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A planidium is a specialized form of
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
seen in the first-
instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ass ...
of a few families of insects that have
parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
al 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 Greek ( el, label=Modern Greek, Ελληνικά, Elliniká, ; grc, Ἑλληνική, Hellēnikḗ) is an independent branch of the Indo-European family of languages, native to Greece, Cyprus, southern Italy (Calabria and Salento), southern Al ...
''πλανής'' (planis) meaning "wanderer". The term
planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores. Some groups of Nemerteans also produce larvae that are very similar to the planula, which ...
was similarly derived in reference to the wandering larvae of certain
Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...
. Accordingly, "planidium" is the general term for such an adaptation, and it is not limited to any particular species or
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
. 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 The insect order Neuroptera, or net-winged insects, includes the lacewings, mantidflies, antlions, and their relatives. The order consists of some 6,000 species. Neuroptera can be grouped together with the Megaloptera and Raphidioptera in th ...
,
Hymenoptera Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...
,
Coleoptera Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
,
Strepsiptera The Strepsiptera are an order of insects with eleven extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never ...
, and
Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
; examples include the neuropteran family
Mantispidae Mantispidae, known commonly as mantidflies, mantispids, mantid lacewings, mantisflies or mantis-flies, is a family of small to moderate-sized insects in the order Neuroptera. There are many genera with around 400 species worldwide, especially in ...
, the
beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
families
Meloidae Blister beetles are beetles of the family Meloidae, so called for their defensive secretion of a blistering agent, cantharidin. About 7,500 species are known worldwide. Many are conspicuous and some are aposematically colored, announcing their ...
and
Ripiphoridae Ripiphoridae (formerly spelled Rhipiphoridae) is a cosmopolitan family of some 450 described species of beetles sometimes called "wedge-shaped beetles". Ripiphoridae are unusual among beetle families in that many species are hypermetamorphic pa ...
, and the
fly Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwing ...
families
Acroceridae The Acroceridae are a small family (biology), family of odd-looking flies. They have a hump-backed appearance with a strikingly small head, generally with a long proboscis for accessing nectar. They are rare and not widely known. The most frequen ...
,
Bombyliidae The Bombyliidae are a family of flies, commonly known as bee flies. Adults generally feed on nectar and pollen, some being important pollinators. Larvae are mostly parasitoids of other insects. Overview The Bombyliidae are a large family of fl ...
,
Nemestrinidae Nemestrinidae, or tangle-veined flies is a family of flies in the superfamily Nemestrinoidea, closely related to Acroceridae. The family is small but distributed worldwide, with about 300 species in 34 genera. Larvae are endoparasitoids of eithe ...
, and
Tachinidae The Tachinidae are a large and variable family of true flies within the insect order Diptera, with more than 8,200 known species and many more to be discovered. Over 1,300 species have been described in North America alone. Insects in this family ...
Among the Hymenoptera examples include the
parasitic wasp Parasitoid wasps are a large group of hymenopteran superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, sooner or later causin ...
families
Eucharitidae The Eucharitidae are a family of parasitic wasps.Ayre, G.L. Pseudometagea schwarzii (Ashm.) (Eucharitidae: Hymenoptera), a parasite of Lasius neoniger Emery (Formicidae: Hymenoptera). Canadian Journal of Zoology 40 (1962) : 157-164. Eucharitid ...
and
Perilampidae The Perilampidae are a small family within the Chalcidoidea, composed mostly of hyperparasitoids. The family is closely related to the Eucharitidae, and the eucharitids appear to have evolved from within the Perilampidae, thus rendering the fam ...
. All
Strepsiptera The Strepsiptera are an order of insects with eleven extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never ...
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 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 Phoresis or phoresy is a non-permanent, Commensalism, commensalistic interaction in which one organism (a phoront or phoretic) attaches itself to another (the host) solely for the purpose of travel. Phoresis has been observed directly in Acari ...
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 feed on the subterranean egg pods of grasshoppers and locusts, and Mantispidae feed on egg purses of spiders. A striking example of phoresy is that planidia of beetles of the genus ''
Meloe The blister beetle genus ''Meloe'' is a large, widespread group commonly referred to as oil beetles. They are known as "oil beetles" because they release oily droplets of hemolymph from their joints when disturbed; this contains cantharidin, a ...
'' will form a group and produce a
pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
that mimics the sex attractant of its host
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyly, monophyletic lineage within the ...
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 some species postpone further development while the host larva grows. Whether after a delay or not, the first
ecdysis Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remna ...
changes the planidial form into an extra larval form that is practically unrecognisably different from the planidium; this reflects the lapsed need for the larva to wander any further, together with an increased need for efficiency in feeding. The changes in
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
usually include de-
sclerotization Sclerotization is a biochemical process that produces the rigid shell of sclerotin that comprises an insect's chitinous exoskeleton. It is prominent in the thicker, armored parts of insects and arachnids, especially in the biting mouthparts and scle ...
, and loss of the legs and eyes of the larvae. Inclusion of the extra, functionally distinct, form of larva into the life history, is an example of
hypermetamorphosis Hypermetamorphosis, or heteromorphosis,P.J. Gullan & P.S. Cranston. 2010. The Insects: An Outline of Entomology, 4th Edition. Wiley-Blackwell. is a term used in entomology that refers to a class of variants of holometabolism, that is to say, com ...


References


External link

{{Commonscatinline Parasitic insects Insect physiology Larvae