Head louse

Description
The head louse (Pediculus humanus capitis) is an obligate, ectoparasitic, wingless insect spending its entire life on human scalp and feeding exclusively on human blood. Humans are the only known host of this parasite. Humans can also be infested with the pubic or crab louse (Pthirus pubis) and/or with the body louse (Pediculus humanus humanus).

Morphology
The dorso-ventrally flattened body of the louse is divided into head, thorax and abdomen. On the head, one pair of eyes and one pair of antennae are clearly visible. The mouthparts are adapted to piercing the skin and sucking blood. The legs, with their terminal claws, are adapted to holding the hair-shaft. They cannot jump from head to head and being wingless insects they also cannot fly. In males (Fig.1), the first pair of legs are slightly larger and also used for holding the female during copulation. Males are slightly smaller than females and are characterized by the pointed end of the abdomen and the well-developed genital apparatus visible inside the abdomen. Females are characterized by two gonopods in the shape of a W at the end of their abdomen (see figure above). The eggs (Fig.3) are oval-shaped and ca. 0.8 mm in length. Immediately after oviposition they are shiny, round, and transparent. Head lice are 1-3 mm in size, varying according to their stage of development. They are usually grayish in color but depending on the time since their previous blood-meal they can be also reddish-brown.

Biology
During its lifespan of 4 weeks a female louse lays 50-150 eggs (nits). The egg hatches to the first nymphal stage, which after three moltings develop to nymph 2, nymph 3 and eventually to either a male or female louse. Adult lice copulate frequently and the females lay an average of 3-4 eggs daily. A generation lasts for about 1 month. All stages are blood-feeders and they bite the skin 4-5 times daily to feed. During oviposition the female excretes a glue-like substance from a gland located at the posterior end of the body and attaches the eggs on the hair of the host. Although any part of the scalp may be colonized, lice favor the nape of the neck and the area behind the ears, where the eggs are usually laid.

General recommendations for treatment
The number of cases of human louse infestations (or pediculosis) has increased worldwide since the mid-1960s, reaching hundreds of millions annually. There is no product or method which assures 100% destruction of the eggs and hatched lice after a single treatment. However, there are a number of treatment modalities that can be employed with varying degrees of success. These methods include chemical treatments, natural products, combs, shaving, hot air, and silicone-based lotions.

Ancient lice - Use in archaeogenetics
Lice are also important in the field of Archaeogenetics. Because most "modern" human diseases have in fact recently jumped from animals into humans through close agricultural contact, and also given fact that Neolithic human populations were too scattered to support contagious "crowd" diseases, lice (along with such parasites as intestinal tapeworms) are considered to be one of the few ancestral disease infestations of humans and other hominids. As such, analysis of mitochondrial lice DNA has been used to map early human and archaic human migrations and living conditions. Because lice can only survive for a few hours or days without a human host, and because lice species are so specific to certain species or areas of the body, the evolutionary history of lice reveals much about human history. It has been demonstrated, for example, that some varieties of human lice went through a population bottleneck about 100,000 years ago (supporting the Single origin hypothesis), and also that hominid lice lineages diverged around 1.18 million years ago (probably infesting Homo erectus) before re-uniting around 100,000 years ago. This recent merging seems to argue against the Multi-regional origin of modern human evolution and argues instead for a close proximity replacement of archaic humans by a migration of anatomically modern humans, either through inter-breeding, fighting, or being more fit to use available resources.