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Spiders ( order (biology)|order Araneae ) are air-breathing arthropod s that have eight legs and chelicera e with fangs that inject venom . They are the largest order of arachnida|arachnids and rank seventh in total species diversity among all other groups of organisms.Sebastin PA & KV Peter ( ed ). (2009) Spiders of India . Universities Press. ISBN 978-81-7371-641-6 Spiders are found worldwide on every continent except for Antarctica , and have become established in nearly every habitat with the exception of air and sea colonization. As of 2008, approximately 40,000 spider species (biology)|species , and 109 Family (biology)|families have been recorded by Taxonomy|taxonomists ; however, there has been confusion within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900.

Anatomically, spiders differ from other arthropods in that the usual body segmentation (biology)|segments are fused into two Tagma (biology)|tagmata , the cephalothorax and abdomen , and joined by a small, cylindrical pedicel (spider)|pedicel . Unlike insects , spiders do not have Antenna (biology)|antennae . In all except the most primitive group, the Mesothelae , spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure.

Their abdomens bear appendages that have been modified into Spinneret (spider)|spinnerets that extrude spider silk|silk from up to six types of silk glands within their abdomen. Spider web s vary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-web spider s. Spider-like arachnid s with silk-producing spigot s appeared in the Devonian Period (geology)|period about , but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from , and are very similar to the most primitive surviving order (biology)|order , the Mesothelae . The main groups of modern spiders, Mygalomorphae and Araneomorphae , first appeared in the Triassic period, before .

A Herbivory|herbivorous species, Bagheera kiplingi , was described in 2008, but all other known species are predator s, mostly preying on insects and on other spiders, although a few large species also take birds and lizards. Spiders use a wide range of strategies to capture prey: trapping it in sticky webs, lasso ing it with sticky bolas , mimicking the prey to avoid detection, or running it down. Most detect prey mainly by sensing vibrations, but the active hunters have acute vision, and hunters of the genus Portia (spider)|Portia show signs of intelligence in their choice of tactics and ability to develop new ones. Spiders' guts are too narrow to take solids, and they liquidize their food by flooding it with digestive enzyme s and grinding it with the bases of their pedipalp s, as they do not have true jaws.

Male spiders identify themselves by a variety of complex courtship rituals to avoid being eaten by the females. Males of most species survive a few matings, limited mainly by their short life spans. Females weave silk egg-cases, each of which may contain hundreds of eggs. Females of many species care for their young, for example by carrying them around or by sharing food with them. A minority of species are social, building communal webs that may house anywhere from a few to 50,000 individuals. Social behavior ranges from precarious toleration, as in the Latrodectus|widow spiders , to co-operative hunting and food-sharing. Although most spiders live for at most two years, tarantula s and other Mygalomorphae|mygalomorph spiders can live up to 25& nbsp;years in captivity.

While the venom of a few species is dangerous to humans, scientists are now researching the use of spider venom in medicine and as non-polluting pesticide s. Spider silk provides a combination of lightness, strength and elasticity (physics)|elasticity that is superior to that of synthetic materials, and spider silk genes have been inserted into mammal s and plant s to see if these can be used as silk factories. As a result of their wide range of behaviors, spiders have become common symbols in art and mythology symbolizing various combinations of patience, cruelty and creative powers.

Description

Body plan

(1) four pairs of arthropod leg|legs

(2) cephalothorax

(3) opisthosoma (abdomen)
Spiders are chelicerate s and therefore arthropod s. As arthropods they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; heads that are composed of several segments that fuse during the development of the embryo . Being chelicerates, their bodies consist of two Tagma (biology)|tagmata , sets of segments that serve similar functions: the foremost one, called the cephalothorax or prosoma , is a complete fusion of the segments that in an insect would form two separate tagmata, the head (anatomy)|head and thorax ; the rear tagma is called the abdomen or opisthosoma . In spiders the cephalothorax and abdomen are connected by a small cylindrical section, the pedicel. The pattern of segment fusion that forms chelicerates' heads is unique among arthropods, and what would normally be the first head segment disappears at an early stage of development, so that chelicerates lack the Antenna (biology)|antennae typical of most arthropods. In fact chelicerates' only appendage s ahead of the mouth are a pair of chelicera e, and they lack anything that would function directly as "jaws". The first appendages behind the mouth are called pedipalp s, and serve different functions within different groups of chelicerates.

