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- The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans.
- Adult head lice are small (1–3 mm long), dorso-ventrally flattened (see anatomical terms of location), and entirely wingless.
- The thoracic segments are fused, but otherwise distinct from the head and abdomen, the latter being composed of seven visible segments.
- Head lice are grey in general, but their precise color varies according to the environment in which they were raised. After feeding, consumed blood causes the louse body to take on a reddish color.
- One pair of antennae, each with five segments, protrude from the insect's head. Head lice also have one pair of eyes. Eyes are present in all species within Pediculidae (the family of which the head louse is a member) but are reduced or absent in most other members of the Anoplura suborder.
- Six legs project from the fused segments of the thorax. As is typical in Anoplura, these legs are short and terminate with a single claw and opposing "thumb". Between its claw and thumb, the louse grasps the hair of its host. With their short legs and large claws, lice are well adapted to clinging to the hair of their host.
- There are seven visible segments of the louse abdomen. The first six segments each have a pair of spiracles through which the insect breathes. The last segment contains the anus and (separately) the genitalia.
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