Uropods are posterior appendages found on a wide variety of crustaceans. They have functions in locomotion. Uropods are defined as the appendages of the last body segment of a crustacean. An alternative definition suggested by Frederick R. Schram restricts the term to those structures arising from the segment before the anal segment. Under this latter definition, the appendages of the anal segment are caudal rami, which are analogous to uropods. Uropods are biramous – comprising an endopod and an exopod; the exopod is the larger, may be divided in two by a transverse suture known as the diaeresis. The uropods may work in concert with the telson to form a "tail fan"
The arthropod leg is a form of jointed appendage of arthropods used for walking. Many of the terms used for arthropod leg segments are of Latin origin, may be confused with terms for bones: coxa, femur, tarsus, metatarsus, dactylus, patella. Homologies of leg segments between groups are difficult to prove and are the source of much argument; some authors posit up to eleven segments per leg for the most recent common ancestor of extant arthropods but modern arthropods have eight or fewer. It has been argued that the ancestral leg need not have been so complex, that other events, such as successive loss of function of a Hox-gene, could result in parallel gains of leg segments; the appendages of arthropods may be either uniramous. A uniramous limb comprises a single series of segments attached end-to-end. A biramous limb, branches into two, each branch consists of a series of segments attached end-to-end; the external branch of the appendages of crustaceans is known as the exopod or exopodite, while the internal branch is known as the endopod or endopodite.
Other structures aside from the latter two are termed endites. Exopodites can be distinguished from exites by the possession of internal musculature; the exopodites can sometimes be missing in some crustacean groups, they are absent in insects. The legs of insects and myriapods are uniramous. In crustaceans, the first antennae are uniramous, but the second antennae are biramous, as are the legs in most species. For a time, possession of uniramous limbs was believed to be a shared, derived character, so uniramous arthropods were grouped into a taxon called Uniramia, it is now believed that several groups of arthropods evolved uniramous limbs independently from ancestors with biramous limbs, so this taxon is no longer used. Arachnid legs differ from those of insects by the addition of two segments on either side of the tibia, the patella between the femur and the tibia, the metatarsus between the tibia and the tarsus, making a total of seven segments; the situation is identical with the addition of a pre-tarsus beyond the tarsus.
The claws of the scorpion are not legs, but are pedipalps, a different kind of appendage, found in spiders and is specialised for predation and mating. In Limulus, there are no pretarsi, leaving six segments per leg; the legs of crustaceans are divided primitively into seven segments, which do not follow the naming system used in the other groups. They are: coxa, ischium, carpus and dactylus. In some groups, some of the limb segments may be fused together; the claw of a lobster or crab is formed by the articulation of the dactylus against an outgrowth of the propodus. Crustacean limbs differ in being biramous, whereas all other extant arthropods have uniramous limbs. Myriapods have seven-segmented walking legs, comprising coxa, prefemur, tibia, a tarsal claw. Myriapod legs show a variety of modifications in different groups. In all centipedes, the first pair of legs is modified into a pair of venomous fangs called forcipules. In most millipedes, one or two pairs of walking legs in adult males are modified into sperm-transferring structures called gonopods.
In some millipedes, the first leg pair in males may be reduced to tiny hooks or stubs, while in others the first pair may be enlarged. Insects and their relatives are hexapods, having six legs, connected to the thorax, each with five components. In order from the body they are the coxa, femur and tarsus; each is a single segment, except the tarsus which can be from three to seven segments, each referred to as a tarsomere. A representative insect leg, such as that of a housefly or cockroach, has the following parts, in sequence from most proximal to most distal: coxa trochanter femur tibia tarsus pretarsus. Associated with the leg itself there are various sclerites around its base, their functions are articular and have to do with how the leg attaches to the main exoskeleton of the insect. Such sclerites differ between unrelated insects; the coxa is the proximal functional base of the leg. It articulates with the pleuron and associated sclerites of its thoracic segment, in some species it articulates with the edge of the sternite as well.
