Hexapoda

Hexapods Temporal range: 411–0  Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
A flesh-fly, Sarcophaga sp.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Pancrustacea
Subphylum:	Hexapoda Latreille, 1825[2]
Classes
Clade Elliplura Class Collembola (springtails) Order Protura (coneheads) Clade Cercophora Order Diplura (two-pronged bristletails) Class Insecta (insects)

The

Hexapods are named for their most distinctive feature: a three-part

Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head,

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

Collembolans and diplurans have segmented antenna; each segment has its own set of muscles. The antennea of insects consist of just three segments; the scape, the pedicel and the flagellum. Muscles occur only in the first two segments. The third segment, the flagellum, don't have any muscles and is composed of a various number of annuli. This type of antenna is therefore called annulated antenna. Johnston's organ, which is found on the pedicel, is absent in the Entognatha.^[12][13]

The thorax is composed of three segments, each of which bears a single pair of legs.^[14] 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.^[15] In most insects the second and third thoracic segments also support wings.^[16] 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.^[17]

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

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity.^[23] These were then considered subclasses of a subphylum called Uniramia or Atelocerata.^[24] In the first decade of the 21st century, however, this was called into question, and it appears the hexapods' closest relatives may be the crustaceans.^{[25][26][27][28]}

The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha,^[29] 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).^[30]

Molecular analysis suggests that the hexapods diverged from their sister group, the

Since then remipedians has been revealed as closest living relative of hexapods. Several hypoteses about their internal relationships has been suggested over the years, with proturans as the sister group to the other hexapods and collembolans and diplurans belonging together in Antennomusculata as the latest suggestion:^[32]

• Entognatha (proturans, collembolans and diplurans) and Ectognatha (insects)

• Elliplura (proturans and collembolans) and Cercophora (diplurans and insects)

• Collembolans, Nonoculata (proturans and diplurans) and insects

• Proturans, Antennomusculata (collembolans and diplurans) and insects

The following cladogram is given by Kjer et al. (2016):^[33]

An incomplete possible insect fossil,

External links

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.