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Nit on human hair

Kevin Mackenzie

Download this image from Wellcome Collection.

Scanning electron micrograph of a nit or head louse egg (coloured green) attached to a strand of human hair (coloured brown). Head lice feed on human blood and live in close proximity to the scalp. Female lice lay eggs in sacs that attach firmly to individual strands of hair near the base of the hair shaft. Most will hatch within seven to ten days, and the newly emerged immature louse (nymph) will then need to feed on blood to survive. The width of the image is 1.5 mm.

How do nits stay attached to the hair?

Nits are attached to hair shafts using a glue-like substance made by the female louse. This firmly anchors the nit near the base of the hair, holding it in place so that it cannot be easily brushed away. Even after the louse has hatched, the empty eggshell stays attached. The glue-like substance is thought to be similar in composition to human hair, and together with the eggshell provides a protective sheath for the developing louse. Head lice bites and secretions on the scalp can cause itchiness, and treatment involves using either chemicals (to kill the lice) or physical methods (to remove the lice and nits) such as repeated combing with a fine-toothed comb.

What is the life cycle of a head louse?

A head louse develops from an egg to an adult in 16 to 21 days. Head lice start out life as eggs, which are attached to the hair near the scalp to stay warm. Eggs usually hatch within seven to ten days, and the newly emerged immature lice (nymphs) then need to feed on blood from the scalp to survive. Nymphs go through three stages before maturing into adults, which can take around a week. The adult louse is about the size of a sesame seed and has six legs with claws that help it cling on to the hair. Adult lice can live for three to four weeks, but will only survive for one or two days away from a person's head.