Class Insecta (insects)
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,
Diplura (all of these were once considered
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
2 Evolution and relationships
4 External links
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. 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
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
has eleven segments (often reduced in number in many insect species),
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.
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
Atelocerata. 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.
The non-insect hexapods have variously been considered a single
evolutionary line, typically treated as Class Entognatha, or
several lines with different relationships with the Class Insecta. In
Diplura may be more closely related to the Insecta
than to the
Collembola (spring tails) or the Protura. There is
also some evidence suggesting that the hexapod groups may not share a
common origin, and in particular that the
elsewhere.[better source needed]
Molecular analysis suggests that the hexapods diverged from their
sister group, the
Anostraca (fairy shrimps), at around the start of
Silurian period 440 million years ago - coinciding with the
appearance of vascular plants on land.
The following cladogram is given by Kjer et al. (2016):
Diplura (two-pronged bristletails)
Archaeognatha (jumping bristletails)
Pterygota (winged insects)
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.
See also: Phylogeny of insects
^ 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 .
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^ "Hexapods -
Hexapoda - Overview - Encyclopedia of Life".
Encyclopedia of Life.
Hexapoda - Hexapods - BugGuide.Net". bugguide.net.
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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".
^ Boundless (2016-05-26). "Subphyla of Arthropoda". Boundless.
^ "Humble bug plugs gap in fossil record".
Hexapoda (Insects) (hexa, six + podus, feet) Biology Boom".
^ 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. doi:10.1086/277920. JSTOR 2453748.
^ "Checklist of the Collembola: Are
^ "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 (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.
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
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^ 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:
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doi:10.1098/rspb.2004.2917. PMC 1634985 .
^ "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.
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Hexapoda Oxbridge Notes the United Kingdom".
^ 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):
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^ 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:
^ Shear, William A. (2012-08-02). "Palaeontology: An insect to fill
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^ The Web page cites Garrouste, R; Clément, G; Nel, P; Engel, MS;
Grandcolas, P; D'Haese, C; Lagebro, L; Denayer, J; Gueriau, P;
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from the Late
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Data related to
Hexapoda at Wikispecies
Dichotomous key to the
Hexapoda at Wikibooks
"Hexapoda. Insects, springtails, diplurans, and proturans". Tree of
Life Web Project.
Extant Arthropoda classes by subphylum
Pycnogonida (sea spiders)
Merostomata (horseshoe crabs)¹
Arachnida (spiders, scorpions, ticks, mites)
Symphyla (symphylans or garden centipedes)
Ostracoda (seed shrimps)
Pentastomida (tongue worms)
Branchiura (fish lice)
Malacostraca (woodlice, shrimps, crayfish, lobsters, crabs)
Thecostraca (barnacles and relatives) + Tantulocarida
Cephalocarida (horseshoe shrimps)
Branchiopoda (fairy, tadpole, clam shrimps, water fleas)
Diplura (two-pronged bristletails)³
¹contains the only extant order Xiphosura
italic are paraphyletic groups
Sources: Edgecombe et al. (2014), Petrunina (2012) for pancrustaceans.
Fauna Europaea: 3