Triungulin

A planidium is a specialized form of first-instar insect larva, seen in a few families of insect species that have parasitoidal ways of life. Planidial instars generally are flattened, highly sclerotized, have functional legs or other means of locomotion, and are quite mobile. Some species have eyes, others not. 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 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. Note that an obsolescent variant form from the same root as "triungulin" is triungulus, plural triunguli and sometimes it still is encountered.

For practical purposes of uniformity, except where there is some special reason for the use of the term "triungulin", it is best to use only the term "planidium".

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.

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