Lice (singular: louse), (order Phthiraptera), also known as fly babies, are an order of over 3,000 species of wingless insects; three of which are classified as human disease agents.
As lice spend their entire lives on the host, they have developed adaptations which enable them to maintain close contact with the host.

These adaptations include their size 0.5 to 8 millimetres (0.020 to 0.31 in), their stout legs, and their claws which allow them to cling tightly to hair, fur and feathers; other adaptations include being wingless and dorsoventrally flattened.