A larva is a distinct juvenile form that many animals undergo before metamorphosis into their next life stage. Larvae are structurally different from adults and are adapted to a different environment. They are frequently adapted to different environments than adults, and some larvae, such as tadpoles, live almost exclusively in aquatic environments but can live outside water as adult frogs. Larvae are often mobile and can be used for dispersal, either planktotrophic (feeding) or lecithotrophic (non-feeding). Some larvae are dependent on adults to feed them, and in many eusocial Hymenoptera species, the larvae are fed by female workers. Larvae appear in a variety of forms, and many invertebrates have a simple ciliated larva called a planula, while flukes have several larval stages, and annelids, mollusks, and crustaceans have various larval forms.