What are head lice?

Head lice are tiny greyish-brown wingless insects, about the size of a sesame seed when fully grown

They have 6 legs that end in hook-shaped claws that grasp the hair very tightly until they are either dead or physically removed from the hair. Vigorous hair brushing will not remove them and is also unlikely to break their legs, as some people think.

They cannot jump, hop, fly or swim, but they can crawl very fast.

Head lice are parasites that feed by taking regular blood meals from the human scalp. Before they feed, they inject a substance into the scalp to stop the blood from clotting before they have finished feeding.

Some people are allergic to this substance, that's why lice often cause itching.

Head lice breathe through seven sets of holes called spiracles, which are found along the body. They can close these spiracles for long periods, that's why they don't drown during hair washing.

They can only survive in a warm and humid environment, so they tend to stay close to the scalp where body heat and natural sweating maintain the ideal temperature and humidity.

In the diagram below you can see how the louse develops over its life cycle of around 30 days.

The eggs are cemented to the base of individual hairs close to the scalp. Live eggs are a pearly white colour and are only found at the base of the hair. Dead, unhatched eggs are brownish. Empty egg cases, also known as 'nits' are white and may be mistaken for dandruff. Dead eggs and nits may be found further along the hair, still cemented in place as the hair grows.