Lice are divided into three groups: sucking lice, elephant lice and chewing lice. Lice with sucking mouthparts belong to the order Anoplura (although some taxonomists call this a suborder in the order Pthiraptera). Sucking lice are parasites of mammals. There are about 500 species of sucking lice worldwide and the majority (about two thirds) occur on rodents. Most lice are species specific and do not migrate to different species.
Lice have incomplete life cycles: egg, nymph, adult and do not go through a pupal stage. The females are prolific and can lay up to 100 eggs in a lifetime. Eggs take between 7 to ten days to hatch. Nymphs or larvae are smaller versions of the adults. They go through several moults and then mature sexually. Both adults and nymphs feed by inserting their stylets into the host's skin and then sucking blood. Most lice spend their lives on one host. However, if the host touches another member of the same species, either sexually or while caring for young, the lice can move from one body to another. Lice cannot survive more than a few hours off their host.
What do sucking lice look like? They are small and wingless but otherwise have normal insect characteristics: short antennae and piercing, sucking 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. They are between a half and 8 mm in length.
Humans are the host to three species of sucking lice: the head louse, the body louse and the genital or crab louse. Although the head lice and crabs are annoying, itchy and unpleasant, they are not known to carry disease. However the body louse spreads typhus and has therefore been the cause of death for millions of people over the centuries. In fact, in the book Rats, Lice and History, the author, H. Zinsser, maintains that rats and lice together have caused more wars to be won or lost than the battles that were fought, due to the diseases they have spread among the troops.
Head lice are particularly common on school children and unfortunately are developing resistance to insecticides. Besides the insects themselves, the white eggs attached to the hairs can be a giveaway to their presence. Schoolchildren should be checked regularly and encouraged to not share hats with their friends. Unfortunately it is also best if they don't hug each other as this gives the lice a chance to spread. When another head of hair passes close by, lice will grab on to the new hair and let go of the old, thus moving to a new host.
For more information: http://tolweb.org/Anoplura/13871 http://www.bumblebee.org/invertebrates/Anoplura.htm