30-453: Tethytheria is a clade of mammals that includes the sirenians and proboscideans , as well as the extinct order Embrithopoda . Though there is strong anatomical and molecular support for the monophyly of Tethytheria, the interrelationships between the included taxa remain disputed. The tethytheres are united by several characters, including anteriorly facing orbits and more or less bilophodont cheek teeth (double transverse ridges on
60-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
90-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
120-552: A hundred species, are marine. Insects such as snow scorpionflies flourish in cold habitats including the Arctic and at high altitude. Insects such as desert locusts , ants, beetles, and termites are adapted to some of the hottest and driest environments on earth, such as the Sonoran Desert . Insects form a clade , a natural group with a common ancestor, among the arthropods . A phylogenetic analysis by Kjer et al. (2016) places
150-538: A pupa, developing through a series of increasingly adult-like nymphal stages. The higher level relationship of the insects is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonfly-like insects with wingspans of 55 to 70 cm (22 to 28 in). The most diverse insect groups appear to have coevolved with flowering plants . Adult insects typically move about by walking and flying; some can swim. Insects are
180-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
210-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
240-418: A system of paired openings along their sides, connected to small tubes that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an open hemocoel . Insect vision is mainly through their compound eyes , with additional small ocelli . Many insects can hear, using tympanal organs , which may be on the legs or other parts of
270-525: A variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate , or rub their wings together, to attract a mate and repel other males. Lampyrid beetles communicate with light. Humans regard many insects as pests , especially those that damage crops, and attempt to control them using insecticides and other techniques. Others are parasitic , and may act as vectors of diseases . Insect pollinators are essential to
300-438: Is a stub . You can help Misplaced Pages by expanding it . Clade In biological phylogenetics , a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes
330-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
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#1732852147968360-471: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
390-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
420-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
450-641: The Latin form cladus (plural cladi ) is used rather than the English form. Clades are the fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
480-783: The Latin word insectum from in , "cut up", as insects appear to be cut into three parts. The Latin word was introduced by Pliny the Elder who calqued the Ancient Greek word ἔντομον éntomon "insect" (as in entomology ) from ἔντομος éntomos "cut in pieces"; this was Aristotle 's term for this class of life in his biology , also in reference to their notched bodies. The English word insect first appears in 1601 in Philemon Holland 's translation of Pliny. In common speech, insects and other terrestrial arthropods are often called bugs . Entomologists to some extent reserve
510-491: The arthropod phylum . Insects have a chitinous exoskeleton , a three-part body ( head , thorax and abdomen ), three pairs of jointed legs , compound eyes , and a pair of antennae . Insects are the most diverse group of animals, with more than a million described species ; they represent more than half of all animal species. The insect nervous system consists of a brain and a ventral nerve cord . Most insects reproduce by laying eggs . Insects breathe air through
540-618: The Hemiptera (true bugs), Lepidoptera (butterflies and moths), Diptera (true flies), Hymenoptera (wasps, ants, and bees), and Coleoptera (beetles), each with more than 100,000 described species. Insects are distributed over every continent and almost every terrestrial habitat. There are many more species in the tropics , especially in rainforests , than in temperate zones. The world's regions have received widely differing amounts of attention from entomologists. The British Isles have been thoroughly surveyed, so that Gullan and Cranston 2014 state that
570-459: The body. Their sense of smell is via receptors, usually on the antennae and the mouthparts. Nearly all insects hatch from eggs . Insect growth is constrained by the inelastic exoskeleton, so development involves a series of molts . The immature stages often differ from the adults in structure, habit and habitat. Groups that undergo four-stage metamorphosis often have a nearly immobile pupa . Insects that undergo three-stage metamorphosis lack
600-665: The crowns of the teeth). Proboscidea and Sirenia are linked together based on auditory characters in their petrosal bones , but this link may be a homoplasy . Desmostylians , traditionally considered tethytheres, have been tentatively assigned to Perissodactyla , along with the Early Eocene family Anthracobunidae , which was considered a sister group to Tethytheria. Cladogram modified from Rose 2006 and Cooper et al. 2014 . Hyracoidea † Embrithopoda Sirenia Proboscidea Afroinsectiphilia Classification modified from Rose 2006 . This Afrotheria -related article
630-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
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#1732852147968660-592: The insects among the Hexapoda , six-legged animals with segmented bodies; their closest relatives are the Diplura (bristletails). Collembola (springtails) [REDACTED] Protura (coneheads) [REDACTED] Diplura (two-pronged bristletails) [REDACTED] Insecta (=Ectognatha) [REDACTED] The internal phylogeny is based on the works of Wipfler et al. 2019 for the Polyneoptera , Johnson et al. 2018 for
690-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
720-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
750-402: The name "bugs" for a narrow category of " true bugs ", insects of the order Hemiptera , such as cicadas and shield bugs . Other terrestrial arthropods, such as centipedes , millipedes , woodlice , spiders , mites and scorpions , are sometimes confused with insects, since they have a jointed exoskeleton. Adult insects are the only arthropods that ever have wings, with up to two pairs on
780-554: The only invertebrates that can achieve sustained powered flight; insect flight evolved just once. Many insects are at least partly aquatic , and have larvae with gills; in some species, the adults too are aquatic. Some species, such as water striders , can walk on the surface of water. Insects are mostly solitary, but some, such as bees , ants and termites , are social and live in large, well-organized colonies . Others, such as earwigs , provide maternal care, guarding their eggs and young. Insects can communicate with each other in
810-535: The reproduction of many flowering plants and so to their ecosystems. Many insects are ecologically beneficial as predators of pest insects, while a few provide direct economic benefit. Two species in particular are economically important and were domesticated many centuries ago: silkworms for silk and honey bees for honey . Insects are consumed as food in 80% of the world's nations, by people in roughly 3000 ethnic groups. Human activities are having serious effects on insect biodiversity . The word insect comes from
840-504: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Insect Insects (from Latin insectum ) are hexapod invertebrates of the class Insecta . They are the largest group within
870-527: The thorax. Whether winged or not, adult insects can be distinguished by their three-part body plan, with head, thorax, and abdomen; they have three pairs of legs on the thorax. Estimates of the total number of insect species vary considerably, suggesting that there are perhaps some 5.5 million insect species in existence, of which about one million have been described and named. These constitute around half of all eukaryote species, including animals , plants , and fungi . The most diverse insect orders are
900-518: The total of around 22,500 species is probably within 5% of the actual number there; they comment that Canada's list of 30,000 described species is surely over half of the actual total. They add that the 3000 species of the American Arctic must be broadly accurate. In contrast, a large majority of the insect species of the tropics and the southern hemisphere are probably undescribed. Some 30–40,000 species inhabit freshwater ; very few insects, perhaps
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