to the hair shaft using a strong, water-insoluble glue-like substance. This attachment distinguishes a nit from other common scalp debris like dandruff or hair spray droplets, which can be easily flicked away. Viable eggs are almost always found very close to the scalp, usually within one-quarter inch of the skin. This proximity to the scalp is a requirement because the eggs need the host’s body heat to develop and hatch.
The Hatching Timeline
The hatching period for a nit is remarkably consistent, generally occurring between seven and ten days after being laid by the adult female louse. The warmth and humidity of the scalp are necessary environmental factors that allow the louse embryo to develop within the egg shell. If a nit is dislodged from the hair and falls away from the scalp, it is unlikely to survive and hatch.
Once the louse emerges, the empty shell remains glued to the hair shaft; these empty shells appear whiter or more translucent than the developing eggs. Because hair grows, the location of an empty nit farther down the hair strand can offer clues about how long the infestation has been present.
Nymphs and the Full Life Cycle
The insect that emerges from the nit is called a nymph, which is essentially an immature louse. Nymphs look like smaller versions of adult lice, and they immediately begin feeding on blood from the scalp.
This stage of development lasts approximately seven to twelve days, during which the nymph must feed multiple times daily to survive. Over this period, the nymph undergoes three molts, or shed its skin, as it grows larger. Once the final molt is complete, the insect is considered a fully mature adult louse. Adult females can begin mating and laying their own eggs just a day or two after reaching full maturity, starting the reproductive cycle over again.
Why Timing Matters for Treatment
The timeline of egg hatching and nymph maturation is the reason why a single treatment application is rarely sufficient to eliminate an infestation. Most over-the-counter treatments are designed to kill live lice, including adults and nymphs, but they often fail to penetrate the hard, protective shell of the nit. If the nits survive the initial treatment, they will hatch over the next several days, leading to a renewed population of nymphs.
A second treatment application is therefore necessary, typically advised seven to ten days after the first. This second application is timed to kill all the newly hatched nymphs before they can mature into adults and begin laying eggs, successfully breaking the cycle of reproduction.