The homologies of the various basal sclerites are open to debate. Some authorities suggest. In many species the coxa has two lobes; the posterior lobe is the meron, the larger part of the coxa. A meron is well developed in Periplaneta, the Isoptera and Lepidoptera; the trochanter articulates with the coxa but is attached rigidly to the femur. In some insects its appearance may be confusing. In parasitic Hymenoptera the base of the femur has the appearance of a second trochanter. In most insects the femur is the largest region of the leg; the tibia is the fourth section of the typical insect leg. As a rule the tibia of an insect is slender in comparison to the femur, but it is at least as long and longer. Near the dis
In biology, an organism is any individual entity that exhibits the properties of life. It is a synonym for "life form". Organisms are classified by taxonomy into specified groups such as the multicellular animals and fungi. All types of organisms are capable of reproduction and development, some degree of response to stimuli. Humans are multicellular animals composed of many trillions of cells which differentiate during development into specialized tissues and organs. An organism may be either a eukaryote. Prokaryotes are represented by two separate domains -- archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles. Fungi and plants are examples of kingdoms of organisms within the eukaryotes. Estimates on the number of Earth's current species range from 10 million to 14 million, of which only about 1.2 million have been documented. More than 99% of all species, amounting to over five billion species, that lived are estimated to be extinct.
In 2016, a set of 355 genes from the last universal common ancestor of all organisms was identified. The term "organism" first appeared in the English language in 1703 and took on its current definition by 1834, it is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings, going back at least to Immanuel Kant's 1790 Critique of Judgment. An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. Dictionary definitions can be broad, using phrases such as "any living structure, such as a plant, fungus or bacterium, capable of growth and reproduction". Many definitions exclude viruses and possible man-made non-organic life forms, as viruses are dependent on the biochemical machinery of a host cell for reproduction. A superorganism is an organism consisting of many individuals working together as a single functional or social unit. There has been controversy about the best way to define the organism and indeed about whether or not such a definition is necessary.
Several contributions are responses to the suggestion that the category of "organism" may well not be adequate in biology. Viruses are not considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism; this controversy is problematic because some cellular organisms are incapable of independent survival and live as obligatory intracellular parasites. Although viruses have a few enzymes and molecules characteristic of living organisms, they have no metabolism of their own; this rules out autonomous reproduction: they can only be passively replicated by the machinery of the host cell. In this sense, they are similar to inanimate matter. While viruses sustain no independent metabolism and thus are not classified as organisms, they do have their own genes, they do evolve by mechanisms similar to the evolutionary mechanisms of organisms; the most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly.
Some scientists argue. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible; this is not true for cells. If viruses did not exist, the direction of cellular evolution could be different, but cells would be able to evolve; as for the reproduction, viruses rely on hosts' machinery to replicate. The discovery of viral metagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong in the tree of life; the presence of these genes suggested. However, it was found that the genes coding for energy and protein metabolism have a cellular origin. Most these genes were acquired through horizontal gene transfer from viral hosts. Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival; the same laws that govern non-living chemistry govern the chemical processes of life.
It is the phenomena of entire organisms that determine their fitness to an environment and therefore the survivability of their DNA-based genes. Organisms owe their origin and many other internal functions to chemical phenomena the chemistry of large organic molecules. Organisms are complex systems of chemical compounds that, through interaction and environment, play a wide variety of roles. Organisms are semi-closed chemical systems. Although they are individual units of life, they are not closed to the environment around them. To operate they take in and release energy. Autotrophs produce usable energy using light from the sun or inorganic compounds while heterotrophs take in organic compounds from the environment; the primary chemical element in these compounds is carbon. The chemical properties of this element such as its grea
Insects or Insecta are hexapod invertebrates and the largest group within the arthropod phylum. Definitions and circumscriptions vary; as used here, the term Insecta is synonymous with Ectognatha. Insects have a chitinous exoskeleton, a three-part body, three pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals; the total number of extant species is estimated at between ten million. Insects may be found in nearly all environments, although only a small number of species reside in the oceans, which are dominated by another arthropod group, crustaceans. Nearly all insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts; the immature stages differ from the adults in structure and habitat, can include a passive pupal stage in those groups that undergo four-stage metamorphosis. Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of 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 dragonflies with wingspans of 55 to 70 cm; the most diverse insect groups appear to have coevolved with flowering plants. Adult insects move about by walking, flying, or sometimes swimming; as it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles, composed of the front & rear on one side with the middle on the other side. Insects are the only invertebrates to have evolved flight, all flying insects derive from one common ancestor. Many insects spend at least part of their lives under water, with larval adaptations that include gills, some adult insects are aquatic and have adaptations for swimming; some species, such as water striders, are capable of walking on the surface of water. Insects are solitary, but some, such as certain bees and termites, are social and live in large, well-organized colonies.
