A planidium is a specialized form of first-instar insect larva, seen in groups that are parasitoids; they are generally flattened, highly sclerotized, have legs, and are quite mobile. Some species have eyes, others not. The function of the planidial stage is to find a host on which the larva may feed until it attains adulthood.
The name "planidium" is derived from the Greek language πλάνος (planos) meaning "roaming", and also, for similar reasons, the root of planula. Accordingly it is the general term for such an adaptation, and not limited to any particular species or morphology. Planidia in different species may differ greatly 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. For practical purposes of uniformity, unless there is some special reason for the use of the term, it is best to use only the term "planidium". Note that an obsolescent variant form from the same root as "triungulin" is triungulus, plural trunguli and sometimes it still is encountered.
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 referring to the planidia of the beetle family Meloidae, is commonly applied to similar-looking planidial larvae of other families of beetles or Strepsiptera. It is purely descriptive and of no theoretical importance as differing conceptually from other planidia.
Planidial larvae either wait for a passing host or actively seek one out. In many cases they are phoretic, and ride on the adult form of the host or on an intermediate vector in order to gain access to the actual host life stage. Typically they enter the body of the host larva, but some of them attack host eggs, such as Meloidae that feed on the subterranean egg pods of grasshoppers and locusts, and Mantispidae that feed on egg purses of spiders.
An striking example of phoresy is that planidia of beetles of the genus Meloe will form a group and produce a pheromone that mimics the sex attractant of its host bee species; when the male bee arrives and attempts to mate with the mass of larvae, they climb onto his abdomen, and from there they transfer in turn to a female bee, and finally to the bee nest, where they attack the bee larvae as their hosts.
It is common for planidia to molt shortly after entering the host body or nest, but they often postpone further development while the larva grows. Whether after a delay or not, ecdysis changes their form into an extra larval form, one practically unrecognisably different from the planidium, because the wandering function no longer exists, whereas efficiency in feeding is necessary. The changes usually include loss of the legs and eyes of the larvae, and de-sclerotization. Inclusion of the extra form of larva into the life history is called hypermetamorphosis
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