What is a defining characteristic of lice?

A defining characteristic of lice is that they feed on human blood. Lice are ectoparasites, meaning they live on the exterior of the host and rely on their blood for nutrition. This feeding behavior is what makes them a concern for human health, particularly in terms of itching and potential transmission of infections. Lice infestations are commonly associated with close personal contact, which facilitates their spread from one individual to another.

Other options do not accurately describe lice. For example, lice are wingless insects, which makes the first option incorrect. The second option is irrelevant, as lice do not feed on decomposing plants but specifically target blood from living hosts. While pesticides can be used to treat lice infestations, not all lice species respond equally well to pesticide treatment, so this characteristic does not adequately define lice as a group. Therefore, the fact that lice feed on human blood is the most defining trait.