Each species of louse has its own mammal species it feeds off of, and only rarely will one suck blood from another mammal species.
The Pediculidae family are parasitic on primates including man, and are apparently the only sucking lice to have functioning eyes.
Pediculus humanus, the human louse
On the right is Pediculus humanus, the human louse. It is usually found on the head gripping tightly on to the hair. There are two races of human louse, the head louse (Pediculus humanus capitis) and the body louse (Pediculus humanus corporis). Some experts say these should be split into two species, however they can interbreed. Both suck blood, and the body louse can transmit diseases, e.g. typhus and trench fever.
Outbreaks of body louse infestations usually only occur where hygiene is poor. Outbreaks of head lice are common in children even where there is good hygiene and frequent combing. For advice on getting rid of head lice see below. The human body louse has been found living and breeding on domestic pigs.
Luckily the head louse does not transmit diseases. The head louse ranges in colour from dirty white to greyish black, and the colour of the adult depends on the colour of the hair of the host it was feeding on during its nymphal stages.
Each egg is cemented to the base of the hair near the scalp by the female, and will resist washing and combing with a normal comb. The cement is so strong that the shell of the egg usually remains fixed to the hair until it grows out or is cut. A female can lay up to 10 tubular eggs a day. The eggs are pearly white, 0.8 x 0.3 mm.
The nymphs are just a smaller version of the adults. The adults range from 2 - 6 mm, about the size of a sesame seed. They can reach maturity just eight days after hatching from an egg. There are 3 nymphal stages. During the first stage the insect is almost transparent until it takes a blood meal when it turns a shining red colour. Then as the blood darkens in the gut it turns a purple colour. The nymph feeds at least twice a day.
Feeding. Both nymphs and adults feed the same way. They press the front of the head against the skin of the host. Then curved teeth around the mouth fasten on to the skin, and the piercing stylets goes through the skin which has been lubricated by saliva.
Lifecycle. The egg hatches in 8 or 9 days, and will reach adulthood in less than 3 weeks. A female can lay up to 60 eggs, and it has been found that more than 80% of these go on to hatch. Head lice will die of starvation if they do not drink a blood meal for 55 hours. Transmission is though direct contact or sharing combs or brushes.
Personal experience When I was a child, once a week my mother washed my hair, sat me on the floor while she sat in a straight-backed chair and got out the nit-comb. For the next half hour she'd comb through my long hair till she was satisfied. I never had head lice.