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Head lice are grey (translucent), brown or black, or sometimes even red (if they have recently fed). Adult head lice are small, only 2-3 mm long and translucent. Their feet have claws so they can easily grasp the hair. Nymphs resemble adults but they are smaller and lighter in colour. The smallest nymphs are 1 mm long.
The eggs are 0.3 x 0.8 mm and they attach firmly to the hair strands. They are yellowish and translucent before hatching and paper-like white afterwards. Each egg has a lid that falls off during hatching and empty eggs are purer white. Eggs that are killed with insecticides keep their lid and become brown when their content dries. Eggs are visible with the naked eye but a magnifying glass is needed to see details. Dead or empty eggs can be present at hear strands even though all lice have been removed. It is not necessary to remove these eggs.
Head lice need human blood to survive, and therefore live only on humans. Animals have other species of lice that do not live on humans. Head lice suck blood at least five times a day. They do not transmit germs or viruses.
Lice development stages include egg, three nymph stages and adult. An adult female louse lives for about 25 days, during which time she lays about 90 eggs which hatch after 8 days. During the next 9-12 days, the nymphs change skin three times before they reach adulthood. When the female has been adult for 1-2 days she will begin to lay eggs. The development time for a generation from egg to egg is approximately three weeks.
In 2008 the Norwegian Institute of Public Health conducted two investigations regarding head lice in households of Norwegian school children. Some of the information above is taken from the four articles that were published: