How many millimeters long is a louse? - briefly
A typical head louse measures between 2 mm and 4 mm in length. Body lice are similar, averaging around 3 mm.
How many millimeters long is a louse? - in detail
A common head louse (Pediculus humanus capitis) measures between 2.2 mm and 3.5 mm when fully grown. Body lice (Pediculus humanus humanus) are slightly larger, ranging from 2.5 mm to 4.0 mm. Nymphs progress through three instars; their lengths increase from about 1.0 mm (first instar) to 2.0 mm (third instar).
Key factors influencing size:
- Species: Head versus body lice differ by up to 0.5 mm.
- Developmental stage: Each molt adds roughly 0.5 mm to body length.
- Sex: Adult females are typically 0.2–0.3 mm longer than males.
- Environmental conditions: Temperature and host nutrition can cause minor variations (±0.1 mm).
Measurement methods:
- Collect live specimens onto a microscope slide.
- Immobilize with a drop of ethanol or CO₂.
- Use a calibrated ocular micrometer or digital imaging software to record length from head to abdomen tip.
- Record measurements for a sample of at least 30 individuals to obtain a reliable mean and standard deviation.
Typical reported averages:
- Head louse: 2.8 mm ± 0.3 mm.
- Body louse: 3.3 mm ± 0.4 mm.
These values provide a precise reference for identification, epidemiological studies, and control program assessments.