Spiders and scorpion s are members of one chelicerate group, the arachnid s. Scorpions' chelicerae have three sections and are used in feeding. Spiders' chelicerae have two sections and terminate in fang s that are generally venom ous, and fold away behind the upper sections while not in use. The upper sections generally have thick "beards" that filter solid lumps out of their food, as spiders can take only liquid food. Scorpions' pedipalps generally form large claws for capturing prey, while those of spiders are fairly small appendages whose bases also act as an extension of the mouth; in addition those of male spiders have enlarged last sections used for sperm transfer.

In spiders the cephalothorax and abdomen are joined by a small, cylindrical pedicel (spider)|pedicel , which enables the abdomen to move independently when producing silk . The upper surface of the cephalothorax is covered by a single, wikt:convex|convex carapace while the underside is covered by two rather flat plates. The abdomen is soft and egg-shaped. It shows no sign of segmentation, except that the primitive Mesothelae , whose living members are the Liphistiidae , have segmented plates on the upper surface.

Circulation and respiration

Like other arthropods, spiders are coelomate s in which the coelom is reduced to small areas round the reproductive and excretory systems. Its place is largely taken by a hemocoel , a cavity that runs most of the length of the body and through which blood flows. The heart is a tube in the upper part of the body, with a few ostia that act as non-return valves allowing blood to enter the heart from the hemocoel but prevent it from leaving before it reaches the front end. However, in spiders it occupies only the upper part of the abdomen, and blood is discharged into the hemocoel by one artery that opens at the rear end of the abdomen and by branching artery|arteries that pass through the pedicle and open into several parts of the cephalothorax. Hence spiders have open circulatory system s. The blood of many spiders that have book lung s contains the respiratory pigment hemocyanin to make oxygen transport more efficient.

Spiders have developed several different respiratory anatomies, based on book lungs , a invertebrate trachea|tracheal system, or both. Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen . This is also the case for some basal araneomorph spiders like the family Hypochilidae , but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs. The trachea system has most likely evolved in small ancestors to help resist desiccation . The trachea were originally connected to the surroundings through a pair of openings called spiracle s, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and moved backwards close to the Spinneret (spider)|spinnerets . Spiders that have tracheae generally have higher metabolic rate s and better water conservation.

Feeding, digestion and excretion

Uniquely among chelicerate s, the final sections of spiders' chelicerae are fangs, and the great majority of spiders can use them to inject venom into prey from venom gland s in the roots of the chelicerae. The family Uloboridae has lost its poison glands, and is killing the prey with silk instead. Like most arachnid s including scorpion s, spiders have a narrow gut that can only cope with liquid food and spiders have two sets of filters to keep solids out. They use one of two different systems of external digestion. Some pump digestive enzyme s from the midgut into the prey and then suck the liquified tissues of the prey into the gut, eventually leaving behind the empty husk of the prey. Others grind the prey to pulp using the chelicerae and the bases of the pedipalp s, while flooding it with enzymes; in these species the chelicerae and the bases of the pedipalps form a preoral cavity that holds the food they are processing.

The stomach in the cephalothorax acts as a pump that sends the food deeper into the digestive system. The mid gut bears many digestive cecum|ceca , compartments with no other exit, that extract nutrients from the food; most are in the abdomen, which is dominated by the digestive system, but a few are found in the cephalothorax.

Most spiders convert nitrogen ous waste products into uric acid , which can be excreted as a dry material. Malphigian tubules ("little tubes") extract these wastes from the blood in the hemocoel and dump them into the cloaca l chamber, from which they are expelled through the anus . Production of uric acid and its removal via Malphigian tubules are a water-conserving feature that has evolved independently in several arthropod lineages that can live far away from water, for example the tubules of insect s and arachnids develop from completely different parts of the embryo . However a few primitive spiders, the sub- Order (biology)|order Mesothelae and infra-order Mygalomorphae , retain the ancestral arthropod nephridia ("little kidney s"), which use large amounts of water to excrete nitrogenous waste products as ammonia .

Central nervous system

The basic arthropod central nervous system consists of a pair of Ventral nerve cord|nerve cord s running below the gut, with paired ganglia as local control centers in all segments; a brain formed by fusion of the ganglia for the head segments ahead of and behind the mouth, so that the esophagus is encircled by this conglomeration of ganglia. Except for the primitive Mesothelae , of which the Liphistiidae are the sole surviving family, spiders have the much more centralized nervous system that is typical of arachnids: all the ganglia of all segments behind the esophagus are fused, so that the cephalothorax is largely filled with nervous tissue and there are no ganglia in the abdomen; in the Mesothelae, the ganglia of the abdomen and the rear part of the cephalothorax remain unfused.

