|Subphylum:|| Hexapoda |
The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects).The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs (six 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 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 the heads of Myriapoda 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 and with gill on 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 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 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 was called into question, and it appears the hexapoda's closest relatives may be the crustaceans.
The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha, [ better source needed ]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) or the Protura. 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 period- coinciding with the appearance of vascular plants on land.
The cladogram below follows the work of Bernhard Misof et al. (2014)and shows the relationships between the extant orders of Hexapoda:
The following cladogram is given by Kjer et al. (2016):
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,although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect.
Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca, Notostraca and the Devonian Lepidocaris. They are mostly small, freshwater animals that feed on plankton and detritus.
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 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 name Apterygota is sometimes applied to a 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.
Atelocerata is a proposed clade of arthropods that includes Hexapoda and Myriapoda, but excludes Crustacea and Chelicerata. The name is currently used interchangeably with Tracheata. or Uniramia sensu stricto. It is an extensive division of arthropods comprising all those that breathe by tracheae, as distinguished from Crustacea, which breathe by means of gills.
Pancrustacea is the clade that comprises all crustaceans and hexapods. This grouping is contrary to the Atelocerata hypothesis, in which Myriapoda and Hexapoda are sister taxa, and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon is 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 arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa, trochanter, femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus, patella.
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 (springtails), Diplura (“two-tail”) and Protura (“first-tail”), and over 5000 known 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 the hexapodous condition of these animals has evolved independently from that of insects, and independently within each order. The orders may not be closely related, and Entognatha is now considered to be a polyphyletic group.
Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda, Crustacea and Hexapoda. Mandibulata is currently believed to be the sister group of the clade Arachnomorpha, which comprises the rest of arthropods. The mandibulates constitute the largest and most varied arthropod group.
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. Its name means "small creature of the Rhynie chert, a forerunner [of modern hexapoda]". For some time it was believed to be the only hexapod from the Early Devonian. It was possibly the same animal as Rhyniognatha (partim).
Arthropods are invertebrate animals having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. Arthropods are bilaterally symmetrical and their body possesses an external skeleton. 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. Some species have wings. They are an extremely diverse group, with up to 10 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 clade inside the subphylum Crustacea that comprises two classes that were discovered in the 20th century: Remipedia and Cephalocarida. Xenocarida is thought to be the sister clade 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 is a proposed clade of protostome animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations. The cladogram below shows the relationships implied by this hypothesis.
Crustaceans form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, shrimp, krill, prawns, woodlice, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata; because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the clade Pancrustacea other than hexapods. Some crustaceans are more closely related to insects and the other hexapods than they are to certain other crustaceans.
Eumetabola is an unranked clade of Neoptera. Two large unities known as the Paurometabola and Eumetabola are probably from the adelphotaxa of the Neoptera after exclusion of the Plecoptera. The monophyly of these unities appears to be weakly justified.
|Wikisource has the text of the 1911 Encyclopædia Britannica article " Hexapoda ".|