Hexapods are named for their most distinctive feature: a three-partbody plan with a consolidatedthorax and three pairs oflegs. Most other arthropods have more than three pairs of legs.[5] Most recent studies have recovered Hexapoda as a subgroup ofPancrustacea.[6]
Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into ananterior head,thorax, andposterior abdomen.[7][8] Thehead is composed of a presegmentalacron that usually bears eyes (absent inProtura andDiplura),[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 thelabrum (upper lip).[10] In true insects (class Insecta) the mouthparts are exposed orectognathous, while in other groups they are enveloped orendognathous. Similar appendages are found on the heads ofMyriapoda andCrustacea, although the crustaceans have secondaryantennae.[11]
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 theEntognatha.[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 a single walking branch composed of five segments. The legs do not have the gill branches found in some other arthropods.[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 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 inCollembola 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 sensorycerci on the last segment.[20][21][22]
Themyriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity.[23] These were then considered subclasses of a subphylum calledUniramia 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 thecrustaceans.[25][26][27][28]
The non-insect hexapods have variously been considered a single evolutionary line, typically treated as ClassEntognatha,[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 theCollembola (springtails).[30]
A 2002 molecular analysis suggests that the hexapods diverged from their sister group, theAnostraca (fairy shrimps), at around the start of theSilurian period440 million years ago, coinciding with the appearance ofvascular plants on land.[31]
Since thenremipedians 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]
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 followingcladogram is given by Kjeret al. (2016):[33]
An incomplete possible insect fossil,Strudiella devonica, has been recovered from theDevonian period. This fossil may help to fill thearthropod 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 theOrdovician marine fossil siteCastle Bank was reported, although further study is needed.[37]
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