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 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. The first-instar larva in the beetle family Meloidae has three claws on each foot, and is therefore called a triungulin (plural triungula). The term is derived from the Latin tri meaning “three,” and ungula meaning “a claw.” An obsolescent variant triungulus (plural triunguli) may still be encountered. For practical purposes of uniform terminology, except where there is some special reason for the use of the more narrowly specific term “triungulin,” it is best to use only the more general term “planidium.” Function and occurrence Planidia occur among subsets of the members of several orders, including: Neuroptera, Hymenoptera, Coleoptera, Strepsiptera, and Diptera. Examples include: the neuropteran family Mantispidae; the beetle families Meloidae and Ripiphoridae; and the fly families Acroceridae, Bombyliidae, Nemestrinidae, and Tachinidae. Among the Hymenoptera examples include the parasitic wasp families Eucharitidae and Perilampidae. All Strepsiptera have planidial larvae. The term “triungulin,” originally coined in reference to the planidia of the beetle family Meloidae, is commonly applied to similar-looking planidial larvae of other families of beetles or of Strepsiptera. It is purely descriptive, and of no theoretical importance as implying any conceptual difference from other planidia. Planidial behaviour Depending on their species, planidial larvae either wait for a passing host, or actively seek one out. In many species, the planidia depend on phoresy to gain access to the actual host life stage. 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. Typically, such a planidium then enters the body of the host larva, but some of the species attack host eggs; for example, some Meloidae