A planidium is a specialized type 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 the orders Hymenoptera, Coleoptera, Strepsiptera, and Diptera; specifically the beetle families Meloidae and Ripiphoridae, the fly family Acroceridae, and 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 to refer to other planidial beetle or Strepsipteran larvae.

These larvae either wait for a passing host, or actively seek one out. In most cases they are phoretic, and ride on the adult form of the host or an intermediate vector, in order to gain access to the actual life stage attacked (typically, they enter the body of the host larva). In an unusual case, planidium larvae 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 transfer to a female bee, and from there 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 and subsequent molts usually involve a loss of legs and eyes, as well as de-sclerotization, a process called hypermetamorphosis.

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