Hexapoda

Hexapoda

Overview
The subphylum Hexapoda constitutes the largest (in terms of number of species) grouping of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s and includes the insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

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
Springtail
Springtails form the largest of the three lineages of modern hexapods that are no longer considered insects...

s) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs.
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Encyclopedia
The subphylum Hexapoda constitutes the largest (in terms of number of species) grouping of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s and includes the insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

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
Springtail
Springtails form the largest of the three lineages of modern hexapods that are no longer considered insects...

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 head
Arthropod head problem
The arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other...

 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
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 (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
Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial...

 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
Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial...

 have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia
Uniramia
S. M. Manton's polyphyletic Arthropod hypothesis with Uniramia as one of three Phyla.Uniramia is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category. The group is currently used in a narrower sense.Uniramia is one of three...

 or Atelocerata. New work, however, has called this into question, and it appears their closest relatives may be the crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s. The non-insect hexapods have variously been considered a single evolutionarily line, typically treated as Class Entognatha
Entognatha
The Entognatha is a class of ametabolous arthropods, which, together with insects, makes up the subphylum Hexapoda. Their mouthparts are entognathous, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings. The class contains three orders: Collembola , Diplura...

 (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
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 period - coinciding with the appearance of vascular plants on land.

The following cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

s, showing three possible relationships among hexapoda are based on Tree of Life Project; Diplura is shown with an unstable position: