Head lice are small, wingless, blood-sucking insects. They live in the hair on the head and feed off the blood from the scalp. A louse (a single adult) is about the size of a sesame seed. A nit (louse egg) is about the size of a flake of dandruff. Symptoms of head lice include extreme scalp itchiness, feeling like something is crawling on your scalp, sores on the scalp from scratching. There are several head lice treatments available.