page_title: Head louse
section_title: Development and nymphs
hierarchical_section_title: Head louse / Development and nymphs
caption_attribution_description: This illustration depicts the life cycle of a “sucking louse”, Pediculus humanus var. corporis, and the morphologic changes that take place at each successive developmental plateau reached. All developmental stages of the louse’s life cycle are passed while it resides as an ectoparasite on its mammalian host. The stages include the egg, the first nymph, second nymph, and third nymph, and the adult male and female.
context_page_description: The head louse is an obligate ectoparasite of humans that causes head lice infestation.
  Head lice are wingless insects spending their entire lives on the 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, as well as other parts of the human body.
  Lice differ from other hematophagic ectoparasites such as fleas in spending their entire lifecycle 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 in preferring to attach eggs to scalp hair rather than to clothing. The two subspecies are morphologically almost identical, but do not normally interbreed, although they will do so in laboratory conditions. From genetic studies, they are thought to have diverged as subspecies about 30,000–110,000 years ago, when many humans began to wear a significant amount of clothing.
context_section_description: Head lice, like other insects of the order Phthiraptera, are hemimetabolous. Newly hatched nymphs will moult three times before reaching the sexually-mature adult stage. Thus, mobile head lice populations contain members of up to four developmental stages: three nymphal instars, and the adult (imago). Metamorphosis during head lice development is subtle. The only visible differences between different instars and the adult, other than size, is the relative length of the abdomen, which increases with each molt. Aside from reproduction, nymph behavior is similar to the adult. Nymphs feed only on human blood (hematophagia), and cannot survive long away from a host.
  The time required for head lice to complete their nymph development to the imago depends on feeding conditions. At minimum, eight to nine days are required for lice having continuous access to a human host. This experimental condition is most representative of head lice conditions in the wild. Experimental conditions where the nymph has more limited access to blood produces more prolonged development, ranging from 12 to 24 days.
  Nymph mortality in captivity is high—about 38%—especially within the first two days of life. In the wild, mortality may instead be highest in the third instar. Nymph hazards are numerous. Failure to completely hatch from the egg is invariably fatal and may be dependent on the humidity of the egg's environment. Death during molting can also occur, although it is reportedly uncommon. During feeding, the nymph gut can rupture, dispersing the host's blood throughout the insect. This results in death within a day or two. Whether the high mortality recorded under experimental conditions is representative of conditions in the wild is unclear.
caption: Development of Pediculus humanus humanus (body lice), which is similar to that of head lice (Pediculus humanus capitis)
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