Hexapods Temporal range: | |
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A flesh-fly, Sarcophaga sp. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Clade: | Mandibulata |
Clade: | Pancrustacea |
Subphylum: | Hexapoda Latreille, 1825 [2] |
Classes | |
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The subphylum Hexapoda (from Greek for 'six legs') or hexapods comprises the largest clade of arthropods and includes most of the extant arthropod species. It includes the crown group class Insecta (true insects), as well as the much smaller class Entognatha, which includes three groups of wingless arthropods that were once considered insects: Collembola (springtails), Protura (coneheads) and Diplura (two-pronged bristletails). [3] [4] The insects and springtails are very abundant and are some of the most important pollinators, basal consumers, scavengers/detritivores and micropredators in terrestrial environments.
Hexapods are named for their most distinctive feature: a three-part body plan with a consolidated thorax and three pairs of legs. Most other arthropods have more than three pairs of legs. [5] Most recent studies have recovered Hexapoda as a subgroup of Crustacea. [6]
Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax, and posterior abdomen. [7] [8] The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), [9] followed by six segments, all closely fused together, with the following appendages:
The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). [10] 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 and Crustacea, although the crustaceans have secondary antennae. [11]
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 Collembola only six (sometimes reduced to only four). [18] [19] The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment. [20] [21] [22]
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]
A 2002 molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago, coinciding with the appearance of vascular plants on land. [31]
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: [32]
The following cladogram is given by Kjer et al. (2016): [33]
Hexapoda |
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An incomplete possible insect fossil, Strudiella devonica, has been recovered from the Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago, [34] [35] although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect. [36] In 2023, a hexapod-like arthropod fossil from the Ordovician marine fossil site Castle Bank was reported, although further study is needed. [37]
The order Diplura is one of three orders of non-insect hexapods within the class Entognatha. The name "diplura", or "two tails", refers to the characteristic pair of caudal appendages or filaments at the terminal end of the body.
The Protura, or proturans, and sometimes nicknamed coneheads, are very small, soil-dwelling animals, so inconspicuous they were not noticed until the 20th century. The Protura constitute an order of hexapods that were previously regarded as insects, and sometimes treated as a class in their own right.
Antennae, sometimes referred to as "feelers", are paired appendages used for sensing in arthropods.
Remipedia is a class of blind crustaceans, closely related to hexapods, found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The first described remipede was the fossil Tesnusocaris goldichi. Since 1979, at least seventeen living species have been identified in subtropical regions around the world.
The telson is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on account of not arising in the embryo from teloblast areas as other segments. It never carries any appendages, but a forked "tail" called the caudal furca may be present. The shape and composition of the telson differs between arthropod groups.
The name Apterygota is sometimes applied to a former subclass of small, agile insects, distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are the silverfish, the firebrat, and the jumping bristletails. Their first known occurrence in the fossil record is during the Devonian period, 417–354 million years ago. The group Apterygota is not a clade; it is paraphyletic, and not recognized in modern classification schemes. As defined, the group contains two separate clades of wingless insects: Archaeognatha comprises jumping bristletails, while Zygentoma comprises silverfish and firebrats. The Zygentoma are in the clade Dicondylia with winged insects, a clade that includes all other insects, while Archaeognatha is sister to this lineage.
Caecilius is a genus of Psocoptera from the family Caeciliusidae, the lizard barklice. Species are found all around the world.
The most recent understanding of the evolution of insects is based on studies of the following branches of science: molecular biology, insect morphology, paleontology, insect taxonomy, evolution, embryology, bioinformatics and scientific computing. It is estimated that the class of insects originated on Earth about 480 million years ago, in the Ordovician, at about the same time terrestrial plants appeared. Insects are thought to have evolved from a group of crustaceans. The first insects were landbound, but about 400 million years ago in the Devonian period one lineage of insects evolved flight, the first animals to do so. The oldest insect fossil has been proposed to be Rhyniognatha hirsti, estimated to be 400 million years old, but the insect identity of the fossil has been contested. Global climate conditions changed several times during the history of Earth, and along with it the diversity of insects. The Pterygotes underwent a major radiation in the Carboniferous while the Endopterygota underwent another major radiation in the Permian.
Pancrustacea is the clade that comprises all crustaceans, including hexapods. This grouping is contrary to the Atelocerata hypothesis, in which Hexapoda and Myriapoda are sister taxa, and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon was considered well accepted, with most studies recovering Hexapoda within Crustacea. The clade has also been called Tetraconata, referring to having four cone cells in the ommatidia. This name is preferred by some scientists as a means of avoiding confusion with the use of "pan-" to indicate a clade that includes a crown group and all of its stem group representatives.
The Entognatha are a class of wingless and ametabolous arthropods, which, together with the insects, makes up the subphylum Hexapoda. Their mouthparts are entognathous, meaning that they are retracted within the head, unlike the insects. Entognatha are apterous, meaning that they lack wings. The class contains three orders: Collembola, Diplura and Protura. These three groups were historically united with the now-obsolete order Thysanura to form the class Apterygota, but it has since been recognized that the hexapodous condition of these animals has evolved independently from that of insects, and independently within each order. The orders might not be closely related, and Entognatha is now considered to be a paraphyletic group.
Mandibulata, is one of two major clades of living arthropods alongside Chelicerata. It comprises the extant groups Myriapoda and Pancrustacea. The name "Mandibulata" refers to the mandibles, a modified pair of limbs used in food processing, the presence of which are characteristic of most members of the group. Members of the group are referred to as mandibulates.
Rhyniella is a genus of fossil springtails (Collembola) from the Rhynie chert, which formed during the Pragian stage of the Early Devonian. One species has been described, Rhyniella praecursor. For some time it was believed to be the only hexapod from the Early Devonian
Arthropods are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated (metameric) segments, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They form an extremely diverse group of up to ten million species.
Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects. Although the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be any more closely related to one another than they are to all insects, which have external mouthparts.
Xenocarida is a proposed clade inside the subphylum Crustacea that comprises two classes that were discovered in the 20th century: Remipedia and Cephalocarida. Both groups are marine hermaphrodites. The clade was recovered as the sister groups to Hexapoda.
Wingertshellicus is an extinct genus of arthropod that has been found in Hunsrück Slate, that is located in the Rhenish Massif in Germany, and lived about 405 million years ago, during the Lower Emsian.
Tactopoda or Arthropodoidea is a proposed clade of ecdysozoan animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations. The cladogram below shows the relationships implied by this hypothesis.
Crustaceans are invertebrate animals that constitute one group of arthropods that are a part of the subphylum Crustacea, a large, diverse group of mainly aquatic arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods emerged deep in the Crustacean group, with the completed pan-group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans.
Dryotribini is a tribe of beetles in the subfamily Cossoninae.