Some insects, such as earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in 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 certain insects as pests, attempt to control them using insecticides, a host of other techniques; some insects damage crops by feeding on sap, fruits, or wood. Some species are parasitic, may vector diseases; some insects perform complex ecological roles. Insect pollinators are essential to the life cycle of many flowering plant species on which most organisms, including humans, are at least dependent. Many insects are considered ecologically beneficial as predators and a few provide direct economic benefit. Silkworms produce silk and honey bees produce honey and both have been domesticated by humans.
Insects are consumed as food in 80% of the world's nations, by people in 3000 ethnic groups. Human activities have effects on insect biodiversity; the word "insect" comes from the Latin word insectum, meaning "with a notched or divided body", or "cut into", from the neuter singular perfect passive participle of insectare, "to cut into, to cut up", from in- "into" and secare "to cut". A calque of Greek ἔντομον, "cut into sections", Pliny the Elder introduced the Latin designation as a loan-translation of the Greek word ἔντομος or "insect", Aristotle's term for this class of life in reference to their "notched" bodies. "Insect" first appears documented in English in 1601 in Holland's translation of Pliny. Translations of Aristotle's term form the usual word for "insect" in Welsh, Serbo-Croatian, etc; the precise definition of the taxon Insecta and the equivalent English name "insect" varies. In the broadest circumscription, Insecta sensu lato consists of all hexapods. Traditionally, insects defined in this way were divided into "Apterygota" —the wingless insects—and Pterygota—the winged insects.
However, modern phylogenetic studies have shown that "Apterygota" is not monophyletic, so does not form a good taxon. A narrower circumscription restricts insects to those hexapods with external mouthparts, comprises only the last three groups in the table. In this sense, Insecta sensu stricto is equivalent to Ectognatha. In the narrowest circumscription, insects are restricted to hexapods that are either winged or descended from winged ancestors. Insecta sensu strictissimo is equivalent to Pterygota. For the purposes of this article, the middle definition is used; the evolutionary relationship of insects to other animal groups remains unclear. Although traditionally grouped with millipedes and centiped
A homeobox is a DNA sequence, around 180 base pairs long, found within genes that are involved in the regulation of patterns of anatomical development in animals and plants. These genes encode homeodomain protein products that are transcription factors sharing a characteristic protein fold structure that binds DNA; the "homeo-" prefix in the words "homeobox" and "homeodomain" stems from the mutational phenotype known as "homeosis", observed when these genes are mutated in animals. Homeosis is a term coined by William Bateson to describe the outright replacement of a discrete body part with another body part. Homeobox genes are not only found in animals, but have been found in fungi, for example the unicellular yeasts, in plants, numerous single cell eukaryotes. Homeoboxes were discovered independently in 1983 by Ernst Hafen, Michael Levine, William McGinnis working in the lab of Walter Jakob Gehring at the University of Basel, Switzerland; the existence of homeobox genes were first discovered in Drosophila, where mutations in homeobox genes caused the radical alterations known as "homeotic transformations".
One of the most famous such mutation is antennapedia, in which legs grow from the head of a fly instead of the expected antennae. A homeobox is about 180 DNA base pairs long and encodes a protein domain that binds DNA; the following shows the consensus homeodomain: Helix 1 Helix 2 Helix 3/4 ______________ __________ _________________ RRRKRTAYTRYQLLELEKEFHFNRYLTRRRRIELAHSLNLTERHIKIWFQNRRMKWKKEN....|....|....|....|....|....|....|....|....|....|....|....| 10 20 30 40 50 60 The characteristic homeodomain protein fold consists of a 60-amino acid long domain composed of three alpha helixes. Helix 2 and helix 3 form a so-called helix-turn-helix structure, where the two alpha helices are connected by a short loop region; the N-terminal two helices of the homeodomain are antiparallel and the longer C-terminal helix is perpendicular to the axes established by the first two. It is this third helix that interacts directly with DNA via a number of hydrogen bonds and hydrophobic interactions, as well as indirect interactions via water molecules, which occur between specific side chains and the exposed bases within the major groove of the DNA.
