A female head louse lays eggs by gripping a strand of hair near the scalp and secreting a fast-hardening protein glue that locks each egg in place. She can produce roughly 10 eggs per day throughout her 30-day lifespan, meaning a single louse may deposit around 300 eggs before she dies. Understanding exactly how this process works helps explain why lice eggs (called nits) are so notoriously difficult to remove.
Where the Glue Comes From
Inside a female louse’s abdomen sit specialized cement glands dedicated entirely to producing the adhesive that attaches eggs to hair. These glands secrete a mix of proteins that harden almost immediately upon contact with air. Enzymes called peroxidases, also produced in the cement glands, cross-link those proteins together to form a rigid, waterproof seal. The glands produce cement only, not eggs themselves. The egg develops separately in the reproductive tract and meets the cement at the moment of laying.
The Egg-Laying Process Step by Step
When a female louse is ready to lay, she crawls to a hair shaft close to the scalp, typically about a quarter inch from the skin surface. She straddles the hair with her clawed legs and positions her abdomen so the opening of her reproductive tract faces the shaft. She then deposits a single egg while simultaneously releasing a stream of liquid cement around it.
The cement flows down and around the hair, forming a cylindrical sheath that encases the base of the egg and grips the shaft like a sleeve. As the protein mixture is exposed to air, it polymerizes rapidly, hardening into a structure that physically clamps onto the hair. Lab analysis shows there is no chemical bond between the glue and the hair itself. The attachment is purely mechanical: the sheath shrinks and tightens as it solidifies, creating a vise-like grip around the shaft. This is why nits feel cemented in place when you try to slide them off with your fingernails.
The entire process takes only seconds per egg. The louse then moves to another hair strand and repeats. She lays eggs one at a time, never clustering multiple eggs on the same strand.
Why Eggs Are Placed So Close to the Scalp
Lice are extremely sensitive to temperature. Nit production peaks at about 84°F, and eggs need a range of 72°F to 82°F with at least 70% humidity to successfully incubate. The surface of the human scalp provides exactly these conditions. By cementing each egg roughly 1 millimeter to a quarter inch from the skin, the female louse ensures the developing embryo stays warm and humid enough to survive.
This placement also explains a practical trick for gauging how long an infestation has been present. Because hair grows, nits found far from the scalp were laid weeks or months ago and have likely already hatched or died. Nits found right at the scalp line are the fresh, viable ones.
What the Egg Looks Like Up Close
A louse egg is a tiny, oval capsule about the size of a sesame seed. At the top sits a cap called the operculum, which has small breathing holes (aeropyles) that allow the developing embryo inside to take in oxygen. The cement sheath covers the bottom and sides of the egg but deliberately leaves the operculum exposed so air can reach the embryo. This design is remarkably precise: the glue seals the egg firmly to the hair without suffocating the organism growing inside.
Biochemical analysis of the sheath reveals it is made of at least four distinct bands of protein, likely cross-linked with fatty components and arranged in a layered, sheet-like structure. This composition makes the sheath both rigid and resistant to water, shampoo, and most solvents. It is the reason ordinary washing does almost nothing to dislodge nits.
From Egg to Nymph
Once laid, a nit incubates for six to nine days, with most hatching at about the one-week mark. When the nymph is ready to emerge, it pushes open the operculum cap and crawls out onto the scalp to begin feeding. The empty shell stays glued to the hair and gradually turns white or clear, which is why old nits are easier to spot than fresh ones (which tend to be a yellowish-brown color that blends with lighter hair).
The nymph goes through three molts over the next nine to twelve days before becoming a mature adult. A female louse can begin laying her own eggs within one to two days of reaching adulthood, which is how infestations grow so quickly. Within a few weeks, a handful of lice can produce hundreds of nits, each one individually cemented to its own strand of hair with the same resilient protein glue.
Why Nits Are So Hard to Remove
The mechanical grip of the cement sheath is the core challenge. Because the bond is physical rather than chemical, dissolving it requires breaking down the protein structure itself. Water, regular shampoo, and even many over-the-counter lice treatments do not dissolve the sheath. Fine-toothed nit combs work by physically shearing the sheath off the hair, which requires dragging the comb slowly from root to tip under tension. Some removal products use enzymes or acidic solutions designed to weaken the protein cross-links, making combing more effective.
The fact that each egg is laid individually on a separate hair strand means there is no shortcut. Every nit must be physically removed or destroyed one by one, which is why thorough combing sessions, repeated over multiple days to catch newly hatched nymphs, remain the most reliable way to clear an infestation.