What is the size of a louse in millimeters? - briefly
«A typical head louse measures approximately 2–3 mm in length». «Body lice are slightly larger, reaching up to about 4 mm».
What is the size of a louse in millimeters? - in detail
The adult head louse (Pediculus humanus capitis) measures approximately 2.2–3.5 mm in length and 0.8–1.0 mm in width. The body louse (Pediculus humanus humanus) is slightly larger, ranging from 2.5–4.0 mm long and 1.0–1.2 mm wide. The pubic louse (Pthirus pubis) is the smallest, typically 1.2–2.0 mm long and 0.3–0.4 mm wide.
Nymphs, which undergo three molts before reaching adulthood, display proportional size reductions:
- First‑instar nymph: 0.8–1.2 mm long.
- Second‑instar nymph: 1.2–1.8 mm long.
- Third‑instar nymph: 1.8–2.5 mm long.
Measurements are obtained with stereomicroscopes equipped with calibrated ocular micrometers or digital imaging software that provides pixel‑to‑millimeter conversion. Variability arises from factors such as host species, environmental temperature, and nutritional status of the parasite.
Average mass of an adult head louse is about 0.02 mg, correlating with the linear dimensions described. The slender, dorsoventrally flattened body contributes to the small cross‑sectional area, facilitating movement through hair shafts.
In summary, lice are minute ectoparasites whose lengths span roughly 1.2 mm to 4.0 mm, depending on species and developmental stage, with corresponding widths of 0.3 mm to 1.2 mm.