The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans. Head lice are wingless insects spending their entire life on human scalp and feeding exclusively on human blood. Humans are the only known hosts of this specific parasite, while chimpanzees host a closely related species Pediculus schaeffi. Other species of lice infest most orders of mammals and all orders of birds.
Like all lice, head lice differ from other hematophagic ectoparasites such as the flea in that lice spend their entire life cycle on a host. Head lice cannot fly, and their short stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces.
The non-disease-carrying head louse differs from the related disease-carrying body louse (Pediculus humanus humanus) in preferring to attach eggs to scalp hair rather than to clothing. Although the two subspecies are morphologically almost identical, they do not normally interbreed, although they will interbreed in laboratory conditions. From genetic studies of them, they are thought to have diverged as species about 30,000–110,000 years ago, when many humans began to wear a significant amount of clothing. A yet more distantly related species of hair-clinging louse, the pubic or crab louse (Pthirus pubis), also infests humans. It is visually different from the other two species and is much closer in appearance to the lice which infest other primates. Lice infestation of any part of the body is known as pediculosis.
Historically, head lice (especially in children) have been, and still are, subject to various eradication campaigns. However, except for rare secondary infections that result from scratching at bites, head lice are harmless, and have been regarded by some as essentially a cosmetic rather than a medical problem. Unlike body lice, head lice are not the vectors of any diseases, and it has even been suggested that head lice infections are beneficial in helping to foster a natural immune response against lice which helps humans in defense against the far more dangerous body louse, which is capable of transmission of a number of dangerous diseases.
Read more about Head Louse: Adult Morphology, Louse Eggs, Nits, Development and Nymphs, Reproduction, Lifespan and Colony Persistence, Distribution, Treatment, Prevention, Archaeogenetics, Possible Mutualism Between Humans and Headlice
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