An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-
The Mygalomorphae or mygalomorphs are an infraorder of spiders. The name is derived from meaning "shrew", plus morphē meaning form or shape. An older name for the group is Orthognatha, derived from the orientation of the fangs which point straight down and do not cross each other; this group of spiders comprises heavy-bodied, stout-legged spiders including tarantulas, Australian funnel-web spiders, mouse spiders, various families of spiders called trapdoor spiders. Like the "primitive" suborder of spiders Mesothelae, they have two pairs of book lungs, downward-pointing chelicerae; because of this, the two groups were once believed to be related. It was realized that the common ancestors of all spiders had these features. Following the branching into the suborders of Mesothelae and Opisthothelae, the mygalomorphs retained them, while their fellow Opisthothelae members, the araneomorphs, evolved new "modern" features, including a cribellum and cross-acting fangs. Mesotheles retain the external abdominal segmentation of ancestral arachnids and have at least vestiges of four pairs of spinnerets, whereas mygalomorphs lack abdominal segmentation and have a reduced number of spinnerets only two pairs.
Like spiders in general, most species of Mygalomorphae have eight eyes, one pair of principal and three pairs of secondary eyes. Their chelicerae and fangs are large and powerful and have ample venom glands that lie within their chelicerae; these weapons, combined with their size and strength, make. Many of these spiders are well adapted to killing other large arthropods and will sometimes kill small mammals and reptiles. Despite their fearsome appearance and reputation, most mygalomorph spiders are not harmful to humans, with the exception of the Australian funnel-web spiders those of the genus Atrax. While the world's biggest spiders are mygalomorphs —Theraphosa blondi has a body length of 10 cm and a leg span of 28 cm —some species are less than one millimeter long. Mygalomorphs are capable of spinning at least adhesive silk, some build elaborate capture webs that approach a meter in diameter. Unlike Araneomorphae, which die after about a year, Mygalomorphae can live for up to 25 years, some do not reach maturity until they are about six years old.
Some flies in the family Acroceridae that are endoparasites of mygalomorphs may remain dormant in their book lungs for as long as 20 years before beginning their development and consuming the spider. One female trapdoor spider, first recorded in a survey in 1974 in Western Australia, is known to have lived for 43 years. Megarachne servinei was thought to be a giant mygalomorph from the Upper Carboniferous, but was found to be a eurypterid; the oldest known mygalomorph is Rosamygale grauvogeli from the Triassic of north-east France. No mygalomorphs from the Jurassic have yet been found; the number of families and their relationships have both been undergoing substantial changes since a cladogram showing family relationships was published in 2005, with two significant studies in 2018. The division of Mygalomorphae into two superfamilies and Avicularioidea, has been established in many studies; the Atypoidea retain some vestiges of abdominal segmentation in the form of dorsal tergites. Molecular phylogenetic studies undertaken between 2012 and 2017 have found somewhat different relationships within the Avicularioidea.
Some families appear not to be monophyletic and further changes are possible in future. AtypoideaAntrodiaetidae Atypidae – purseweb spiders Mecicobothriidae – dwarf tarantulas AvicularioideaActinopodidae Atracidae – Australian funnel-web spiders Barychelidae – brushed trapdoor spiders Ctenizidae Cyrtaucheniidae – wafer trapdoor spiders Dipluridae – curtain-web spiders Euctenizidae Halonoproctidae Hexathelidae Idiopidae – armoured trapdoor spiders Macrothelidae Microstigmatidae Migidae – tree trapdoor spiders Nemesiidae Paratropididae – baldlegged spiders Porrhothelidae Theraphosidae – tarantulas Most members of this infraorder occur in the tropics and subtropics, but their range can extend farther north, e.g. into the southern and western regions of the United States. Only a few species occur in Europe; these are of the families Atypidae, Ctenizidae, Macrothelidae and Cyrtaucheniidae, together with only a dozen species. However, it is suggested that the Mygalomorphae were distributed worldwide before the breakup of Pangaea.
Raven, R. J.. The spider infraorder Mygalomorphae: Cladistics and systematics. Bull. Am. Mus. Nat. Hist. 182:1-180. Goloboff, P. A.. A Reanalysis of Mygalomorphae Spider Families. American Museum Novitates 3056. PDF Taxonomy and captive breeding of Dipluridae Mygalomorphae or primitive spiders Suborder Mygalomorphae
Sea spiders called Pantopoda or pycnogonids, are marine arthropods of class Pycnogonida. They are cosmopolitan, found in oceans around the world. There are with a leg span ranging from 1 mm to over 70 cm. Most are toward the smaller end of this range in shallow depths. Although "sea spiders" are not true spiders, or arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans; this is in dispute, however, as genetic evidence suggests they may be the sister group to all other living arthropods. Sea spiders have long legs in contrast to a small body size; the number of walking legs is eight, but species with five and six pairs exist. Because of their thin body and legs, no respiratory system is necessary, with gases moving by diffusion. A proboscis allows them to suck nutrients from soft-bodied invertebrates, their digestive tract has diverticula extending into the legs. Certain Pycnogonids are so small that each of their tiny muscles consists of only one single cell, surrounded by connective tissue.
The anterior region consists of the proboscis, which has limited dorsoventral and lateral movement, three to four appendages including the ovigers, which are used in caring for young and cleaning as well as courtship. In some species, the chelifores and ovigers can be reduced or missing in adults. In those species that lack chelifores and palps, the proboscis is well developed and more mobile and flexible equipped with numerous sensory bristles and strong rasping ridges around the mouth; the last segment includes the tubercle, which projects dorsally. In total, pycnogonids have four to six pairs of legs for walking as well as other appendages which resemble legs. A cephalothorax and much smaller abdomen make up the reduced body of the pycnogonid, which has up to two pairs of dorsally located simple eyes on its non-calcareous exoskeleton, though sometimes the eyes can be missing among species living in the deep oceans; the abdomen does not have any appendages, in most species it is reduced and vestigial.
