Head louse. Animated coloured scanning electron micrograph (SEM) of a head louse (Pediculus humanus capitis) clinging to a strand of human hair. The head louse is one of three lice species that feed exclusively on humans. It ranges in size from 2 to 3 millimetres in length. Each of its six legs ends in a claw. These claws are used to grasp the shaft of a hair, and as such render the louse unable to jump. An infestation will cause itching due to an allergic reaction against louse saliva, and if not treated can lead to fever and secondary infections.