Topics below:

• What the heck are those creatures?
• Life Cycle of Lice
• The gross stuff: Feeding
• Who gets ‘em?
• Lice and your great, great, great, great, great grandparents

What the heck are those creatures?

Head lice are wingless insects spending their entire life on human scalp and feeding exclusively on human blood.  Humans are the only known host of this parasite.  Head lice are closely related to the body lice, and more distantly related to the pubic or crab louse.

Head lice cannot fly, and their short stumpy legs render them quite incapable of jumping, or even walking efficiently on flat surfaces.

Adult head lice are small are only about 1-3 millimeters long.  Head lice are grey in general, but their precise color varies according to the environment in which they were raised.  After feeding, blood causes the louse body to take on a reddish color.

With their short legs and large claws, lice are well adapted to clinging to the hair of their host.

Life Cycle of Lice

Eggs are generally laid on a hair shaft within 1 cm of the scalp surface. To attach each egg, the adult female secretes a glue from her reproductive organ.  Each egg is oval-shaped and about 0.8 mm in length. They are brown so long as they contain an embryo, but appear white after hatching.

Viable louse eggs hatch 6 to 9 days after laying.  After hatching and before becoming able to reproduce, they’re called “nymphs”.  Once hatched, head lice to reach sexual maturity in 8 to 9 days.

The hazards for a young nymph are numerous, and about 1/3 die during this stage. For example, failure to completely hatch from the egg is common and invariably fatal.  During feeding, the nymph gut can rupture, dispersing the host’s blood throughout the insect. This results in death within a day or two.  The ones that survive are either tough little buggers or lucky.

Once molting into adulthood, the louse wastes no time before reproducing.  Pairing can begin within the first 10 hours of adult life.

Females lay an average of 3-4 eggs daily. During its lifespan of 4 weeks a female louse lays 50-150 eggs (nits).

The gross stuff: Feeding

All stages are blood-feeders and they bite the skin 4-5 times daily to feed. To feed, the louse bites through the skin and injects saliva which prevents blood from clotting; it then sucks blood into its digestive tract. Bloodsucking may continue for a long period if the louse is not disturbed. While feeding, lice may excrete dark red feces onto the skin.

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. Head lice are repelled by light, and will move towards shadows or dark-colored objects in their vicinity.

Normally head lice infest a new host only by close contact between individuals, making social contacts among children and parent child interactions more likely routes of infestation than shared combs, brushes, towels, clothing, beds or closets. Head-to-head contact is by far the most common route of lice transmission.

Who gets ‘em?

Girls are 2-4 times more frequently infested than boys. Children between 4 and 13 years of age are the most frequently infested group.

About 6-12 million people, mainly children, are treated annually for head lice in the United States alone.

Head lice are not known to be vectors of diseases.  That means they’re icky, but never deadly.

Lice and your great, great, great, great, great grandparents

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.

(thanks for that, Wikipedia!)