Planidium

thumb|Planidia and larva of a parasitoid wasp of the Perilampidae family. thumb|Triungulin, later larval, and other instars of a Blister beetle|Meloid beetle. thumb|Planidia of a Meloidae|Meloid beetle in opportunistic phoresy on a male solitary bee ([[Andrena carlini), awaiting contact with a female, whose nest they then could invade.]] 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. They are usually flattened, highly sclerotized (hardened), and quite mobile. The function of the planidial stage is to find a host on which the later larval instars may feed, generally until the insect pupates.

==Etymology== The term "planidium" is derived from the Greek language πλανής (planis) meaning "wanderer", the same origin of the word planet. The term planula was similarly derived in reference to the wandering larvae of certain Cnidaria. Accordingly, "planidium" is the general term for such an adaptation, and it is not limited to any particular species or morphology. Planidia of different species differ variously from each other in form.