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, are quite mobile, and sometimes have eyes. They occur in 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 first instar larva in Meloidae has three claws on each foot, and is therefore called a triungulin (plural triungula), but otherwise they are typical planidia. The term "triungulin" is sometimes used loosely in referring to other planidial larvae of beetles or Strepsiptera.
These 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).
An unusual example 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 to parasitize the bee larvae. It is common for planidia to molt shortly after entering the host body, but they often postpone further development while the larva grows. This process of molting into an extra larval form is called hypermetamorphosis and usually involves a loss of the legs and eyes of the larvae, as well as de-sclerotization.
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