Hexapoda
62163
222503505
2008-06-29T18:10:20Z
JD
30533
/* Hexapod morphology */ Corrected maxilla and mandible to point at insect specific articles. If you want something more general, those lead to that.
{{Taxobox
| name = Hexapods
| fossil_range = Early [[Devonian]]<ref name=Gaunt2002/> - Recent
| image = diptera_01gg.jpg
| image_caption = A [[flesh-fly]]
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| subphylum = '''Hexapoda'''
| subphylum_authority = [[Pierre André Latreille|Latreille]], 1825
| subdivision_ranks = Classes & Orders
| subdivision =
Class [[Insect]]a (insects)<br>
Class [[Entognatha]] <br>
}}
The subphylum '''Hexapoda''' (from the [[Greek language|Greek]] for ''six legs'') constitutes the largest (in terms of number of species) grouping of [[arthropod]]s and includes the [[insect]]s as well as three much smaller groups of wingless arthropods: [[Collembola]], [[Protura]], and [[Diplura]] (all of these were once considered insects). The Collembola (or [[springtail]]s) 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.
==Hexapod morphology==
Hexapods have bodies divided into an anterior head, thorax, and posterior abdomen. The [[Arthropod head problem|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:<br><small>
:Segment I. None
:Segment II. [[Antenna (biology)|Antennae]] (sensory), absent in [[Protura]]
:Segment III. None
:Segment IV. [[Labium (insect)#Mandible|Mandible]]s (crushing jaws)
:Segment V. [[Labium (insect)#Maxilla|Maxillae]] (chewing jaws)
:Segment VI. [[Labium (insect)|Labium]] (lower lip)</small>
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 herein) 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. 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 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). The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory '''''cerci''''' on the last segment.
==Hexapod relationships==
The [[Myriapoda|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. New work, however, has called this into question, and it appears their closest relatives may be the [[crustacean]]s.<ref>{{cite journal|author=[[Gonzalo Giribet|Giribet, G.]], [[Gregory D. Edgecombe|Edgecombe, G.D.]] and [[Ward C. Wheeler|Wheeler, W.C.]]| year=2001 |title= Arthropod phylogeny based on eight molecular loci and morphology | journal=[[Nature (journal)|Nature]]|volume=413| pages=157–161 |doi= 10.1038/35093097}}</ref> The non-insect hexapods have variously been considered a single evolutionarily line, typically treated as Class [[Entognatha]] (cladogram A), or several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than the Collembola 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.
Molecular analysis suggests that the hexapods diverged from their sister group, the [[Anostraca]] (fairy shrimps), at around the start of the [[Silurian]] period {{Ma|440}} - coinciding with the appearance of [[vascular plants]] on land.<ref name=Gaunt2002>{{cite journal
| author = Gaunt, M.W.
| coauthors = Miles, M.A.
| date = [[2002-05-01]]
| title = An Insect Molecular Clock Dates the Origin of the Insects and Accords with Palaeontological and Biogeographic Landmarks
| journal = Molecular Biology and Evolution
| volume = 19
| pages = 748–761
| issn = 1537-1719
| url = http://www.mbe.oupjournals.org/cgi/content/abstract/19/5/748
| accessdate = 2007-07-14
| pmid = 11961108
}}</ref>
The following [[cladogram]]s, showing three possible relationships among hexapoda are based on Tree of Life Project;<ref name=tree>[http://www.tolweb.org/Hexapoda/2528 Tree of Life, Hexapoda]</ref> [[Diplura]] shown a unstable position:
<div style="-moz-column-count:3; column-count:3; font-size:71%">
{{Clade | style=font-size:100%;line-height:100%
| label1=[[Hexapoda]]
| 1={{Clade
|label1= [[Entognatha]]
| 1={{clade
| label1=[[Ellipura]]
| 1= {{clade
| label1=
| 1= [[Collembola]]
| 2=[[Protura]]}}
| 2=[[Diplura]]}}
| label2=[[Ectognatha]]
| 2=[[Insecta]]
}} }}
'''A'''
{{Clade | style=font-size:100%;line-height:100%
| label1=[[Hexapoda]]
| 1={{Clade
| label1=[[Ellipura]]
| 1={{clade
| label1=
| 1=[[Collembola]]
| 2=[[Protura]]}}
| 2={{clade
| 1=[[Diplura]]
| 2=[[Insecta]]
}} }} }}
'''B'''
{{Clade | style=font-size:100%;line-height:100%
| label1=[[Hexapoda]]
| 1={{Clade
| label1=[[Ellipura]]
| 1={{clade
| label1=
| 1=[[Collembola]]
| 2=[[Protura]]}}
| 2=[[Diplura]]
| 3=[[Insecta]]
}} }}
'''C'''
</div>
==References==
<references/>
==External links==
{{Wikispecies|Hexapoda}}
{{Wikibookspar|Dichotomous Key|Hexapoda}}
* [http://tolweb.org/tree?group=Hexapoda&contgroup=Arthropoda Tree of Life Project] – Hexapoda
{{Arthropods}}
[[Category:Arthropods]]
[[ca:Hexàpode]]
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[[hr:Heksapodi]]
[[io:Artikopodo]]
[[is:Sexfætlur]]
[[it:Hexapoda]]
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[[li:Hexapoda]]
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[[oc:Hexapoda]]
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