There are thousands of species of small wingless parasites out of which three organisms feed on human blood and head lice are one of them.
They are pale, beige or dark grey coloured ectoparasites that turn dark brown after feeding on blood. Head Lice cannot survive outside the host and soon die when taken away from them.
Along with lice, their eggs, called "nits" are also found in infested hair. They are usually more in the hair behind the ears and the back of the head. This is because grooming of the hair (combing, shampooing) is not as rigorous in these parts as in other parts of the head.