Tarantulas comprise a group of large and hairy arachnids belonging to the Theraphosidae family of spiders, of which about 900 species have been identified. This article only describes members of the Theraphosidae, although some other members of the same infraorder are referred to as "tarantulas"; some species have become popular in the exotic pet trade. New World species kept as pets have urticating hairs that can cause irritation to the skin and, in extreme cases, cause damage to eyes. Like all arthropods, the tarantula is an invertebrate that relies on an exoskeleton for muscular support. Like other Arachnida, a tarantula's body comprises the prosoma and the opisthosoma; the prosoma and opisthosoma are connected by the pregenital somite. This waist-like connecting piece is part of the prosoma and gives the opisthosoma a wide range of motion relative to the prosoma. Tarantula sizes range from as small as a fingernail to as large as a dinner plate when the legs are extended. Depending on the species, the body length of tarantulas ranges from 2.5 to 10 cm, with leg spans of 8–30-centimetre.
Leg span is determined by measuring from the tip of the back leg to the tip of the front leg on the opposite side. Some of the largest species of tarantula may weigh over 85 g; the fang size of this tarantula reaches a maximum of 3.8 cm. Theraphosa apophysis was described 187 years after the goliath birdeater, so its characteristics are not as well attested. Theraphosa blondi is thought to be the heaviest tarantula, T. apophysis to have the greatest leg span. Two other species, Lasiodora parahybana and Lasiodora klugi, rival the size of the two goliath spiders. Most species of North American tarantulas are brown. Elsewhere, species have been found that variously display cobalt blue, black with white stripes, yellow leg markings, metallic blue legs with vibrant orange abdomen and green prosoma, their natural habitats include savanna, grasslands such as the pampas, deserts, scrubland and cloud forests. They are classed among the terrestrial types, they are burrowers. Tarantulas are becoming popular as pets and some species are available in captivity.
The spider bearing the name "tarantula" was Lycosa tarantula, a species of wolf spider native to Mediterranean Europe. The name derived from that of the southern Italian town of Taranto; the term "tarantula" subsequently was applied to any large, unfamiliar species of ground-dwelling spider, in particular to the Mygalomorphae and to the New World Theraphosidae. Compared to tarantulas, wolf spiders are not large or hairy, so among English speakers in particular, the usage shifted in favour of the Theraphosidae, though they are related to the wolf spiders, being in a different infraorder; when theraphosids were encountered in the Americas, they were named "tarantulas", causing usage of the term to shift to the tropical spiders. These spiders belong to the infraorder Mygalomorphae, are not related to wolf spiders; the name "tarantula" is incorrectly applied to other large-bodied spiders, including the purseweb spiders or atypical tarantulas, the funnel-webs, the "dwarf tarantulas". These spiders are classified in different families.
Huntsman spiders of the family Sparassidae have been termed "tarantulas" because of their large size. In fact, they are not related. Tarantulas of various species occur throughout the United States, Mexico, in Central America, throughout South America. Other species occur variously throughout Africa, much of Asia, all of Australia. In Europe, some species occur in Spain, Turkey, south Italy, Cyprus; some genera of tarantulas hunt prey in trees. All tarantulas can produce silk – while arboreal species reside in a silken "tube tent", terrestrial species line their burrows with silk to stabilize the burrow wall and facilitate climbing up and down. Tarantulas eat large insects and other arthropods such as centipedes and other spiders, using ambush as their primary method of prey capture. Armed with their massive, powerful chelicerae tipped with long chitinous fangs, tarantulas are well-adapted to killing other large arthropods; the biggest tarantulas sometimes kill and consume small vertebrates such as lizards, bats and small snakes.
The eight legs, the two chelicerae with their fangs, the pedipalps are attached to the prosoma. The chelicerae are two double-segmented appendages located just below the eyes and directly forward of the mouth; the chelicerae contain. The fangs are hollow extensions of the chelicerae that inject venom into prey or animals that the tarantula bites in defense, they are used to masticate; these fangs are articulated so that they can extend downward and outward in preparation to bite or can fold back toward the chelicerae as a pocket knife blade folds back into its handle. The chelicerae of a tarantula contain the venom glands and the muscles that
Simple eye in invertebrates
A simple eye refers to a type of eye form or optical arrangement that contains a single lens. A "simple eye" is so called in distinction from a multi-lensed "compound eye", is not at all simple in the usual sense of the word; the eyes of humans and large animals, camera lenses are classed as "simple" because in both cases a single lens collects and focuses light onto the retina or film. Many insects have compound eyes consisting of multiple lenses, each focusing light onto a small number of retinula cells; the structure of an animal's eye is determined by the environment in which it lives, the behavioural tasks it must fulfill to survive. Arthropods differ in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, avoiding predators. An enormous variety of eye designs are found in arthropods: they possess a wide variety of novel solutions to overcome visual problems or limitations; some jellyfish, sea stars and ribbonworms bear the simplest eyes, pigment spot ocelli, which have pigment distributed randomly and which have no additional structures such as a cornea and lens.
