Temporal range: 411–0 Ma
|Subphylum:|| Hexapoda |
The subphylum Hexapoda (from the Greek for six legs) constitutes the largest number of 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.
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 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.
Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Cladocera, Notostraca and the Devonian Lepidocaris. They are mostly small, freshwater animals that feed on plankton and detritus.
Uniramia is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category. The group is currently used in a narrower sense.
The order Diplura is one of three orders 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.
Marrella is an extinct genus of arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale.
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.
In biology, a tagma is a specialized grouping of multiple segments or metameres into a coherently functional morphological unit. Familiar examples are the head, the thorax, and the abdomen of insects. The segments within a tagma may be either fused or so jointed as to be independently moveable.
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 may have evolved from a group of crustaceans. The first insects were land bound, 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 a clade, comprising 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. The clade has also been called Tetraconata, referring to having four cone cells in the ommatidia. That 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. 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.
The (pan)arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other. While the dispute has historically centered on the exact make-up of the insect head, it has been widened to include other living arthropods such as the crustaceans and chelicerates; and fossil forms, such as the many arthropods known from exceptionally preserved Cambrian faunas. While the topic has classically been based on insect embryology, in recent years a great deal of developmental molecular data has become available. Dozens of more or less distinct solutions to the problem, dating back to at least 1897, have been published, including several in the 2000s.
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).
The order Symphypleona is one of the three main groups of springtails (Collembola), tiny hexapods related to insects. When the springtails were still believed to be an order of insects, the Symphypleona were ranked as a suborder.
The Entomobryomorpha are one of the three main groups (order) of springtails (Collembola), tiny hexapods related to insects. This group was formerly treated as a superfamily, the Entomobryoidea.
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora.
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.
Wingertshellicus is an extinct genus of arthropod, of average size, 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. The body consists of just two main parts, a head and a trunk that comprises a long row of similar segments. The relatively small head is dominated by large eyes, and three pairs of long legs, making it look like a damselfly nymph, although W. backesi has long antennae, unlike damselflies.
Crustaceans form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, shrimps, prawns, krill, woodlice, and barnacles. 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 other hexapods than they are to certain other crustaceans.
|Wikisource has the text of the 1911 Encyclopædia Britannica article Hexapoda .|