A planidium is a specialized form of insect larva seen in the first-instar of a few families of insects that have parasitoidal ways of life. Wikipedia

The first-instar larva of any species of parasitoidal insect in which the life cycle is an example of hypermetamorphosis; so called because the first instar wanders in search of the host, whereas the rest of the instars remain stationary while they feed until they pupate. Wiktionary