Hexapoda

The subphylum Hexapoda (from the Greek

Greek language

Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...

 for six legs) constitutes the largest (in terms of number of species) grouping of arthropod

Arthropod

An arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s and includes the insect

Insect

Insects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...

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.

The subphylum Hexapoda (from the Greek

Greek language

Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...

 for six legs) constitutes the largest (in terms of number of species) grouping of arthropod

Arthropod

An arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s and includes the insect

Insect

Insects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...

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 and crustaceans. More recently, the term has also been applied to cilium structures present in many 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...

 or Atelocerata. New work, however, has called this into question, and it appears their closest relatives may be the crustacean

Crustacean

Crustaceans are a very large group of arthropods, comprising almost 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles...

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 hexapods. 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 and Protura...

 (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 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . 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 cladograms, showing three possible relationships among hexapoda are based on Tree of Life Project; Diplura is shown with an unstable position:

External links

• Tree of Life Project – Hexapoda