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17-520: 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 clade Entognatha , which includes three classes of wingless arthropods that were once considered insects: Collembola (springtails), Protura (coneheads) and Diplura (two-pronged bristletails). The insects and springtails are very abundant and are some of

34-413: A pouch in the head capsule, called the gnathal pouch, so only the tips of the mandibles and maxillae are exposed beyond the cavity. This pouch is created in the embryo by a flap or lateral head sclerite near the mouth on each side of the head which fuses with the labium. Other differences from insects are that each antennal segment is musculated; in insects, only the two basal segments are. Sperm transfer

51-503: 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. 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 and with gill on

68-534: 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: 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

85-418: Is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used the rank of subphylum, for instance monocotyledons as a subphylum of phylum Angiospermae and vertebrates as a subphylum of phylum Chordata . Subphylum is: Where convenient, subphyla in turn can be divided into infraphyla ; in turn such an infraphylum also would be superordinate to any classes or superclasses in

102-589: The Devonian 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 marine fossil site Castle Bank was reported, although further study is needed. Subphylum The taxonomic rank of " subdivision " in fungi and plant taxonomy

119-607: The hierarchy . Not all fauna phyla are divided into subphyla. Those that are include: Examples of infraphyla include the Mycetozoa , the Gnathostomata and the Agnatha . Entognatha Insecta (insects) 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

136-401: The abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment. 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

153-417: The abdominal segments of some immature aquatic insects. 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 follow epimorphic development, where all segments are already present at the end of embryonic development in all

170-405: The head, unlike the insects. Entognatha are apterous , meaning that they lack wings . The class contains three orders : Collembola ( springtails , 9000 species), Diplura ("two-tail", 1000 species) and Protura ("first-tail", 800 species). These three groups were historically united with the now-obsolete order Thysanura to form the class Apterygota , but it has since been recognized that

187-407: The heads of Myriapoda and Crustacea , although the crustaceans have secondary antennae . 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

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204-521: 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). The appendages on

221-425: 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. These minute arthropods are apterous, unlike some orders of insects that have lost their wings secondarily (but are derived from winged ancestors). Their mouthparts are enclosed within

238-512: 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. 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

255-564: 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. Most recent studies have recovered Hexapoda as a subgroup of Crustacea . 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. The head

272-533: The sister group to the other hexapods and collembolans and diplurans belonging together in Antennomusculata as the latest suggestion: The following cladogram is given by Kjer et al. (2016): Collembola (springtails) Protura (coneheads) Diplura (two-pronged bristletails) Archaeognatha (jumping bristletails) Zygentoma (silverfish) Pterygota (winged insects) An incomplete possible insect fossil, Strudiella devonica , has been recovered from

289-605: Was called into question, and it appears the hexapods' closest relatives may be the crustaceans . 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

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