The body louse (Pediculus humanus humanus) is indistinguishable in appearance from the head louse, but they occupy different habitats. In particular, body lice have evolved to attach their eggs to clothes, whereas head lice attach their eggs to the base of hairs. The body louse and head louse are exclusive to mammals.

Body lice have three forms: the egg (also called a nit), the nymph, and the adult. Nits are lice eggs. They are generally easy to see in the seams of an infested person’s clothing, particularly around the waistline or under armpits. They are oval and usually yellow to white in color. Nits may take 1–2 weeks to hatch. A nymph is an immature louse that hatches from the nit (egg). A nymph looks like an adult, but smaller. Nymphs mature into adults about 9–12 days after hatching. To live, it must feed on blood. The adult body louse is about the size of a sesame seed, has 6 legs, and is tan to greyish-white. To live, lice must feed on blood. If separated from its host, lice die at room temperature.