The apparent eye color in these animals is therefore black. However, other cnidaria have more complex eyes, including those of Cubomedusae which have distinct retina and cornea. Many snails and slugs have ocelli, either at the tips or at the bases of the tentacles. However, some other gastropods, such as the Strombidae, have much more sophisticated eyes. Giant clams have ocelli. Spiders do not have compound eyes, but instead have several pairs of simple eyes with each pair adapted for a specific task or tasks; the principal and secondary eyes in spiders are arranged in four or more pairs. Only the principal eyes have moveable retinas; the secondary eyes have a reflector at the back of the eyes. The light-sensitive part of the receptor cells is next to this, so they get direct and reflected light. In hunting or jumping spiders, for example, a forward-facing pair possesses the best resolution to see the prey at a large distance. Night-hunting spiders' eyes are sensitive in low light levels with a large aperture, f/0.58.
The term "ocellus" is derived from the Latin oculus, means "little eye". Two distinct ocellus types exist: dorsal ocelli, found in most insects, lateral ocelli, which are found in the larvae of some insect orders, they are structurally and functionally different. Simple eyes of other animals, e.g. cnidarians, may be referred to as ocelli, but again the structure and anatomy of these eyes is quite distinct from those of the dorsal ocelli of insects. Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects, e.g. Hymenoptera, Diptera and Orthoptera; the ocelli coexist with the compound eyes. The number and functions of the dorsal ocelli vary markedly throughout insect orders, they tend to be larger and more expressed in flying insects, where they are found as a triplet. Two lateral ocelli are directed to the left and right of the head while a central ocellus is directed frontally. In some terrestrial insects, only two lateral ocelli are present: the median ocellus is absent.
The labelled "lateral ocelli" here refers to the sideways-facing position of the ocelli, which are of the dorsal type. They should not be confused with the lateral ocelli of some insect larvae. A dorsal ocellus consists of a layer of photoreceptors; the ocellar lens may be curved or flat. The photoreceptor layer may not be separated from the lens by a clear zone; the number of photoreceptors varies but may number in the hundreds or thousands for well-developed ocelli. Two somewhat unusual features of the ocelli are notable and well conserved between insect orders; the refractive power of the lens is not sufficient to form an image on the photoreceptor layer. Dorsal ocelli ubiquitously have massive convergence ratios from first-order to second-order neurons; these two factors have led to the conclusion that the dorsal ocelli are incapable of perceiving form, are thus suitable for light-metering functions. Given the large aperture and low f-number of the lens, as well as high convergence ratios and synaptic gains, the ocelli are considered to be far more sensitive to light than the compound eyes.
Additionally, given the simple neural arrangement of the eye, as well as the large diameter of some ocellar interneurons, the ocelli are considered to be "faster" than the compound eyes. One common theory of ocellar function in flying insects holds that they are used to assist in maintaining flight stability. Given their underfocused nature, wide fields of view, high light-collecting ability, the ocelli are superbly adapted for measuring changes in the perceived brightness of the external world as an insect rolls or pitches arou
Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, crayfish, krill, amphipods, mantis shrimp and many other, less familiar animals, they have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments, divided into a head and abdomen; the name Malacostraca was coined by a French zoologist Pierre André Latreille in 1802. He was curator of the arthropod collection at the National Museum of Natural History in Paris; the name comes from the Greek roots μαλακός and ὄστρακον. The name is misleading, since the shell is only soft after moulting, is hard. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, named after the obsolete taxon Entomostraca; the class Malacostraca includes about 40,000 species, "arguably... contains a greater diversity of body forms than any other class in the animal kingdom".
Its members are characterised by the presence of three tagmata – a five-segmented head, an eight-segmented thorax and an abdomen with six segments and a telson, except in the Leptostraca, which retain the ancestral condition of seven abdominal segments. Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa except Remipedia; each body segment bears a pair of jointed appendages. The head bears two pairs of antennae, the first of, biramous and the second pair bear exopods which are flattened into antennal scales known as scaphocerites; the mouthparts consist of pairs each of mandibles and maxillae. A pair of stalked compound eyes is present, although in some taxa the eyes are unstalked, reduced or lost. Up to three thoracic segments may be fused with the head to form a cephalothorax. A carapace may be absent, present or secondarily lost, may cover the head, part or all of the thorax and some of the abdomen, it is variable in form and may be fused dorsally with some of the thoracic segments or be in two parts, hinged dorsally.
