A planidium is a specialized form of insect larva seen in the first-instar of a few families of insects that have parasitoidal 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.
The term “planidium” is derived from the Greek language πλανής (planis) meaning “wanderer”, the same origin of the word planet. [1] 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.” An obsolescent variant triungulus (plural triunguli) may still be encountered. [2]
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.”
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. [3] Among the Hymenoptera examples include the parasitic wasp families Eucharitidae and Perilampidae. All Strepsiptera have planidial larvae.
The term “triungulin,” originally coined in reference 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; without implying any conceptual difference from other planidia.
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 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 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. [4]
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 changes the planidial form into an extra larval form that is remarkably 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 usually include de-sclerotization, 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
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 emerge from the host after entering its body, finally dying inside it. The early-stage larvae do emerge because they must find an unoccupied living host, and the short-lived males must emerge to seek a receptive female in her host. They are believed to be most closely related to beetles, from which they diverged 300–350 million years ago, but do not appear in the fossil record until the mid-Cretaceous around 100 million years ago.
Hypermetamorphosis, or heteromorphosis, is a term used mainly in entomology; it refers to a class of variants of holometabolism, that is to say, complete insect metamorphosis. Hypermetamorphosisis is exceptional in that some instars, usually larval instars, are functionally and visibly distinct from the rest. The differences between such instars usually reflect transient stages in the life cycle; for instance, one instar might be mobile while it searches for its food supply, while the following instar immediately sheds its locomotory organs and settles down to feed until it is fully grown and ready to change into the reproductive stage, which in turn, does not have the same nutritional requirements as the larvae.
A pupa is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages thereof being egg, larva, pupa, and imago. The processes of entering and completing the pupal stage are controlled by the insect's hormones, especially juvenile hormone, prothoracicotropic hormone, and ecdysone. The act of becoming a pupa is called pupation, and the act of emerging from the pupal case is called eclosion or emergence.
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 toxicity to would-be predators.
The Acroceridae are a small 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 frequently applied common names are small-headed flies or hunch-back flies. Many are bee or wasp mimics. Because they are parasitoids of spiders, they also are sometimes known as spider flies.
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 tropics and subtropics. Only five species of Mantispa occur in Europe. As their names suggest, members of the group possess raptorial forelimbs similar to those of the praying mantis, a case of convergent evolution.
Nomadinae is a subfamily of bees in the family Apidae. They are known commonly as cuckoo bees.
Meloe proscarabaeus is a European oil beetle. It lives in meadows, field margins and other warm sites in all but the far north of the continent. It lacks hind wings and the elytra are correspondingly reduced in size.
Xenos vesparum is a parasitic insect species of the order Strepsiptera that are endoparasites of paper wasps in the genus Polistes that was first described in 1793. Like other members of this family, X. vesparum displays a peculiar lifestyle, and demonstrates extensive sexual dimorphism.
Meloe is a genus of blister beetles commonly referred to as oil beetles. The name derives from their defensive strategy: when threatened by collectors or predators they release oily droplets of hemolymph from their joints. This fluid is bright orange and contains cantharidin, a poisonous chemical compound. Wiping the chemical on skin can cause blistering and painful swelling of the skin. This defensive strategy is not exclusive to this genus; all meloids possess and exude cantharidin upon threat.
Anthrenus (Anthrenus) scrophulariae, also known as the common carpet beetle or buffalo carpet beetle, is a species of beetle originally found in Europe, the Middle East and the Nearctic, which has now spread to most of the world. Adult beetles feed on pollen and nectar, but the larvae feed on animal fibres and can be damaging pests to carpets, fabrics and museum specimens.
Phoresis or phoresy is a non-permanent, commensalistic interaction in which one organism attaches itself to another solely for the purpose of travel. Phoresis has been observed directly in ticks and mites since the 18th century, and indirectly in fossils 320 million years old. It is not restricted to arthropods or animals; plants with seeds that disperse by attaching themselves to animals are also considered to be phoretic.
Lebia grandis is a ground beetle in the family Carabidae found in North America. It is a specialist predator on the eggs and larvae of Colorado potato beetles, and its larvae are obligate parasitoids of Colorado potato beetle pupae.
Meloe violaceus, the violet oil beetle, is a species of oil beetle belonging to the family Meloidae subfamily Meloinae.
Berberomeloe majalis, the red-striped oil beetle, is an insect in the genus Berberomeloe, in the family of Blister Beetles. It is native to the western Mediterranean Basin.
Meloe variegatus is a European oil beetle. It is commonly known as the variegated oil beetle. Adult beetles feed on leaves and can be pests of crops, while the larvae are parasitic on solitary bees.
Ceroctis capensis, or spotted blister beetle, is diurnal and endemic to Southern Africa occurring in diverse habitats, and belonging to the Meloidae or Blister beetle family. It secretes a toxic liquid from its leg joints when roughly handled, blistering human skin. This species somewhat resembles Mylabris oculata, a member of the same family.
Stenoria analis, the ivy bee blister beetle, is a species of blister beetle from the family Meloidae which is found in western Europe and North Africa and is a specialist cleptoparasite of the ivy bee larvae. Its occurrence in regions outside of the known range of the ivy bee, for example in North Africa, suggest that it has other hosts.
Meloe franciscanus is a species of blister beetle in the family Meloidae. It is found in the deserts of the southwestern United States. The larvae are parasites of bee larvae, eating them and consuming their provisions.
Apalus bimaculatus, the early blister beetle, is a species of blister beetle from the family Meloidae. It is the type species of the genus Apalus.
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