The function of the planidial stage is to find a host on which the later larval instars may feed, generally until the insect pupates.
The term "planidium" is derived from the Greek language πλανής (planis) meaning "wanderer", the same origin of the word planet.
The first-instar larva in the beetle family Meloidae has three claws on each foot, and is therefore called a triungulin (plural triungula).
For instance, they may ride on the adult form of the host, or on an intermediate vector that might carry them to where their later instars might feed till they are ready for pupation.
Inclusion of the extra, functionally distinct form of larva into the life history is an example of hypermetamorphosis Media related to Planidium at Wikimedia Commons