Planidium
Encyclopedia
A planidium is a specialized type of first-instar
insect
larva
, seen in groups that are parasitoids; they are generally flattened, highly sclerotize
d, 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.http://esa.confex.com/esa/2007/techprogram/paper_32157.htm 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
.
Instar
An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...
insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
larva
Larva
A larva 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...
, seen in groups that are parasitoids; they are generally flattened, highly sclerotize
Sclerosis (medicine)
In medicine, sclerosis refers to the stiffening of a structure, usually caused by a replacement of the normal organ-specific tissue with connective tissue.Types include:...
d, have legs, are quite mobile, and sometimes have eyes. They occur in the orders Hymenoptera
Hymenoptera
Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...
, Coleoptera, Strepsiptera
Strepsiptera
The Strepsiptera are an order of insects with ten families making up about 600 species...
, and Diptera
Diptera
Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...
; specifically the beetle
Beetle
Coleoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...
families Meloidae and Ripiphoridae
Ripiphoridae
The family Ripiphoridae is a cosmopolitan group of beetles commonly known as wedge-shaped beetles containing some 450 species...
, the fly
Fly
True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...
family Acroceridae
Acroceridae
Acroceridae is a small family of odd looking flies most closely related to Nemestrinidae. There are about 520 species in 50 genera. They are characterized by a humpbacked appearance and a small head, sometimes with a long proboscis for nectar. As such, acrocerids are commonly known as small-headed...
, and the parasitic wasp
Parasitic wasp
The term parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. They are primarily parasitoids of other animals, mostly other arthropods...
families Eucharitidae
Eucharitidae
Eucharitidae is a family of parasitic wasps known as Eucharitid wasps. Eucharitid wasps are members of the superfamily Chalcidoidea and consist of three subfamilies: Oraseminae, Eucharitinae, and Gollumiellinae. There are 53 genus and 473 species of Eucharitidae; most of which are members of the...
and Perilampidae
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 family paraphyletic...
. 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
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 poisonous chemical causing blistering of the skin and...
will form a group and produce a pheromone
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 outside the body of the secreting individual to impact the behavior of the receiving individual...
that mimics the sex attractant of its host bee
Apidae
The Apidae are a large family of bees, comprising the common honey bees, stingless bees , carpenter bees, orchid bees, cuckoo bees, bumblebees, and various other less well-known groups...
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.http://esa.confex.com/esa/2007/techprogram/paper_32157.htm 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
Hypermetamorphosis
Hypermetamorphosis is a kind of complete insect metamorphosis in which the different larval instars represent two or more different forms of larva. As the larva molts its morphology can change from that of a campodeiform to scarabaeiform or to vermiform...
.