Parasitic lice are divided into two major groups according to their method of feeding. Here, we are dealing with the chewing lice. Some authorities consider them a full order, the Mallophaga, while others call the Mallophaga a suborder of the order Phthiraptera. Either way, they are an important group of ectoparasites that plague the existences of many mammals and birds.
Ectoparasites live on the surfaces of their hosts, on the skin or in the hair. In this habitat, they have an abundance of food and not much worry about predators, since their large hosts offer them safe sanctuary. For these reasons, lice have secondarily lost the capacity to fly and their eyes are small or absent altogether. They are probably descended from the ancestors of the book lice, order Psocoptera, which they resemble. Somewhere in the Cretaceous period about a hundred million years ago, their forebearers lived in the nests of early mammals and birds, where they could feed off shed skin and feathers or the fungus that grows on them. To reach other nests, these insects hitch-hiked on the nest builders and eventually ceased to leave the host.
While the lice do not have to worry about other predators, they do have to contend with the grooming and cleaning efforts of their hosts. Lice have developed strong gripping claws to protect themselves from being groomed off their hosts. Their bodies are flattened dorso-ventrally and their antennae are short, all of which helps them evade grooming. They also lay sticky eggs that are cemented to the host's hair or feathers. When the young louse is ready to hatch, it swallows air which builds up pressure and blows the lid off the egg cap. After hatching the nymph sheds several times before maturing.
Lice can be a way of discovering evolutionary paths. Lice are usually specific to their hosts and evolve more slowly, so that they show the relationships better than the hosts do. Because lice are host specific, you don't have to worry about lice moving from your cat to your budgie to you. Usually lice only outlive their host by a few days and cannot survive off the body to find a new host. The usual way to move to a new host is through direct contact, either between the sexes during mating or from mother to child.
What do lice look like? They are small and wingless but otherwise have normal insect characteristics: short antennae and chewing mouthparts on the head, followed by a thorax that supports six stubby jointed legs that end in gripping claws, followed by a segmented abdomen. Lice are usually grey or white and slightly hairy. They are flattened dorso-ventrally instead of sideways like fleas, and unlike fleas, they cannot hop. Humans are affected by three species of lice but they are all sucking rather than biting lice and so will be dealt with in a separate article about the Anoplura.
Biting lice are annoying to their hosts but seldom fatal and you have to give them credit for being tough little survivors in a hostile world.