Each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments. In decapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous. There is the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, gas exchange, creating a current or brooding eggs; the first and second abdominal pleopods may be modified in the male to form gonopods. The appendages of the last segment are flattened into uropods, which together with the terminal telson, make up the "tail fan", it is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is used in steering. In Leptostraca, the appendages on the telson instead form caudal rami.
The digestive tract is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like "gastric mill" for grinding food. The walls of this have chitinous ridges and calcareous ossicles; the fine particles and soluble material are moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca. The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson. Like other crustaceans, malacostracans have an open circulatory system in which the heart pumps blood into the hemocoel where it supplies the needs of the organs for oxygen and nutrients before diffusing back to the heart; the typical respiratory pigment in malacostracans is haemocyanin. Structures that function as kidneys are located near the base of the antennae. A brain exists in the form of ganglia close to the antennae, there are ganglia in each segment and a collection of major ganglia below the oesophagus.
Sensory organs include compound eyes, ocelli and sensory bristles. The naupliar eye is a characteristic of the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness. Malacostracans live in a wide range of marine and freshwater habitats, three orders have terrestrial members: Amphipoda and Decapoda, they are abundant in all marine ecosystems, most species are scavengers, although some, such as the porcelain crabs, are filter feeders, some, such as mantis shrimps, are carnivores. Most species of malacostracans have distinct sexes; the female genital openings or gonopores are located on the sixth t
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders and several extinct lineages, such as the eurypterids; the Chelicerata originated as marine animals in the Middle Cambrian period. The surviving marine species include the four species of xiphosurans, the 1,300 species of pycnogonids, if the latter are indeed chelicerates. On the other hand, there are over 77,000 well-identified species of air-breathing chelicerates, there may be about 500,000 unidentified species. Like all arthropods, chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; the chelicerate bauplan consists of two tagmata, the prosoma and the opisthosoma, except that mites have lost a visible division between these sections. The chelicerae, which give the group its name, are the only appendages. In most sub-groups, they are modest pincers used to feed. However, spiders' chelicerae form fangs; the group has the open circulatory system typical of arthropods, in which a tube-like heart pumps blood through the hemocoel, the major body cavity.
Marine chelicerates have gills, while the air-breathing forms have both book lungs and tracheae. In general, the ganglia of living chelicerates' central nervous systems fuse into large masses in the cephalothorax, but there are wide variations and this fusion is limited in the Mesothelae, which are regarded as the oldest and most primitive group of spiders. Most chelicerates rely on modified bristles for touch and for information about vibrations, air currents, chemical changes in their environment; the most active hunting spiders have acute eyesight. Chelicerates were predators, but the group has diversified to use all the major feeding strategies: predation, herbivory and eating decaying organic matter. Although harvestmen can digest solid food, the guts of most modern chelicerates are too narrow for this, they liquidize their food by grinding it with their chelicerae and pedipalps and flooding it with digestive enzymes. To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by Malpighian tubules, structures that evolved independently in insects.
While the marine horseshoe crabs rely on external fertilization, air-breathing chelicerates use internal but indirect fertilization. Many species use elaborate courtship rituals to attract mates. Most lay eggs that hatch as what look like miniature adults, but all scorpions and a few species of mites keep the eggs inside their bodies until the young emerge. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young; the evolutionary origins of chelicerates from the early arthropods have been debated for decades. Although there is considerable agreement about the relationships between most chelicerate sub-groups, the inclusion of the Pycnogonida in this taxon has been questioned, the exact position of scorpions is still controversial, though they were long considered the most primitive of the arachnids. Venom has evolved three times in the chelicerates. In addition there have been undocumented descriptions of venom glands in Solifugae.
Chemical defense has been found in whip scorpions, shorttailed whipscorpions, beetle mites and sea spiders. Although the venom of a few spider and scorpion species can be dangerous to humans, medical researchers are investigating the use of these venoms for the treatment of disorders ranging from cancer to erectile dysfunction; the medical industry uses the blood of horseshoe crabs as a test for the presence of contaminant bacteria. Mites can cause allergies in humans, transmit several diseases to humans and their livestock, are serious agricultural pests; the Chelicerata are arthropods as they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. Chelicerates' bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the prosoma or cephalothorax, the rear tagma is called the opisthosoma or abdomen. However, in the Acari there is no visible division between these sections; the prosoma is formed in the embryo by fusion of the acron, which carries the eyes, with segments two to seven, which all have paired appendages, while segment one is lost during the embryo's development.