Homeodomain proteins are found in eukaryotes. Through the HTH motif, they share limited sequence similarity and structural similarity to prokaryotic transcription factors, such as lambda phage proteins that alter the expression of genes in prokaryotes; the HTH motif shows some sequence similarity but a similar structure in a wide range of DNA-binding proteins. One of the principal differences between HTH motifs in these different proteins arises from the stereo-chemical requirement for glycine in the turn, needed to avoid steric interference of the beta-carbon with the main chain: for cro and repressor proteins the glycine appears to be mandatory, whereas for many of the homeotic and other DNA-binding proteins the requirement is relaxed. Homeodomains can bind both and nonspecifically to B-DNA with the C-terminal recognition helix aligning in the DNA's major groove and the unstructured peptide "tail" at the N-terminus aligning in the minor groove; the recognition helix and the inter-helix loops are rich in arginine and lysine residues, which form hydrogen bonds to the DNA backbone.
Homeodomain proteins show a preference for the DNA sequence 5'-TAAT-3'. Through the DNA-recognition properties of the homeodomain, homeoproteins are believed to regulate the expression of targeted genes and direct the formation of many body structures during early embryonic development. Many homeodomain proteins induce cellular differentiation by initiating the cascades of coregulated genes required to produce individual tissues and organs. Other proteins in the family, such as NANOG are involved in maintaining pluripotency. Homeobox genes are critical in the establishment of body axes during embryogenesis. Homeoprotein transcription factors switch on cascades of other genes; the homeodomain binds DNA in a sequence-specific manner. However, the specificity of a single homeodomain protein is not enough to recognize only its desired target genes. Most of the time, homeodomain proteins act in the promoter region of their target genes as complexes with other transcription factors; such complexes have a much higher target specificity than a single homeodomain protein.
Homeodomains are encoded both by genes of the Hox gene clusters and by other genes throughout the genome. The homeobox domain was first identified in a number of Drosophila homeotic and segmentation proteins, but is now known to be well-conserved in many other animals, including vertebrates. Specific members of the Hox family have been implicated in vascular remodeling and disease by orchestrating changes in matrix degradation and components of the ECM. HoxA5 is implicated in atherosclerosis. HoxD3 and HoxB3 are proinvasive, angiogenic genes that upregulate b3 and a5 integrins and Efna1 in ECs, respectively. HoxA3 induces endothelial cell migration by upregulating MMP14 and uPAR. Conversely, HoxD10 and HoxA5 have the opposite effect of suppressing EC migration and angiogenesis, stabilizing adherens junctions by upregulating TIMP1/downregulating uPAR and MMP14, by upregulating Tsp2/downregulating VEGFR2, Efn
Crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, crayfish, krill and barnacles. The crustacean group is treated as a subphylum, because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, comprises all animals in the Pancrustacea clade other than hexapods; some crustaceans are more related to insects and other hexapods than they are to certain other crustaceans. The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm, to the Japanese spider crab with a leg span of up to 3.8 m and a mass of 20 kg. Like other arthropods, crustaceans have an exoskeleton, they are distinguished from other groups of arthropods, such as insects and chelicerates, by the possession of biramous limbs, by their larval forms, such as the nauplius stage of branchiopods and copepods. Most crustaceans are free-living aquatic animals, but some are terrestrial, some are parasitic and some are sessile; the group has an extensive fossil record, reaching back to the Cambrian, includes living fossils such as Triops cancriformis, which has existed unchanged since the Triassic period.
