When do lice lose their hair? - briefly
Lice shed their outer covering during molting, which occurs several times in their development, typically every 4–5 days. The last molt, when a nymph becomes an adult, results in a permanent exoskeleton that the insect retains.
When do lice lose their hair? - in detail
Head lice (Pediculus humanus capitis) and body lice (Pediculus humanus corporis) possess a chitinous exoskeleton that must be replaced as the insect grows. The replacement process, called ecdysis, occurs at the end of each immature stage. Consequently, lice lose their outer cuticular hairs during these molting events.
The life cycle consists of three developmental phases: egg (nit), nymph, and adult. Nymphs pass through three instars, each ending with a molt. The sequence is:
- First instar – hatches from the egg and retains the embryonic cuticle for about 1–2 days before shedding it.
- Second instar – follows the first molt; the cuticle is replaced after an additional 2–3 days.
- Third instar – after another 2–3 days, the third molt produces the adult form.
- Adult – does not molt further; the cuticle remains until death.
Overall, a louse discards its hair and old cuticle roughly every 2–3 days, completing three molts within 7–10 days after hatching. The exact timing varies with species, ambient temperature (optimal range 30–32 °C), and host conditions such as humidity.
Higher temperatures accelerate metabolic activity, shortening the interval between ecdyses, whereas cooler environments prolong development. Nutritional status of the host does not directly affect molting but can influence lice survival after each molt.
Understanding the molting schedule is critical for treatment planning. Insecticidal agents that target the nervous system are most effective when applied before the first molt, while products that disrupt chitin synthesis are most lethal during the molting windows. Re‑treatment after 7 days aligns with the expected emergence of new adults that have just completed their final molt.