Which is larger, a flea or a louse? - briefly
A flea exceeds a louse in size. Fleas range from roughly 1.5 mm to 4 mm in length, whereas lice are generally 2 mm to 3 mm long.
Which is larger, a flea or a louse? - in detail
Fleas and lice are both ectoparasitic insects, but their body dimensions differ markedly. Adult fleas typically range from 1.5 mm to 4 mm in length, depending on species. The common cat flea (Ctenocephalides felis) averages about 2.5 mm, while the human flea (Pulex irritans) can reach up to 4 mm. In contrast, adult lice are considerably smaller. Head lice (Pediculus humanus capitis) measure 2 mm to 3 mm, and body lice (Pediculus humanus corporis) are roughly 2 mm to 3.5 mm. The smallest chewing lice, such as those in the genus Menacanthus, may be as short as 1 mm.
Key size attributes:
- Length: Fleas 1.5–4 mm; lice 1–3.5 mm.
- Width: Fleas 0.5–1 mm; lice 0.3–0.5 mm.
- Weight: Fleas approximately 0.5–2 mg; lice around 0.2–0.6 mg.
Morphologically, fleas possess a laterally compressed body adapted for jumping, which contributes to a bulkier appearance. Lice have a flatter, elongated form suited for clinging to hair shafts or feathers. Both groups lack wings, but fleas have enlarged hind legs that store elastic energy, while lice rely on claws for anchorage.
Habitat influences size variation. Fleas that infest mammals often develop larger specimens to accommodate blood meals from larger hosts. Lice, restricted to a single host species, maintain a size that balances mobility on hair or feathers with efficient feeding.
Overall, fleas exceed lice in length, width, and mass across the majority of species, making them the larger of the two ectoparasites.