The organs of this chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone. The morphology of the sea spider creates an well suited surface-area to volume ratio for any respiration to occur through direct diffusion. Oxygen is transported via the hemolymph to the rest of the body; the most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a moult. The small, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure; the beating of the sea spider heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs. Hemolymph circulation in the legs is driven by the peristaltic movement in the part of the gut that extends into every leg; these creatures possess an open circulatory system as well as a nervous system consisting of a brain, connected to two ventral nerve cords, which in turn connect to specific nerves.
All pycnogonid species have separate sexes, except for one species, hermaphroditic. Females possess a pair of ovaries, while males possess a pair of testes located dorsally in relation to the digestive tract. Reproduction involves external fertilisation after "a brief courtship". Only males care for young; the larva has a blind gut and the body consists of a head and its three pairs of cephalic appendages only: the chelifores and ovigers. The abdomen and the thorax with its thoracic appendages develop later. One theory is. At least four types of larvae have been described: the typical protonymphon larva, the encysted larva, the atypical protonymphon larva, the attaching larva; the typical protonymphon larva is most common, is free living and turns into an adult. The encysted larva is a parasite that hatches from the egg and finds a host in the shape of a polyp colony where it burrows into and turns into a cyst, will not leave the host before it has turned into a young juvenile. Not much is known about the development of the atypical protonymphon larva.
The adults are free living, while the larvae and the juveniles are living on or inside temporary hosts such as polychaetes and clams. When the attaching larva hatches it still looks like an embryo, attaches itself to the ovigerous legs of the father, where it will stay until it has turned into a small and young juvenile with two or three pairs of walking legs ready for a free-living existence; these small animals live in many different parts of the world, from Australia, New Zealand, the Pacific coast of the United States, to the Mediterranean Sea and the Caribbean Sea, to the north and south poles. They are most common in shallow waters, but can be found as deep as 7,000 metres, live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines. Sea spiders either walk along the bottom with their stilt-like legs or swim just above it using an umbrella pulsing motion. Sea spiders are carnivorous predators or scavengers that feed on c
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
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
Horseshoe crabs are marine and brackish water arthropods of the family Limulidae, suborder Xiphosurida, order Xiphosura. Their popular name is a misnomer, for they are not true crabs. Horseshoe crabs live in and around shallow coastal waters on soft sandy or muddy bottoms, they tend to spawn in the intertidal zone at spring high tides. They are eaten in Asia, used as fishing bait, in fertilizer and in science. In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. Tetrodotoxin may be present in Carcinoscorpius rotundicauda; because of their origin 450 million years ago, horseshoe crabs are considered living fossils. A 2019 molecular analysis places them as the sister group of Ricinulei within Arachnida. Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods and are related to arachnids. Horseshoe crabs are related to the extinct eurypterids, which include some of the largest arthropods to have existed, the two may be sister groups.
The earliest horseshoe crab fossils are found in strata from the late Ordovician period 450 million years ago. The Limulidae are the only recent family of the order Xiphosura, contains all four living species of horseshoe crabs: Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in South and Southeast Asia Limulus polyphemus, the Atlantic or American horseshoe crab, found along the American Atlantic coast and in the Gulf of Mexico Tachypleus gigas, the Indo-Pacific, Indian or southern horseshoe crab, found in South and Southeast Asia Tachypleus tridentatus, the Chinese, Japanese or tri-spine horseshoe crab, found in Southeast and East Asia The entire body of the horseshoe crab is protected by a hard carapace, it has two compound lateral eyes, each composed of about 1,000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, a pair of rudimentary lateral eyes on the top. The latter become functional.
A pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson. The horseshoe crab has five additional eyes on top of its shell. Despite having poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans', their eyes are a million times more sensitive to light at night than during the day; the mouth is located in the center of the legs, whose bases are referred to as gnathobases and have the same function as jaws and help grind up food. The horseshoe crab has five pairs of legs for walking and moving food into the mouth, each with a claw at the tip, except for the last pair. Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, are occasionally used for swimming; as in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills. They are more found on the ocean floor searching for worms and molluscs, which are their main food.
They may feed on crustaceans and small fish. Females are about 20–30% larger than males; the smallest species is the largest is T. tridentatus. On average, males of C. rotundicauda are about 30 cm long, including a tail, about 15 cm, their carapace is about 15 cm wide. Some southern populations of L. polyphemus are somewhat smaller, but otherwise this species is larger. In the largest species, T. tridentatus, females can reach as much as 79.5 cm long, including their tail, up to 4 kg in weight. This is only about 10–20 cm longer than the largest females of L. polyphemus and T. gigas, but twice the weight. The juveniles grow about 33% larger with every molt until reaching adult size. During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male selects a female and clings to her back. Several males surround the female and all fertilize together, which makes it easy to spot and count females as they are the large center carapace surrounded by 3-5 smaller ones; the female lays her eggs while the male fertilize them.
The female can lay between 120,000 eggs in batches of a few thousand at a time. In L. polyphemus, the eggs take about two weeks to hatch. The larvae molt six times during the first year. Natural breeding of horseshoe crabs in captivity has proven to be difficult; some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. It is not known with certainty what is in the sand how they sense it. Artificial insemination and induced spawning have been done on a large scale in captivity, eggs and juveniles collected from the wild are raised to adulthood in captivity. Horseshoe crabs use hemocyanin to carry oxygen through their blood; because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a similar role to the white blood cells of vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, used for the detection of bacterial endotoxins in medical applications.
This means there is a high demand for the blood, the harvest of which involves collecting and bleeding the animal