Why do lice crawl on the head? - briefly
Lice inhabit human scalps because the warm, moist environment and hair shafts supply shelter and a continuous blood source for feeding. Their specialized claws grip individual hairs, allowing them to move and stay attached.
Why do lice crawl on the head? - in detail
Lice are obligate ectoparasites that depend on mammalian blood for nutrition. Their anatomy includes mandibles designed for piercing skin, and a small, lightweight body that can move easily through hair shafts. These traits make the human scalp an optimal feeding site.
The scalp offers a stable temperature around 33‑35 °C, high humidity, and a continuous supply of blood vessels close to the skin surface. Hair provides a three‑dimensional network that shields the insects from external disturbances while allowing them to navigate between feeding points. This combination of warmth, moisture, and protection creates an environment where lice can survive and reproduce efficiently.
Lice locate their host by detecting several cues:
- Elevated carbon‑dioxide concentration exhaled through the nose and mouth.
- Heat gradients generated by the head’s metabolic activity.
- Volatile compounds emitted by skin oils and sweat.
- Mechanical vibrations caused by head movements.
These sensory inputs trigger directed crawling toward the scalp, where the insects can attach and feed.
During their life cycle, nymphs and adult lice must move periodically to locate fresh feeding sites and to spread eggs (nits) along hair strands. Mobility also facilitates dispersion between individuals during close contact, ensuring population maintenance and expansion.
In summary, the scalp’s thermal stability, moisture level, and protective hair structure, combined with the insects’ sensory mechanisms and reproductive requirements, explain why these parasites actively traverse the head.