Hexapoda

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Hexapods
Temporal range: Early Devonian–Recent[1]
Diptera 01gg.jpg
A flesh-fly
Scientific classification e
Kingdom: Animalia
Phylum: Arthropoda
Clade: Pancrustacea
Subphylum: Hexapoda
Latreille, 1825[2]
Classes & Orders

Class Insecta (insects)
Class Entognatha

The subphylum Hexapoda (from the Greek for six legs) constitutes the largest number of species 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).[3][4] 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.[5]

Morphology[edit]

Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax, and posterior abdomen.[6][7] The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura),[8] 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).[9] In true insects (class Insecta) 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.[10]

The thorax is composed of three segments, each of which bears a single pair of legs.[11] 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 and with gill on the abdominal segments of some immature aquatic insects.[12] In most insects the second and third thoracic segments also support wings.[13] 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.[14]

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).[15][16] The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.[17][18][19]

Evolution and relationships[edit]

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity.[20] These were then considered subclasses of a subphylum called Uniramia or Atelocerata.[21] New work, however, has called this into question, and it appears the hexapoda's closest relatives may be the crustaceans.[22][23][24][25] The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha (cladogram A),[26] or several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (spring tails)[27] 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.[28]\[better source needed]

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 the Tree of Life Web Project;[29][better source needed] 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



C

An incomplete possible insect fossil, Strudiella devonica, has been recovered from the Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago.[30][31]

References[edit]

  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. doi:10.1093/oxfordjournals.molbev.a004133. ISSN 1537-1719. PMID 11961108. 
  2. ^ "Hexapods - Hexapoda". 
  3. ^ "Hexapods - Hexapoda - Overview - Encyclopedia of Life". Encyclopedia of Life. 
  4. ^ "Subphylum Hexapoda - Hexapods - BugGuide.Net". bugguide.net. 
  5. ^ "Hexapoda". tolweb.org. 
  6. ^ "Hexapoda facts, information, pictures | Encyclopedia.com articles about Hexapoda". www.encyclopedia.com. 
  7. ^ "Hexapoda". biosurvey.ou.edu. 
  8. ^ "Hexapoda". comenius.susqu.edu. 
  9. ^ "Hexapoda (Insecta): General Characteristics | easybiologyclass". www.easybiologyclass.com. 
  10. ^ Boundless (2016-05-26). "Subphyla of Arthropoda". Boundless. 
  11. ^ "Humble bug plugs gap in fossil record". 
  12. ^ "Class Hexapoda (Insects) (hexa, six + podus, feet) | Biology Boom". biologyboom.com. 
  13. ^ Walton, L. B. (1901-01-01). "The Metathoracic Pterygoda of the Hexapoda and Their Relation to the Wings". The American Naturalist. 35 (413): 357–362. JSTOR 2453748. 
  14. ^ "Checklist of the Collembola: Are Collembola terrestrial Crustacea?". www.collembola.org. 
  15. ^ "GeoKansas--Fossil Isects". www.kgs.ku.edu. 
  16. ^ "HEXAPODA". comenius.susqu.edu. 
  17. ^ Böhm, Alexander; Szucsich, Nikolaus U.; Pass, Günther (2012-01-01). "Brain anatomy in Diplura (Hexapoda)". Frontiers in Zoology. 9: 26. doi:10.1186/1742-9994-9-26. ISSN 1742-9994. PMC 3585824Freely accessible. PMID 23050723. 
  18. ^ "The Hexapods". projects.ncsu.edu. 
  19. ^ "A Devonian hexapod". Pharyngula. 2012-08-02. 
  20. ^ Dessi, Giancarlo. "Notes on Entomology: Flies. Morphology and anatomy of adults: Antennae - giand.it". www.giand.it. 
  21. ^ "GEOL 331 Principles of Paleontology". www.geol.umd.edu. 
  22. ^ Giribet, G., Edgecombe, G.D. and Wheeler, W.C. (2001). "Arthropod phylogeny based on eight molecular loci and morphology". Nature. 413 (6852): 157–161. doi:10.1038/35093097. PMID 11557979. 
  23. ^ Kazlev, M.Alan. "Palaeos Arthropods: Hexapoda". palaeos.com. 
  24. ^ "How do insects breathe? An outline of the tracheal system | Teaching Biology". Teaching Biology. 2012-11-26. 
  25. ^ Regier, J. C.; Shultz, J. W.; Kambic, R. E. (2005-02-22). "Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic". Proceedings of the Royal Society B: Biological Sciences. 272 (1561): 395–401. doi:10.1098/rspb.2004.2917. PMC 1634985Freely accessible. PMID 15734694. 
  26. ^ "HEXAPODA". comenius.susqu.edu. 
  27. ^ Engel, Michael S.; Grimaldi, David A. (2004-02-12). "New light shed on the oldest insect". Nature. 427 (6975): 627–630. doi:10.1038/nature02291. ISSN 0028-0836. 
  28. ^ "Hexapoda | Oxbridge Notes the United Kingdom". www.oxbridgenotes.co.uk. 
  29. ^ Tree of Life, Hexapoda.
  30. ^ Shear, William A. (2012-08-02). "Palaeontology: An insect to fill the gap". Nature. 488 (7409): 34–35. doi:10.1038/488034a. ISSN 0028-0836. 
  31. ^ The Web page cites Garrouste R, Clément G, Nel P, Engel MS, Grandcolas P, D’Haese C, Lagebro L, Denayer J, Gueriau P, Lafaite P, Olive S, Prestianni C, Nel A (2012) A complete insect from the Late Devonian period. Nature 488, 82–85.

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