The Dicondylia are a taxonomic group (
taxon
In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
) that includes all
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 pa ...
s except the jumping bristletails (
Archaeognatha
The Archaeognatha are an order of apterygotes, known by various common names such as jumping bristletails. Among extant insect taxa they are some of the most evolutionarily primitive; they appeared in the Middle Devonian period at about the sa ...
). Dicondylia have a
mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
attached with two hinges to the head capsule (dicondyl), in contrast to a hypothetical ancestral mandible with a single ball joint (monocondyl); the members of Archaeognatha do in fact have dicondylic mandibles, though they are not identical to the structure seen in "true" dicondylic insects.
[Blanke, A., Machida, R., Szucsich, N.U., Wilde, F. and Misof, B. (2015) Mandibles with two joints evolved much earlier in the history of insects: dicondyly is a synapomorphy of bristletails, silverfish and winged insects. ''Syst. Entomol.'' 40: 357-364. https://doi.org/10.1111/syen.12107]
Dicondyle mandible and other features
The taxon is distinguished by the possession of a modified mandible with an additional joint canal, which also changes the muscle attachments of the mouth tools and allows a modified mandible movement compared to other mandibles (crustaceans, centipedes). This so-called dicondyle mandible has two joints with which it is attached to the head capsule, while non-insect taxa have only one single ball joint.
[Merkmale nach Klausnitzer 1997.][Ward C. Wheeler, Michael Whiting, Quentin D. Wheeler, James M. Carpenter: ''The Phylogeny of the Extant Hexapod Orders.'' Cladistics 17, 2001; S. 113–169.]
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. The Archaeognatha were once thought to have only a single articulation, but it has since been shown that they do possess two articulations that are
homologous to those in other insects, though slightly different.
In addition to this feature, all members of the Dicondylia have a number of other group-specific features in their blueprint. They have a continuous occipital seam, and a further joint between the upper and lower limbs.
At the base of the oviposition tube (
ovipositor
The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typical ...
), there is an additional sclerite, the
gonangulum, which allows for the improved coordination of the movement of the
gonapophyses. In addition, all these insects ancestrally have a five-membered tarsus
and styli are present at their maximum at the two last abdominal segments. Another feature relates to embryonic development; all Dicondylia form a closed amniotic cavity around the embryos, producing two complete embryonic shells (the
amnion
The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the develo ...
and
serosa).
Systematics
The Dicondylia includes all of the winged and secondarily wingless insects (
Pterygota
The Pterygota ( grc, πτερυγωτός, pterugōtós, winged) are a subclass of insects that includes the winged insects. It also includes insect orders that are secondarily wingless (that is, insect groups whose ancestors once had wings ...
), along with the
Zygentoma
Zygentoma are an order in the class Insecta, and consist of about 550 known species. The Zygentoma include the so-called silverfish or fishmoths, and the firebrats. A conspicuous feature of the order are the three long caudal filaments. The tw ...
(silverfish, etc.) that were formerly classified with the jumping bristletails in the now deprecated order Thysanura.
Notes and references
Notes
References
* Bernhard Klausnitzer: ''Insecta (Hexapoda), Insekten'' In Westheide, Rieger (Hrsg.): ''Spezielle Zoologie Teil 1: Einzeller und Wirbellose Tiere.'' Gustav Fischer Verlag, Stuttgart, Jena 1997; S. 626f.
{{Taxonbar, from=Q1210274
Insect taxonomy