A larva (plural larvae /ˈlɑrviː/) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The larva's appearance is generally very different from the adult form (e.g. caterpillars and butterflies). A larva often has unique structures and organs that do not occur in the adult form, and may have a considerably different diet.
Larvae are frequently adapted to environments separate from adults. For example, some larvae such as tadpoles live exclusively in aquatic environments, but can live outside water as adult frogs. By living in a distinct environment, larvae may be given shelter from predators and reduce competition for resources with the adult population.
Animals in the larval stage will consume food to fuel their transition into the adult form. Some species such as barnacles are immobile as adults, and use their mobile larval form to distribute themselves.
The larvae of some species (for example, some newts) can become pubescent and not further develop into the adult form. This is a type of neoteny.
It is a misunderstanding that the larval form always reflects the group's evolutionary history. This could be the case, but often the larval stage has evolved secondarily, as in insects. In these cases the larval form may differ more than the adult form from the group's common origin.
Read more about Larva: Selected Types of Larvae