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Human Head Lice.
Introduction to Lice.
Louse Gallery.

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louse nit
Head louse egg (nit) on human hair. x70.

  Human Lice.

"The louse has killed more human beings than has any other insect, with, perhaps, the malaria mosquito excepted. Historically, lice are the major insect vector of disease in temperate and cold lands, as mosquitoes are in hot ones."
The Appreciation of Lice. Maunder J. W. (paper) 1983.
Louse is a term applied to an insect which fulfils two conditions. It must be an external parasite of a warm-blooded animal (bird or mammal), and it must spend its entire life cycle on that animal.
The human head louse attatches its eggs to the base of a hair, close to the scalp. The nymph which hatches from the egg develops to an adult after three moults, and dies within two or three weeks, after a life of constant feeding, mating and egg-laying.

Lice are spread by physical contact between humans. When they are removed from the host's head and placed under the microscope, they appear sluggish and clumsy, but at the temperature of the human scalp and amongst the hairs for which their bodies are adapted, they are quite active, and can easily change hosts during brief moments of contact. Thus they spread most rapidly between children. In this way, the average louse may spend its day on several heads.

The other common lice of the human body are the clothing lice and pubic lice. Interbreeding between head and clothing lice is possible, but under normal conditions, the three types favour different regions of the body, and live distinct and separate lifestyles.

Head lice are increasingly common amongst the well-scrubbed children of the middle classes, having been previously associated with the poorest in society sharing a more crowded and less hygienic lifestyle. Humans are not the only creatures which thrive in a more hygenic environment.

Their removal is beset with misapprehensions and old-wives tales. Washing with medicated shampoos and similar offerings is not much use -- unless the medication is left in contact with the lice long enough to kill them. Anything short of this amounts to using sub-lethal dosage to guarantee the evolution of resistant lice.
Also, it is almost impossible to drown a louse, at least during the course of a normal shower or bath. They have been known to survive four days clinging to the body hairs of animals constantly submerged or drenched with water.

The most effective solution is normal hygiene plus frequent combing of the hair. Fine-tooth combs are effective for removing the lice, and frequent grooming with a normal comb also controls the infestation by removing some lice and inflicting injury on others which cling tenaciously to hairs in their efforts to resist being dislodged. Lice do not regenerate lost limbs, and any louse which has lost a leg due to combing (or scratching) will die very shortly thereafter.
In some societies, head lice are controlled by applying oil to the hair. The oil suffocates the growing louse by sealing the air vents in the operculum of the egg casing.

Click for a diagram of human lice.
Click for Hooke's illustration of a louse from Micrographia (1665).

Louse nit. The egg of a human louse (nit) on a human hair. The female louse attaches the egg close to the scalp with a transparent quick-setting glue. The time taken to hatch depends upon temperature. At an optimum temperature of 31°C, they will take seven days, and 16 days at 22°C, the lowest temperature at which hatching occurs.

The exit-lid (operculum) of the egg is uppermost, and has a raised cluster of about sixteen thick-rimmed apertures which allow the developing louse to breathe.

Brightfield, x70.
Empty louse egg-case The empty eggshell of a human louse, attatched to a hair. The operculum is missing. On a dark-haired person, the empty egg casing (also called a nit) is much more conspicuous than the developing egg, and is usually discovered when further from the scalp.
Since human hair grows at around a centimetre per month, the distance of the empty eggshells from the scalp gives an indication as to when the eggs were first deposited.
Brightfield, x70.
Young human head louse. A young male human louse.
The visual feature which distinguishes the males from the females is the end of the abdomen, which is rounded in the male louse. Each of the legs is equipped with a single strong claw, allowing the louse to grip up to six hairs simultaneously in its efforts to remain on the scalp of the host.
Brightfield, x50.
Adult human head louse An adult female human head louse.
Females are distinguished from the males by the forked appearance of the end of the abdomen. Both males and females have a pair of simple eyes on either side of the head, and a pair of short antennae, each having five segments.
Brightfield, x30.
Young louse showing trachea. The lice in these micrographs were killed and preserved in a 50% mixture of isopropyl alcohol in water. The two specimens in the above pictures were shot after a day had passed, and their internal organs had become somewhat opaque. The picture on the left was taken immediately after killing in the alcohol solution. The louse is still transparent, and the tracheal tubes in the thorax and legs can be clearly seen. Having no lungs as such, it is the contraction of muscles and the movement of internal organs which causes air to move through the tracheal system, thus bringing oxygen to the tissues of the louse.
A fine degree of control over the valves which close off the spiracles can enable a louse to effectively hold its breath and hang on for some days when immersed in water. Washing the hair does not get rid of lice.

This picture was taken using the OU field microscope/digital camera setup described in the article on videomicrography in the field.
Brightfield, x100.

  Many thanks to Ms. Ada Zimmermann for the supply of the specimens.

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