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The Life Cycle of Head Lice

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Head lice have three stages to their life cycle, moving from egg to nymph to adult in about eight to ten days. Known specifically as pediculus humanus capitis, head lice live on or near the scalp of their host and feed throughout their lives (about thirty days) on human blood.


The eggs of head lice are called nits (in fact, the phrase “nit picking” derives from one of the procedures for removing head lice). Because they are clear and difficult to detect, they are frequently misidentified as dandruff or flakes of skin. The eggs hatch about a week after being laid and are typically located within six millimeters of the scalp to maintain an ideal temperature.


Newly hatched eggs produce a nymph, at which point the broken shell darkens and becomes easier to detect, though it remains attached to the shaft of hair. The newly hatched nymph is about the size of a pinhead and when viewed under a microscope, looks like an adult louse. The nymph will undergo three moltings (shedding its exoskeleton as it grows) and reaches maturity at the end of the molting cycle – about seven days after it hatched.


Adult head lice are true insects, having six legs with claws to help them crawl across the scalp. Initially, the louse is clear or off-white so it will blend in with the hair color of its host. Once it begins feeding on the blood of the host, however, it gradually darkens in color, taking on a reddish-brown hue. At this point, the louse is about as big as a sesame seed, with females being the larger of the species. An adult female louse only needs to mate once and can lay as many as eight eggs a day for the remainder of its thirty day lifecycle. Left untreated, head lice will continue to infest the host.

While head lice do not pose an immediate health threat (they do not carry diseases, nor are they a sign of poor hygiene), they are irritating parasites and can cause uncomfortable itching, which can result in secondary infections from constantly scratching the scalp. Treatment of head lice is best accomplished through either over-the-counter or prescription medications that target these pests, diligent application, and regular monitoring by a physician until the all clear is given.

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