Hexapoda

Hexapods
Temporal range: 411–0 Ma[1]
A flesh-fly, Sarcophaga sp.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Clade: Mandibulata
Clade: Pancrustacea
Subphylum: Hexapoda
Latreille, 1825[2]
Classes

The subphylum Hexapoda (from Greek for 'six legs') or hexapods comprises the largest clade of arthropods and includes most of the extant arthropod species. It includes the crown group class Insecta (true insects), as well as the much smaller clade Entognatha, which includes three classes of wingless arthropods that were once considered insects: Collembola (springtails), Protura (coneheads) and Diplura (two-pronged bristletails).[3][4] The insects and springtails are very abundant and are some of the most important pollinators, basal consumers, scavengers/detritivores and micropredators in terrestrial environments.

Hexapods are named for their most distinctive feature: a three-part body plan with a consolidated thorax and three pairs of legs. Most other arthropods have more than three pairs of legs.[5] Most recent studies have recovered Hexapoda as a subgroup of Crustacea.[6]

Morphology

See also: Arthropod head problem

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.[7][8] The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura),[9] 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).[10] 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 the crustaceans have secondary antennae.[11]

Collembolans and diplurans have segmented antenna; each segment has its own set of muscles. The antennea of insects consist of just three segments; the scape, the pedicel and the flagellum. Muscles occur only in the first two segments. The third segment, the flagellum, don't have any muscles and is composed of a various number of annuli. This type of antenna is therefore called annulated antenna. Johnston's organ, which is found on the pedicel, is absent in the Entognatha.[12][13]

The thorax is composed of three segments, each of which bears a single pair of legs.[14] 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.[15] In most insects the second and third thoracic segments also support wings.[16] 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.[17]

The abdomen follow epimorphic development, where all segments are already present at the end of embryonic development in all the hexapod groups except for Protura, which has an anamorphic development where the hatched juveniles has an incomplete complement of segments, and goes through a post-embryonic segment addition with each molting before the final adult number of segments is reached. All true insects have eleven segments (often reduced in number in many insect species), but in Protura there are twelve, and in Collembola only six (sometimes reduced to only four).[18][19] The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.[20][21][22]

Evolution and relationships

See also: Phylogeny of insects
Hexapoda phylogenetic tree

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity.[23] These were then considered subclasses of a subphylum called Uniramia or Atelocerata.[24] In the first decade of the 21st century, however, this was called into question, and it appears the hexapods' closest relatives may be the crustaceans.[25][26][27][28]

The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha,[29] or as 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 (springtails).[30]

A 2002 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.[31]

Since then remipedians have been revealed as closest living relative of hexapods. Several hypotheses about their internal relationships have been suggested over the years, with proturans as the sister group to the other hexapods and collembolans and diplurans belonging together in Antennomusculata as the latest suggestion:[32]

  • Entognatha (proturans, collembolans and diplurans) and Ectognatha (insects)
  • Elliplura (proturans and collembolans) and Cercophora (diplurans and insects)
  • Collembolans, Nonoculata (proturans and diplurans) and insects
  • Proturans, Antennomusculata (collembolans and diplurans) and insects

The following cladogram is given by Kjer et al. (2016):[33]

Hexapoda

Collembola (springtails)

Protura (coneheads)

Diplura (two-pronged bristletails)

Insecta

Archaeognatha (jumping bristletails)

Zygentoma (silverfish)

Pterygota (winged insects)

(Ectognatha)

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,[34][35] although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect.[36] In 2023, a hexapod-like arthropod fossil from the Ordovician marine fossil site Castle Bank was reported, although further study is needed.[37]

