Any insect of the moth family Psychidae, found worldwide, named for the baglike cases the larvae (see larva ) carry with them.
The bag, which ranges in size from 0.25 to 6 in. (6–150 mm), is constructed from silk and bits of leaves, twigs, and other debris. The strong-bodied male has broad, fringed wings with a wingspread averaging 1 in. (25 mm). The wormlike female lacks wings. Bagworm larvae often damage trees, especially evergreens.