There are over 3,000 species of louse – they are insects that live among the feathers of birds and the hairs of mammals and suck their blood.
Mammals without hair, such as the whales, are not susceptible (the ‘whale louse’ is actually a crustacean).
Human lice live on the head, in the clothing, and in the pubic region. Each is a separate type.
The Head Louse and Nits
The Head Louse (Pediculus humanus capitis) lives in hair all over the head, but it prefers the nape of the neck (if the hair is long enough). They particularly like to get behind the ears – where they often lay their eggs. Maybe this is why adults often ask children if they have washed behind their ears?
- Head lice usually move from one person to another when two individuals put their heads together. (Lice can neither fly, nor jump as fleas do, so they have to climb from hair to hair.)
- Girls are more likely to have head lice than boys, probably because they usually have longer hair.
- Over 10 million people a year get head lice in the USA. These are usually children.
- Head Lice can only survive for a short time away from their host (a matter of hours), and, unlike fleas, they do not lay eggs anywhere except among the hairs of their host.
- Head Lice attach their eggs to hair, usually near the scalp, and the eggs (and larvae) are known as nits.
The Body Louse
The Body Louse (Pediculus humanus corporis) is closely related to the Head Louse (they are sub-species of the same species). The two look identical, but they live in different places.
Head Lice prefer head hair, while Body Lice live and lay their eggs in clothing, or bedding, and only come out to feed.
Studies suggest that Body Lice evolved around 100,000 years ago, suggesting that this is when humans first put on clothes. Before that time humans were presumably more hairy, and the lice could wander all over the body – except for the habitat already occupied by the Pubic Louse.
The Pubic Crab Louse
The Pubic Louse (Pthirus pubis) is not closely related to the other two. It specialises in adult pubic hair, although it will sometimes move to a particularly hairy belly, or a beard or moustache.
- Pubic Lice normally pass from adult host to adult host during sexual intercourse, and they are thought to infect between 2 and 20 per cent of the human population – what does this say about monogamy?
- The only other animal to have Pubic Lice is the gorilla – and it is best not to think about this too much!
- ‘Phthiriasis’ is an infestation (normally among the pubic hairs) causing intense itching.
- The itching is caused by a reaction to the louse saliva, and the bites often turn a blue-grey colour ('maculae caeruleae').
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