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Class Insecta (insects) Class Entognatha

The subphylum Hexapoda
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
Diplura
(all of these were once considered insects).[3][4] The Collembola
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 (six legs). Most other arthropods have more than three pairs of legs.[5]

Contents

1 Morphology 2 Evolution and relationships 3 References 4 External links

Morphology[edit] See also: Arthropod
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.[6][7] The head is composed of a presegmental acron that usually bears eyes (absent in Protura
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
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 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 has eleven segments (often reduced in number in many insect species), but in Protura
Protura
it has twelve, and in Collembola
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
Uniramia
or Atelocerata.[21] In the first decade of the 21st century, however, this called 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,[26] or several lines with different relationships with the Class Insecta. In particular, the Diplura
Diplura
may be more closely related to the Insecta than to the Collembola
Collembola
(spring tails)[27] or the Protura. There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola
Collembola
belong elsewhere.[28][better source needed] Molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca
Anostraca
(fairy shrimps), at around the start of the Silurian
Silurian
period 440 million years ago - coinciding with the appearance of vascular plants on land.[29] The following cladogram is given by Kjer et al. (2016):[30]

Hexapoda

Collembola
Collembola
(springtails)

Protura
Protura
(coneheads)

Diplura
Diplura
(two-pronged bristletails)

Ectognatha

Archaeognatha
Archaeognatha
(jumping bristletails)

Zygentoma
Zygentoma
(silverfish)

Pterygota
Pterygota
(winged insects)

An incomplete possible insect fossil, Strudiella devonica, has been recovered from the Devonian
Devonian
period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago.[31][32] See also: Phylogeny of insects References[edit]

^ 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. doi:10.1038/srep38939. PMC 5154178 . PMID 27958352.  ^ "Hexapods - Hexapoda".  ^ "Hexapods - Hexapoda
Hexapoda
- Overview - Encyclopedia of Life". Encyclopedia of Life.  ^ "Subphylum Hexapoda
Hexapoda
- Hexapods - BugGuide.Net". bugguide.net.  ^ "Hexapoda". tolweb.org.  ^ " Hexapoda
Hexapoda
facts, information, pictures Encyclopedia.com articles about Hexapoda". www.encyclopedia.com.  ^ "Hexapoda". biosurvey.ou.edu.  ^ "Hexapoda". comenius.susqu.edu.  ^ " Hexapoda
Hexapoda
(Insecta): General Characteristics easybiologyclass". www.easybiologyclass.com.  ^ Boundless (2016-05-26). "Subphyla of Arthropoda". Boundless.  ^ "Humble bug plugs gap in fossil record".  ^ "Class Hexapoda
Hexapoda
(Insects) (hexa, six + podus, feet) Biology Boom". biologyboom.com.  ^ Walton, L. B. (1901-01-01). "The Metathoracic Pterygoda of the Hexapoda
Hexapoda
and Their Relation to the Wings". The American Naturalist. 35 (413): 357–362. doi:10.1086/277920. JSTOR 2453748.  ^ "Checklist of the Collembola: Are Collembola
Collembola
terrestrial Crustacea?". www.collembola.org.  ^ "GeoKansas--Fossil Isects". www.kgs.ku.edu.  ^ "HEXAPODA". comenius.susqu.edu.  ^ Böhm, Alexander; Szucsich, Nikolaus U.; Pass, Günther (2012-01-01). "Brain anatomy in Diplura
Diplura
(Hexapoda)". Frontiers in Zoology. 9: 26. doi:10.1186/1742-9994-9-26. ISSN 1742-9994. PMC 3585824 . PMID 23050723.  ^ "The Hexapods". projects.ncsu.edu.  ^ "A Devonian
Devonian
hexapod". Pharyngula. 2012-08-02.  ^ Dessi, Giancarlo. "Notes on Entomology: Flies. Morphology and anatomy of adults: Antennae - giand.it". www.giand.it.  ^ "GEOL 331 Principles of Paleontology". www.geol.umd.edu.  ^ 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. CS1 maint: Multiple names: authors list (link) ^ Kazlev, M.Alan. "Palaeos Arthropods: Hexapoda". palaeos.com.  ^ "How do insects breathe? An outline of the tracheal system Teaching Biology". Teaching Biology. 2012-11-26.  ^ 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 1634985 . PMID 15734694.  ^ "HEXAPODA". comenius.susqu.edu.  ^ 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.  ^ " Hexapoda
Hexapoda
Oxbridge Notes the United Kingdom". www.oxbridgenotes.co.uk.  ^ Gaunt, M.W.; Miles, M.A. (1 May 2002). "An Insect
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. Archived from the original on 20 March 2005.  ^ 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. doi:10.1098/rsif.2016.0363.  ^ 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.  ^ 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
Devonian
period". Nature. 488: 82–85. doi:10.1038/nature11281. 

External links[edit]

Arthropods portal

Data related to Hexapoda
Hexapoda
at Wikispecies Dichotomous key to the Hexapoda
Hexapoda
at Wikibooks "Hexapoda. Insects, springtails, diplurans, and proturans". Tree of Life Web Project. 

v t e

Extant Arthropoda classes by subphylum

Kingdom Animalia Subkingdom Eumetazoa (unranked) Bilateria (unranked) Protostomia Superphylum Ecdysozoa

Chelicerata

Pycnogonida (sea spiders)

Euchelicerata

Merostomata
Merostomata
(horseshoe crabs)¹ Arachnida (spiders, scorpions, ticks, mites)

Mandibulata

Myriapoda

Chilopoda (centipedes) Diplopoda (millipedes) Pauropoda
Pauropoda
(pauropods) Symphyla
Symphyla
(symphylans or garden centipedes)

Pancrustacea ( Crustacea
Crustacea
+ + Hexapoda)

Oligostraca

Ostracoda (seed shrimps) Mystacocarida²

Ichthyostraca²

Pentastomida
Pentastomida
(tongue worms) Branchiura
Branchiura
(fish lice)

Altocrustacea

Multicrustacea

Malacostraca
Malacostraca
(woodlice, shrimps, crayfish, lobsters, crabs)

Copepoda (copepods) Thecostraca
Thecostraca
(barnacles and relatives) + Tantulocarida

Allotriocarida

Cephalocarida (horseshoe shrimps) Branchiopoda
Branchiopoda
(fairy, tadpole, clam shrimps, water fleas)

Miracrustacea s.s./ Labiocarida

Remipedia

Hexapoda

Protura
Protura
(coneheads)³ Collembola
Collembola
(springtails)³ Diplura
Diplura
(two-pronged bristletails)³ Insecta (insects)

¹contains the only extant order Xiphosura ²Maxillopoda ³Entognatha italic are paraphyletic groups Sources: Edgecombe et al. (2014), Petrunina (2012) for pancrustaceans.

Taxon identifiers

Wd: Q105146 ADW: Hexapoda BugGuide: 878075 EoL: 2634370 EPPO: 1HEXAQ Fauna Europaea: 3 Fossilworks: 133899 ITIS: 563886 NCBI

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