Head lice (Pediculus humanus capitis) are small, wingless insects that live exclusively on the human scalp, feeding on blood. Infestations are common, particularly among school-age children. The female louse’s reproductive capacity, including the number of eggs laid, drives the speed and persistence of an infestation. These eggs, called nits, are securely attached to the hair, making effective removal and treatment challenging.
The Reproductive Output of Female Lice
The reproductive success of head lice is tied to the output of the adult female. Once mated, a female louse begins laying eggs one to two days after reaching maturity. She only needs to mate once to remain fertile throughout her adult lifespan, which is roughly 30 days.
A female louse lays between six and ten eggs daily. This consistent output allows an infestation to grow rapidly. Over her reproductive period of about 14 to 16 days, a single female can produce 50 to 150 eggs. This potential for growth emphasizes why prompt treatment is necessary.
Since only adult females lay eggs, controlling the population requires targeting both the live insects and the attached nits.
The Structure and Adhesion of Nits
The eggs of the head louse, known as nits, are small, oval structures, typically measuring less than a millimeter in length. Viable nits are usually translucent, yellowish, or tan when laid, darkening as the embryo develops. Hatched or dead nits appear whiter and shriveled, often confused with dandruff.
The most distinctive feature of the nit is its strong attachment to the hair shaft. The female louse secretes a specialized, cement-like substance, composed of keratin-like proteins, that permanently affixes the egg to the hair.
This insoluble “glue” is highly resistant to water, shampoos, and most chemical treatments, making nits difficult to remove. Nits are placed close to the scalp for optimal incubation temperature. The distance of the nit from the scalp can help estimate the duration of the infestation.
The Lifecycle Timeline from Egg to Adult
The lifecycle of the head louse progresses through three distinct stages: the egg, the nymph, and the adult. Understanding the timing of each stage is important for successful treatment.
Egg Stage (Incubation)
The initial egg stage, or incubation period, typically lasts between six and ten days before hatching.
Nymph Stage (Growth)
Once hatched, a young louse, called a nymph, emerges. The nymph is a miniature version of the adult but cannot reproduce. It must feed on blood multiple times daily and undergoes three molts as it grows.
Adult Stage (Reproduction)
It takes approximately seven to twelve days for a nymph to mature into a reproductive adult louse. The entire cycle, from a newly laid egg to an egg-laying adult, takes about 18 to 24 days.
Because most treatments are effective against the nymphs and adults but not the eggs, repeated applications are necessary. Treatment protocols often involve a second application seven to ten days after the first, designed to kill all newly hatched nymphs before they mature and restart the cycle.