Class: Insects
Size: Up to 3mm
Weight: Variable
Age: 3 to 4 weeks
Appearance: 6 legs, wingless, light to greyish colour
Food: Blood of humans
Distribution: Worldwide
Original location: Africa
Habitat: Scalp and hair of humans
Natural enemies: None
Sexual maturity: After 9 to 12 days
Egg laying: Approx. 90 eggs, are attached to the hair
Threatened with extinction: No

• The human louse family consists of six species, two of which are particularly adapted to humans. There are two subspecies of the pubic louse (Pthirus pubis) and the clothes louse (Pediculus humanus humanus), also known as the body louse (Pediculus humanus corporis). The best known, however, is the head louse (Pediculus humanus capitis).
• The natural habitat of head lice is the human scalp and hair. Head lice absorb blood by scratching the skin surface with the stiletto-like continuation of their head and sucking blood from open capillaries. At the same time, they inject saliva into the microscopic wound to prevent blood clotting. As yet unknown components of the lice saliva have immunogenic effects, i.e. they are recognised as foreign by the human immune system.
• The end limbs of the legs are shaped into hook-shaped claws. With these claws, head lice can hold on very well or move along the hair. Usually, head lice hold a hair with some claws and then grab another hair with the free claws. In this way, the parasite can be quickly transferred from one head to another. On smooth surfaces, however, head lice can only move clumsily.
• Head lice undergo only incomplete metamorphosis (semimetabolism). This means that the nymphs that hatch from the eggs already look like adult lice, but are much smaller. The newly hatched nymphs are 1 to 2mm, the adult lice about 3mm long. Head lice are sexually mature in about 9 to 12 days. Only adults try to get onto other heads. Female head lice can lay viable eggs without male fertilisation (parthenogenesis).
• The optimal temperature for egg development is between 34°C and 36°C. At this temperature and high humidity (a characteristic condition of the scalp), young lice usually hatch from their eggs within 7 to 8 days. When the temperature drops, the embryo takes longer to develop. However, about 30% of these eggs do not contain embryos or they could not develop into viable lice.
• Eggs are mainly found in particularly hair-dense areas: Temples, behind the ears and on the neck. The temperature remains relatively constant here and the humidity is extremely high, so there is less danger of the newly hatched lice drying out.
• Head lice have stinging and sucking mouthparts. They take a blood meal for a few minutes every 4 to 6 hours. Head lice have adapted optimally to life on the scalp and in the hair. If the head louse loses contact with the host, it cannot survive for more than one to two days, without a blood meal only three days at room temperature.
• The typical route of transmission of head lice to other people is close contact with the head of a person infected with the animals. Presumably, almost all new diseases develop in this way. Sharing pillows, towels or combs can theoretically cause transmission, but in practice this is an exception. Textile surfaces such as sofa covers, headrests and carpets, as well as smooth surfaces such as tables, desks and floors are not suitable for head lice and do not pose a risk of infection.
• The problem is that a lice infestation is usually not detected early. Over time, the probability increases that the lice stage has been passed on to other people. This is favoured by people from all social classes: In these groups, infected people have more frequent head contact, especially in community settings such as kindergartens and schools, as they do with family and friends.