Segment two has a pair of chelicerae, small appendages that form pincers, segment three has a pair of pedipalps that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments have pairs of legs. In primitive forms the acron has a pair of compound eyes on the sides and four pigment-cup ocelli in the middle; the mouth is between segments three. The opisthosoma consists of twelve or fewer segments that formed two groups, a mesosoma of seven segments and a metasoma of five, terminating with a telson or spike; the abdominal appendages of modern chelicerates
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron. In crustaceans, the carapace functions as a protective cover over the cephalothorax. Where it projects forward beyond the eyes, this projection is called a rostrum; the carapace is calcified to varying degrees in different crustaceans. Zooplankton within the phylum Crustacea have a carapace; these include Cladocera and isopods, but isopods only have a developed "cephalic shield" carapace covering the head. In arachnids, the carapace is formed by the fusion of prosomal tergites into a single plate which carries the eyes, ocularium and diverse phaneres. In a few orders, such as Solifugae and Schizomida, the carapace may be subdivided. In Opiliones, some authors prefer to use the term carapace interchangeably with the term cephalothorax, incorrect usage, because carapace refers only to the dorsal part of the exoskeleton of the cephalothorax.
Alternative terms for the carapace of arachnids and their relatives, which avoids confusion with crustaceans, are prosomal dorsal shield and peltidium. The carapace is the dorsal convex part of the shell structure of a turtle, consisting of the animal's rib cage, dermal armor, scutes
A body plan, Bauplan, or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrate body plan is one of many: invertebrates consist of many phyla; this term applied to animals, envisages a "blueprint" encompassing aspects such as symmetry and limb disposition. Evolutionary developmental biology seeks to explain the origins of diverse body plans. Body plans have been considered to have evolved in a flash in the Cambrian explosion, but a more nuanced understanding of animal evolution suggests gradual development of body plans throughout the early Palaeozoic; the history of the discovery of body plans can be seen as a movement from a worldview centred on the vertebrates, to seeing the vertebrates as one phylum's body plan among many. Among the pioneering zoologists, Linnaeus identified two body plans outside the vertebrates. For comparison, the number of phyla recognised by modern zoologists has risen to 36. In his 1735 book, Systema Naturæ, the Swedish botanist Linnaeus grouped the animals into quadrupeds, birds, "amphibians", fish, "insects" and "worms".
Linnaeus's Vermes included all other groups of animals, not only tapeworms and leeches but molluscs, sea urchins and starfish, jellyfish and cuttlefish. In his 1817 work, Le Règne Animal, the French zoologist Georges Cuvier combined evidence from comparative anatomy and palaeontology to divide the animal kingdom into four body plans. Taking the central nervous system as the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans or embranchements: I. with a brain and a spinal cord II. with organs linked by nerve fibres III. with two longitudinal, ventral nerve cords linked by a band with two ganglia below the oesophagus IV. with a diffuse nervous system, not discernibleGrouping animals with these body plans resulted in four branches: vertebrates, molluscs and zoophytes or radiata. Ernst Haeckel, in his 1866 Generelle Morphologie der Organismen, asserted that all living things were monophyletic, being divided into plants and animals.
His protista were divided into moneres, flagellates, myxomycetes, myxocystodes and sponges. His animals were divided into groups with distinct body plans: he named these phyla. Haeckel's animal phyla were coelenterates and articulates, vertebrates. Stephen J. Gould explored the idea that the different phyla could be perceived in terms of a Bauplan, illustrating their fixity. However, he abandoned this idea in favor of punctuated equilibrium. 20 out of the 36 body plans originated in the Cambrian period, in the "Cambrian explosion", complete body plans of many phyla emerged much in the Palaeozoic or beyond. The current range of body plans is far from exhaustive of the possible patterns for life: the Precambrian Ediacaran biota includes body plans that differ from any found in living organisms though the overall arrangement of unrelated modern taxa is quite similar, thus the Cambrian explosion appears to have more or less replaced the earlier range of body plans. Genes and development together determine the form of an adult organism's body, through the complex switching processes involved in morphogenesis.
Developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes produce morphogens, chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. A key discovery was the existence of groups of homeobox genes, which function as switches responsible for laying down the basic body plan in animals; the homeobox genes are remarkably conserved between species as diverse as the fruit fly and humans, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in humans. The field of animal evolutionary developmental biology, which studies the genetics of morphology in detail, is expanding with many of the developmental genetic cascades in the fruit fly Drosophila, catalogued in considerable detail. Developmental Biology 8e Online: Patterning of the Mesoderm by ActivinVideosThe Science of Evolution: Sean B. Carroll explains the genetics of the fruit fly body plan
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