The Biology Of Lice
Biology Of Lice
The non-disease-carrying head louse attaches its eggs to scalp hair rather. From genetic studies, they are thought to have diverged as subspecies about 30,000–110,000 years ago, when many humans began to wear a significant amount of clothing. Lice infestation of any part of the body is known as pediculosis.
Head lice (especially in children) have been, and still are, subject to various eradication campaigns. However, and unlike body lice, head lice are not the vectors of any known diseases. Except for rare secondary infections that result from scratching at bites, head lice are harmless.
Like most insects, head lice are oviparous. Females lay about 3-4 eggs per day. Louse eggs are attached near the base of a host hair shaft. In cool climates, eggs are generally laid within 3-5 mm of the scalp surface. In warm climates, and especially the tropics, eggs may be laid 6 inches (15 cm) or more down the hair shaft.
To attach an egg, the adult female secretes glue from her reproductive organ. This glue quickly hardens into a “nit sheath” that covers the hair shaft and large parts of the egg except for the operculum, a cap through which the embryo breathes. The glue was previously thought to be chitin-based, but more recent studies have shown it to be made of proteins similar to hair keratin.
Each egg is oval-shaped and about 0.8 mm in length. They are bright, transparent, tan to coffee-colored so long as they contain an embryo but appear white after hatching. After hatching, the louse nymph leaves behind its egg shell (usually known as nit), still attached to the hair shaft. The empty egg shell remains in place until physically removed by abrasion or the host, or until it slowly disintegrates, which may take 6 and more months.
The term “nit” refers to an egg without embryo or a dead egg. With respect to eggs, this rather broad definition includes the following:
- Viable eggs that will eventually hatch
- Remnants of already-hatched eggs (nits)
- Nonviable eggs (dead embryo) that will never hatch
This has produced some confusion in, for example, school policy (see The “no-nit” policy) because, of the three items listed above, only eggs containing viable embryos have the potential to infest or re-infest a host.