The
subphylum
In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum.
The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...
Hexapoda (from
Greek for 'six legs') or hexapods comprises the largest
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
of
arthropod
Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s and includes most of the extant arthropod species. It includes the
crown group class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
Insecta
Insects (from Latin ') are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed leg ...
(true insects), as well as the much smaller clade
Entognatha, which includes three classes of wingless arthropods that were once considered insects:
Collembola (springtails),
Protura (coneheads) and
Diplura
The order Diplura ("two-pronged bristletails") is one of three orders of non-insect hexapods within the class Entognatha (alongside Collembola (springtails) and Protura). The name "diplura", or "two tails", refers to the characteristic pair o ...
(two-pronged bristletails). The insects and springtails are very abundant and are some of the most important
pollinator
A pollinator is an animal that moves pollen from the male anther of a flower to the female carpel, stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.
Insects are ...
s, basal
consumer
A consumer is a person or a group who intends to order, or use purchased goods, products, or services primarily for personal, social, family, household and similar needs, who is not directly related to entrepreneurial or business activities. ...
s,
scavenger
Scavengers are animals that consume Corpse decomposition, dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a he ...
s/
detritivores and
micropredators in
terrestrial environments.
Hexapods are named for their most distinctive feature: a three-part
body plan
A body plan, (), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.
This term, usually app ...
with a consolidated
thorax
The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen.
In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...
and three pairs of
leg
A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element cap ...
s. Most other arthropods have more than three pairs of legs. Most recent studies have recovered Hexapoda as a subgroup of
Pancrustacea.
Morphology
Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an
anterior
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
head,
thorax
The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen.
In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...
, and
posterior abdomen. The
head
A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple ani ...
is composed of a presegmental ''acron'' that usually bears eyes (absent in
Protura and
Diplura
The order Diplura ("two-pronged bristletails") is one of three orders of non-insect hexapods within the class Entognatha (alongside Collembola (springtails) and Protura). The name "diplura", or "two tails", refers to the characteristic pair o ...
), 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.
Mandible
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone i ...
s (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) the mouthparts are exposed or
ectognathous, while in other groups they are enveloped or
endognathous. Similar appendages are found on the heads of
Myriapoda
Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial.
Although molecular evidence and similar fossils suggests a diversifi ...
and
Crustacea, although the crustaceans have secondary
antennae.
Collembola and Diplura have segmented antenna: each segment has its own set of muscles. The antennae 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, has no muscles and is composed of a various number of annuli. This type of antenna is therefore called an annulated antenna.
Johnston's organ, which is found on the pedicel, is absent in the
Entognatha.
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 a single walking branch composed of five segments. The legs do not have 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 follows an epimorphic developmental pattern, where all segments are already present at the end of embryonic development, in all the hexapod groups except for the Protura, which follow an anamorphic developmental pattern, where the hatched juveniles have an incomplete complement of segments and go through a post-embryonic segment addition with each molting before reaching the final adult number of segments. All true insects have eleven segments (often reduced in number in many insect species), but in Protura there are 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.
Evolution and 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. In the first decade of the 21st century, however, this was called into question, and it appears the hexapods' closest relatives may be the
crustacean
Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
s.
The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class
Entognatha, 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).
A 2002 molecular analysis suggests that the hexapods diverged from their sister group, the
Anostraca (fairy shrimps), at around the start of the
Silurian
The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
period , coinciding with the appearance of
vascular plants on land.
Since then
remipedians have been revealed as closest living relative of hexapods. Several hypotheses about their internal relationships have 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:
* 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
A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
is given by Kjer et al. (2016):
An incomplete possible insect fossil, ''
Strudiella devonica,'' has been recovered from the
Devonian
The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
period. This fossil may help to fill the
arthropod gap from 385 million to 325 million years ago, although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect. In 2023, a hexapod-like arthropod fossil from the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
marine fossil site
Castle Bank was reported, although further study is needed.
References
External links
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*
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{{Authority control
Animal subphyla
Extant Early Devonian first appearances
Pancrustacea