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How do insect lice differ from fish lice?

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human head louse — louse Human head louse (*Pediculus humanus*).

Insect lice and fish lice are fundamentally different organisms, primarily distinguished by their biological classification and habitat.

Insect Lice
True lice belong to the order Phthiraptera. They are wingless, dorsoventrally flattened insects that live as permanent ectoparasites exclusively on birds and mammals. Their mouthparts are adapted for either chewing or sucking blood, and they possess specialized legs with claws for clinging to the hair or feathers of their hosts.123

Fish Lice
In contrast, fish lice are not insects but crustaceans. The term generally refers to members of the subclass Branchiura, though it can also include various copepods and isopods. These aquatic invertebrates parasitize marine and freshwater fish, often attaching to the skin, gills, or mouth cavity using powerful suckers or thoracic claws. Unlike insect lice, which are terrestrial-associated, fish lice are adapted for life in water and are often capable of swimming.456