More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as fished, but may be the animals with the greatest biomass on the planet, form a vital part of the food chain; the scientific study of crustaceans is known as carcinology, a scientist who works in carcinology is a carcinologist. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, the pereon or thorax, the pleon or abdomen; the head and thorax may be fused together to form a cephalothorax, which may be covered by a single large carapace. The crustacean body is protected by the hard exoskeleton, which must be moulted for the animal to grow; the shell around each somite can be divided into a dorsal tergum, ventral sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together; each somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae.
The abdomen bears pleopods, ends in a telson, which bears the anus, is flanked by uropods to form a tail fan. The number and variety of appendages in different crustaceans may be responsible for the group's success. Crustacean appendages are biramous, meaning they are divided into two parts, it is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites, for instance possessed biramous appendages; the main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods and branchiopods have haemoglobins; the alimentary canal consists of a straight tube that has a gizzard-like "gastric mill" for grinding food and a pair of digestive glands that absorb food. Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, a collection of major ganglia is found below the gut.
In many decapods, the first pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs; the majority of crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, woodlice. Marine crustaceans are as ubiquitous in the oceans; the majority of crustaceans are motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts, adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will switch hosts from marine to non-marine species. Krill are the bottom layer and the most important part of the food chain in Antarctic animal communities.
Some crustaceans are significant invasive species, such as the Chinese mitten crab, Eriocheir sinensis, the Asian shore crab, Hemigrapsus sanguineus. The majority of crustaceans have separate sexes, reproduce sexually. A small number are hermaphrodites, including barnacles and Cephalocarida; some may change sex during the course of their life. Parthenogenesis is widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male; this occurs in many branchiopods, some os
Mandible (arthropod mouthpart)
The mandible of an arthropod is a pair of mouthparts used either for biting or cutting and holding food. Mandibles are simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda and Hexapoda; these groups make up the clade Mandibulata, believed to be the sister group to the rest of arthropods, the clade Arachnomorpha. Unlike the chelicerae of arachnids, mandibles can be used to chew food. Mandibulates differ by having antennae, by having three distinct body regions: head and abdomen. Insect mandibles are as diverse in form as their food. For instance and many other plant-eating insects have sharp-edged mandibles that move side to side. Most butterflies and moths lack mandibles as they feed on nectar from flowers. Queen bees have mandibles with sharp cutting teeth unlike worker bees. Male dobsonflies have slender mandibles half as long as the insect's main body. Potter wasps use their mandibles to mix droplets of water with clay. Ants have long, serrated jaws, used for digging, collecting food and cutting, are the most important work tool ants possess.
Ants bite each other when fighting. Some ants use mandibles to injure the squirt poison into the wound. Harvester ants use their mandibles to carry seeds. Army ants have sharp mandibles that are better adapted for fighting than obtaining food or nursing the larvae. Carpenter ants make their nests in various wooden structures, which they hollow out with their sharp mandibles; the shape and size of beetle mandibles varies from species to species depending on the food preferences. For example, carnivorous beetles have extended mandibles to crush prey. Tiger beetles' mandibles are well adapted for killing prey. Diving beetle and firefly larvae have hollow mandibles, which can inject digestive fluid to liquefy the tissues of the prey; when this process is over, they suck the digested tissue through the mandibles. The antlerlike jaws of stag beetles are their namesake trait. In some tropical species they can be up to 10 cm, as long as the body of the beetle; these mandibles are used in combat. Caterpillars use sharp mandibles to cut leaves in side-to-side motions.
Only a few moths have functional mandibles in the adult stage. The most notable example are members of the family Micropterigidae, small moths with toothed mandibles used for chewing pollen grains, lacking the most rudimentary proboscis. Among myriapods, centipedes have strong, bristly mandibles, which have a row of teeth in all centipedes except for members of the order Geophilomorpha. Millipedes have small mandibles, their only functioning mouthparts, as the maxillae are fused to the lower lip. Crustaceans have a pair of mandibles that consist of an enlarged basal segment and a palp consisting of all other segments. In some groups, such as the Branchiopoda, the palp is absent. Crustacean mandibles may be equipped with special teeth. Mandibulata Encarta Reference Library Premium 2005 DVD