References

  1. ^ Wang, Yan-hui; Engel, Michael S.; Rafael, José A.; Wu, Hao-yang; Rédei, Dávid; Xie, Qiang; Wang, Gang; Liu, Xiao-guang; Bu, Wen-jun (2016). "Fossil record of stem groups employed in evaluating the chronogram of insects (Arthropoda: Hexapoda)". Scientific Reports. 6: 38939. Bibcode:2016NatSR...638939W. doi:10.1038/srep38939. PMC 5154178. PMID 27958352.
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  3. ^ "Hexapods". The Encyclopedia of Life.
  4. ^ "Subphylum Hexapoda - Hexapods - BugGuide.Net". bugguide.net.
  5. ^ "Hexapoda". tolweb.org.
  6. ^ Schwentner, Martin; Combosch, David J.; Pakes Nelson, Joey; Giribet, Gonzalo (June 2017). "A Phylogenomic Solution to the Origin of Insects by Resolving Crustacean-Hexapod Relationships". Current Biology. 27 (12): 1818–1824.e5. Bibcode:2017CBio...27E1818S. doi:10.1016/j.cub.2017.05.040. PMID 28602656. S2CID 38457877.
  7. ^ "Hexapoda facts, information, pictures | Encyclopedia.com articles about Hexapoda". www.encyclopedia.com.
  8. ^ "Hexapoda". biosurvey.ou.edu.
  9. ^ "Hexapoda". comenius.susqu.edu. Archived from the original on 27 February 2021. Retrieved 20 February 2017.
  10. ^ "Hexapoda (Insecta): General Characteristics | easybiologyclass". www.easybiologyclass.com. 21 October 2015.
  11. ^ Boundless (26 May 2016). "Subphyla of Arthropoda". Boundless. Archived from the original on 21 February 2017. Retrieved 20 February 2017.
  12. ^ Ramani, S.; Mohanraj, Prashanth; HM, Yeshwanth (16 October 2019). Indian Insects: Diversity and Science. CRC Press. ISBN 978-0-429-59201-0 – via Google Books.
  13. ^ Elgar, M. A.; Zhang, D.; Wang, Q.; Wittwer, B.; Thi Pham, H.; Johnson, T. L.; Freelance, C. B.; Coquilleau, M. (2018). "Insect Antennal Morphology: The Evolution of Diverse Solutions to Odorant Perception". The Yale Journal of Biology and Medicine. 91 (4): 457–469. PMC 6302626. PMID 30588211.
  14. ^ "Humble bug plugs gap in fossil record". phys.org.
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  16. ^ Walton, L. B. (1 January 1901). "The Metathoracic Pterygoda of the Hexapoda and Their Relation to the Wings". The American Naturalist. 35 (413): 357–362. doi:10.1086/277920. JSTOR 2453748.
  17. ^ "Checklist of the Collembola: Are Collembola terrestrial Crustacea?". www.collembola.org.
  18. ^ "GeoKansas--Fossil Isects". www.kgs.ku.edu. Archived from the original on 13 February 2017.
  19. ^ "HEXAPODA". comenius.susqu.edu. Archived from the original on 15 March 2017. Retrieved 20 February 2017.
  20. ^ Böhm, Alexander; Szucsich, Nikolaus U.; Pass, Günther (1 January 2012). "Brain anatomy in Diplura (Hexapoda)". Frontiers in Zoology. 9 (1): 26. doi:10.1186/1742-9994-9-26. ISSN 1742-9994. PMC 3585824. PMID 23050723.
  21. ^ "The Hexapods". projects.ncsu.edu. Archived from the original on 22 February 2017. Retrieved 20 February 2017.
  22. ^ "A Devonian hexapod". Pharyngula. 2 August 2012.
  23. ^ Dessi, Giancarlo. "Notes on Entomology: Flies. Morphology and anatomy of adults: Antennae - giand.it". www.giand.it.
  24. ^ "GEOL 331 Principles of Paleontology". www.geol.umd.edu.
  25. ^ Giribet, G.; Edgecombe, G.D. & Wheeler, W.C. (2001). "Arthropod phylogeny based on eight molecular loci and morphology". Nature. 413 (6852): 157–161. Bibcode:2001Natur.413..157G. doi:10.1038/35093097. PMID 11557979. S2CID 4431635.
  26. ^ Kazlev, M.Alan. "Palaeos Arthropods: Hexapoda". palaeos.com.
  27. ^ "How do insects breathe? An outline of the tracheal system". Teaching Biology. 26 November 2012. Archived from the original on 1 August 2020. Retrieved 20 February 2017.
  28. ^ Regier, J. C.; Shultz, J. W.; Kambic, R. E. (22 February 2005). "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 1634985. PMID 15734694.
  29. ^ "Hexapoda". comenius.susqu.edu. Archived from the original on 28 March 2023. Retrieved 20 February 2017.
  30. ^ Engel, Michael S.; Grimaldi, David A. (12 February 2004). "New light shed on the oldest insect". Nature. 427 (6975): 627–630. Bibcode:2004Natur.427..627E. doi:10.1038/nature02291. ISSN 0028-0836. PMID 14961119. S2CID 4431205.
  31. ^ Gaunt, Michael W.; Miles, Michael A. (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. PMID 11961108.
  32. ^ A new story of four Hexapoda classes: Protura as the sister to all other hexapods
  33. ^ Kjer, Karl M.; Simon, Chris; Yavorskaya, Margarita; Beutel, Rolf G. (2016). "Progress, pitfalls and parallel universes: a history of insect phylogenetics". Journal of the Royal Society Interface. 13 (121): 121. doi:10.1098/rsif.2016.0363. PMC 5014063. PMID 27558853.
  34. ^ Shear, William A. (2 August 2012). "Palaeontology: An insect to fill the gap". Nature. 488 (7409): 34–35. Bibcode:2012Natur.488...34S. doi:10.1038/488034a. ISSN 0028-0836. PMID 22859195. S2CID 6703535.
  35. ^ The Web page cites Garrouste, R.; Clément, G.; Nel, P.; Engel, M. S.; 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 (7409): 82–85. Bibcode:2012Natur.488...82G. doi:10.1038/nature11281. PMID 22859205. S2CID 205229663.
  36. ^ Hörnschemeyer, Thomas; Haug, Joachim T.; Bethoux, Olivier; Beutel, Rolf G.; Charbonnier, Sylvain; Hegna, Thomas A.; Koch, Markus; Rust, Jes; Wedmann, Sonja; Bradler, Sven; Willmann, Rainer (21 February 2013). "Is Strudiella a Devonian insect?". Nature. 494 (7437): E3–E4. Bibcode:2013Natur.494E...3H. doi:10.1038/nature11887. PMID 23426326. S2CID 205232661.
  37. ^ Botting, Joseph P.; Muir, Lucy A.; Pates, Stephen; McCobb, Lucy M. E.; Wallet, Elise; Willman, Sebastian; Zhang, Yuandong; Ma, Junye (May 2023). "A Middle Ordovician Burgess Shale-type fauna from Castle Bank, Wales (UK)". Nature Ecology & Evolution. 7 (5): 666–674. Bibcode:2023NatEE...7..666B. doi:10.1038/s41559-023-02038-4. PMID 37127766. S2CID 258438453.
Wikisource has the text of the 1911 Encyclopædia Britannica article "Hexapoda".

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