Head Louse - Louse Eggs

Louse Eggs

Like most insects, head lice are oviparous. Louse eggs contain a single embryo and are attached near the base of a host hair shaft. Egg-laying behavior is temperature dependent and likely seeks to place the egg in a location that will be conducive to proper embyro development (which is, in turn, temperature dependent). In cool climates, eggs are generally laid within 1 cm 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 each egg, the adult female secretes a glue from her reproductive organ. This glue quickly hardens into a "nit sheath" that covers the hair shaft and the entire 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 tan to coffee-colored so long as they contain an embryo but appear white after hatching. Typically, a hatching time of six to nine days after oviposition is cited by authors. However, these data are from work with body lice (not head lice) that show hatching time and hatching probability are extremely temperature dependent. As of 2008, head louse hatching time and probability in situ (i.e., on a human head) have not been carefully examined.

After hatching, the louse nymph leaves behind its egg shell, 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 months or years.