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Hexapods
Fossil range: Early Devonian–Recent
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[1]
A flesh-fly
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Hexapoda
Latreille, 1825
Classes & Orders

Class Insecta (insects)
Class Entognatha

The subphylum Hexapoda (from the Greek for six legs) constitutes the largest (in terms of number of species) grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects). The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.

Contents

Hexapod morphology

Hexapods have bodies divided into an anterior head, thorax, and posterior abdomen. The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), followed by six segments, all closely fused together, with the following appendages:

Segment I. None
Segment II. Antennae (sensory), absent in Protura
Segment III. None
Segment IV. Mandibles (crushing jaws)
Segment V. Maxillae (chewing jaws)
Segment VI. Labium (lower lip)

The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). In true insects (class Insecta herein) the mouthparts are exposed or ectognathous, while in other groups they are enveloped or endognathous. Similar appendages are found on the heads of Myriapoda and Crustacea, although these have secondary antennae.

The thorax is composed of three segments, each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves.

The abdomen consists of eleven segments in all true insects (often reduced in number in many insect species), but in Protura it has twelve, and in Collembola only six (sometimes reduced to only four). The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.

Hexapod relationships

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata. New work, however, has called this into question, and it appears their closest relatives may be the crustaceans.[2] The non-insect hexapods have variously been considered a single evolutionarily line, typically treated as Class Entognatha (cladogram A), or several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than the Collembola or the Protura (cladogram B). There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola belong elsewhere.

Molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago - coinciding with the appearance of vascular plants on land.[1]

The following cladograms, showing three possible relationships among hexapoda are based on Tree of Life Project;[3] Diplura is shown with an unstable position:

Hexapoda
Entognatha
Ellipura
      

Collembola



Protura




Diplura



Ectognatha

Insecta



A

Hexapoda
Ellipura
      

Collembola



Protura





Diplura



Insecta




B

Hexapoda
Ellipura
      

Collembola



Protura




Diplura



Insecta



References

  1. ^ a b Gaunt, M.W.; Miles, M.A. (1 May 2002). "An Insect Molecular Clock Dates the Origin of the Insects and Accords with Palaeontological and Biogeographic Landmarks". Molecular Biology and Evolution 19 (5): 748–761. ISSN 1537-1719. PMID 11961108. http://www.mbe.oupjournals.org/cgi/content/abstract/19/5/748. Retrieved 14 July 2007.  
  2. ^ Giribet, G., Edgecombe, G.D. and Wheeler, W.C. (2001). "Arthropod phylogeny based on eight molecular loci and morphology". Nature 413: 157–161. doi:10.1038/35093097.  
  3. ^ Tree of Life, Hexapoda

External links

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1911 encyclopedia

Up to date as of January 14, 2010
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From LoveToKnow 1911

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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

Contents

Translingual

Etymology

Greek hexa six, New Latin poda foot

Proper noun

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Wikipedia

Hexapoda

  1. a taxonomic subphylum, within phylum Arthropoda — the insects, etc.

Related terms

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Wikispecies

Translations


Wikispecies

Up to date as of January 23, 2010

From Wikispecies

Taxonavigation

Main Page
Cladus: Eukaryota
Supergroup: Unikonta
Cladus: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Protostomia
Cladus: Ecdysozoa
Phylum: Arthropoda
Subphylum: Hexapoda
Classes: Entognatha - Insecta

Overview of extant ordines

Archaeognatha - Blattodea - Coleoptera - Collembola - Dermaptera - Diplura - Diptera - Embiidina - Ephemeroptera - Hemiptera - Hymenoptera - Isoptera - Lepidoptera - Mantodea - Mecoptera - Megaloptera - Neuroptera - Notoptera - Odonata - Orthoptera - Phasmatodea - Phthiraptera - Plecoptera - Protura - Psocoptera - Raphidioptera - Siphonaptera - Strepsiptera - Thysanoptera - Trichoptera - Zoraptera - Zygentoma

Name

Hexapoda

References

  • Gao, Y.; Bu, Y.; Luan, Y.-X. 2008: Phylogenetic relationships of basal hexapods reconstructed from nearly complete 18S and 28S rRNA gene sequences. Zoological science, 25: 1139–1145.
  • Grimaldi, D.; Engel, M.S. 2005: Evolution of the insects. Cambridge University Press, New York, USA. limited preview on Google books
  • Luan, Y., J.M. Mallatt, R. Xie, Y. Yang, and W. Yin. 2005. The phylogenetic positions of three basal-hexapod groups (Protura, Diplura, and Collembola) based on ribosomal RNA gene sequences. Molecular Biology and Evolution, 22(7): 1579-1592.
  • Bitsch, C. and J. Bitsch. 2000: The phylogenetic interrelationships of the higher taxa of apterygote hexapods. Zoologica Scripta, 29: 131-156.

Vernacular names

Български: Шесткраки
Català: Hexàpodes
Česky: Šestinozí
Deutsch: Sechsfüßer
Ελληνικά: Εξάποδα
English: hexapods
Español: hexápodos
Français: Hexapodes
한국어: 육각아문
Ido: Artikopodo
Italiano: Esapodi
Magyar: Hatlábúak
Македонски: Шестоноги
Nederlands: Zespotigen
日本語: 六脚亜門 (ろっきゃくあもん)
Polski: Sześcionogi
Português: Hexápodes
Русский: Шестиногие
Suomi: Kuusijalkaiset
Türkçe: Altı bacaklılar
中文: 六足亞門
Wikimedia Commons For more multimedia, look at Hexapoda on Wikimedia Commons.
Wikibooks Wikibooks has a Dichotomous Key related to this taxon Hexapoda.

Simple English

Hexapods
Fossil range: Devonian — Recent
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Hexapoda
Latreille, 1825
Classes & Orders

Class Insecta (insects)
Class Entognatha

The subphylum Hexapoda, meaning "six legs", is the largest (in terms of number of species) group of arthropods and includes the insects as well as three much smaller groups of arthropods without wings: Collembola, Protura, and Diplura. All of these were called insects before. They form a polyphyletic group because they evolved independently from each other. In other words, hexapoda is divided into the insects and the entognatha. Hexapods are so called because they have a thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.

Alternatively, the Hexapoda could be eliminated as polyphyletic (which it is), the Insecta promoted to sub-phylum, and the minor groups incertae sedis — a Latin term meaning "of uncertain placement". It is used for a taxonomic group when its broader relationships are unknown or undefined.

Look up Hexapoda in Wikispecies, a directory of species



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