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). 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 (six legs). Most other arthropods have more than three pairs of legs.
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. 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) 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 and with gill on the abdominal segments of some immature aquatic insects. 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 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). The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.
Evolution and 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. In the first decade of the 21st century, however, this was called into question, and it appears the hexapoda's closest relatives may be the crustaceans.
The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha, 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) 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 belong elsewhere.[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 - coinciding with the appearance of vascular plants on land.
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.
- 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 - Overview - Encyclopedia of Life". Encyclopedia of Life.
- "Subphylum Hexapoda - Hexapods - BugGuide.Net". bugguide.net.
- "Hexapoda". tolweb.org.
- "Hexapoda facts, information, pictures | Encyclopedia.com articles about Hexapoda". www.encyclopedia.com.
- "Hexapoda". biosurvey.ou.edu.
- "Hexapoda". comenius.susqu.edu.
- "Hexapoda (Insecta): General Characteristics | easybiologyclass". www.easybiologyclass.com.
- Boundless (26 May 2016). "Subphyla of Arthropoda". Boundless.
- "Humble bug plugs gap in fossil record".
- "Class Hexapoda (Insects) (hexa, six + podus, feet) | Biology Boom". biologyboom.com.
- 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.
- "Checklist of the Collembola: Are Collembola terrestrial Crustacea?". www.collembola.org.
- "GeoKansas--Fossil Isects". www.kgs.ku.edu. Archived from the original on 13 February 2017.
- "HEXAPODA". comenius.susqu.edu.
- 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.
- "The Hexapods". projects.ncsu.edu.
- "A Devonian hexapod". Pharyngula. 2 August 2012.
- 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. & Wheeler, W.C. (2001). "Arthropod phylogeny based on eight molecular loci and morphology". Nature. 413 (6852): 157–161. doi:10.1038/35093097. PMID 11557979. S2CID 4431635.
- Kazlev, M.Alan. "Palaeos Arthropods: Hexapoda". palaeos.com.
- "How do insects breathe? An outline of the tracheal system | Teaching Biology". Teaching Biology. 26 November 2012.
- 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.
- "HEXAPODA". comenius.susqu.edu.
- Engel, Michael S.; Grimaldi, David A. (12 February 2004). "New light shed on the oldest insect". Nature. 427 (6975): 627–630. doi:10.1038/nature02291. ISSN 0028-0836. PMID 14961119. S2CID 4431205.
- "Hexapoda | Oxbridge Notes the United Kingdom". www.oxbridgenotes.co.uk.
- 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. 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): 121. doi:10.1098/rsif.2016.0363. PMC 5014063. PMID 27558853.
- Shear, William A. (2 August 2012). "Palaeontology: An insect to fill the gap". Nature. 488 (7409): 34–35. doi:10.1038/488034a. ISSN 0028-0836. PMID 22859195. S2CID 6703535.
- 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 (7409): 82–85. doi:10.1038/nature11281. PMID 22859205. S2CID 205229663.
|Wikisource has the text of the 1911 Encyclopædia Britannica article Hexapoda.|
- Data related to Hexapoda at Wikispecies
- Dichotomous key to the Hexapoda at Wikibooks
- "Hexapoda. Insects, springtails, diplurans, and proturans". Tree of Life Web Project.