Head louse. Coloured scanning electron micrograph SEM of the head of a head louse Pediculus humanus capitis. Its mouthparts are at centre. Head lice measure 2_3 millimetres in length. They live in the hair on the head and suck blood from the scalp, causing intense itching. The itching provokes scratching, which can lead to scalp inflammation or dermatitis. Treatment is with medicated shampoo.