Hexapoda

Taxobox
name = Hexapods
fossil_range = Early Devonian - Recent


image_caption = A flesh-fly
regnum = Animalia
phylum = Arthropoda
subphylum = Hexapoda
subphylum_authority = Latreille, 1825
subdivision_ranks = Classes & Orders
subdivision = Class Insecta (insects)
Class Entognatha
The subphylum Hexapoda (from the Greek for "six legs") constitutes the largest (in terms of number of species) grouping 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. 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 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 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 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 crustaceans. [cite journal|author=Giribet, G., Edgecombe, G.D. and Wheeler, W.C.| year=2001 |title= Arthropod phylogeny based on eight molecular loci and morphology | journal=Nature|volume=413| pages=157–161 |doi= 10.1038/35093097] 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.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 ]

The following cladograms, showing three possible relationships among hexapoda are based on Tree of Life Project; [http://www.tolweb.org/Hexapoda/2528 Tree of Life, Hexapoda] ] Diplura shown a unstable position:

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

References

External links

* [http://tolweb.org/tree?group=Hexapoda&contgroup=Arthropoda Tree of Life Project] – Hexapoda