Sense organs

Eyes

Most spiders have four pairs of eyes on the top-front area of the cephalothorax, arranged in patterns that vary from one family to another. The pair at the front are of the type called pigment-cup ocelli ("little eyes"), which in most arthropod s are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However the main eyes at the front of spiders' heads are pigment-cup ocelli that are capable of forming images. The other eyes are thought to be derived from the compound eye s of the ancestral chelicerate s, but no longer have the separate facets typical of compound eyes. Unlike the main eyes, in many spiders these secondary eyes detect light reflected from a reflective tapetum lucidum , and wolf spider s can be spotted by torch light reflected from the tapeta. On the other hand jumping spiders' secondary eyes have no tapeta. Some jumping spiders' visual acuity exceeds by a factor of ten that of dragonfly|dragonflies , which have by far the best vision among insect s; in fact the human eye is only about five times sharper than a jumping spider's. They achieve this by a telephoto -like series of lenses, a four-layer retina and the ability to swivel their eyes and integrate images from different stages in the scan. The downside is that the scanning and integrating processes are relatively slow.

Other senses

As with other arthropods, spiders' cuticle s would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly bristles called seta e, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and Olfaction|smell , often by means of setae. Spiders also have in the joints of their limbs slit sensilla e that detect forces and vibrations. In web-building spiders all these mechanical and chemical sensors are more important than the eyes, while the eyes are most important to spiders that hunt actively.

Like most arthropods, spiders lack balance and acceleration sensors and rely on their eyes to tell them which way is up. Arthropods' proprioceptor s, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. On the other hand little is known about what other internal sensors spiders or other arthropods may have.

Locomotion

Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, spiders and a few other groups still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors. The only extensor muscles in spider legs are located in the three hip joints (bordering the coxa and the trochanter). As a result a spider with a punctured cephalothorax cannot extend its legs, and the legs of dead spiders curl up. Spiders can generate pressures up to eight times their resting level to extend their legs, and jumping spiders can jump up to 50 times their own length by suddenly increasing the blood pressure in the third or fourth pair of legs. Unlike smaller jumping spiders, though larger spiders use hydraulics to straighten their legs, they depend on their flexor muscles to generate the propulsive force for their jumps.

Most spiders that hunt actively, rather than relying on webs, have dense tufts of fine hairs between the paired claws at the tips of their legs. These tufts, known as scopulae , consist of bristles whose ends are split into as many as 1,000 branches, and enable spiders with scopulae to walk up vertical glass and upside down on ceilings. It appears that scopulae get their grip from contact with extremely thin layers of water on surfaces. Spiders, like most other arachnid s, keep at least four legs on the surface while walking or running.

Silk production

The abdomen has no appendages except those that have been modified to form one to four (usually three) pairs of short, movable Spinneret (spider)|spinnerets , which emit spider silk|silk . Each spinneret has many spigot s, each of which is connected to one silk gland . There are at least six types of silk gland, each producing a different type of silk.

Silk is mainly composed of a protein very similar to that used in insect silk. It is initially a liquid, and hardens not by exposure to air but as a result of being drawn out, which changes the internal structure of the protein. It is similar in tensile strength to nylon and biological materials such as chitin , collagen and cellulose , but is much more Deformation (engineering)|elastic , in other words it can stretch much further before breaking or losing shape.

Some spiders have a cribellum , a modified spinneret with up to 40,000 spigots, each of which produces a single very fine fiber. The fibers are pulled out by the calamistrum , a comb-like set of bristles on the jointed tip of the cribellum, and combined into a composite woolly thread that is very effective in snagging the bristles of insects. The earliest spiders had cribella, which produced the first silk capable of capturing insects, before spiders developed silk coated with sticky droplets. However most modern groups of spiders have lost the cribellum.

Tarantulas also have silk glands in their feet.

Even species that do not build webs to catch prey use silk in several ways: as wrappers for sperm and for fertilized eggs; as a " Safety harness|safety rope "; for nest-building; and as " parachute s" by the young of some species.

anchor|ReproductionReproduction and life cycle

Spiders reproduce sex ually and fertilization is internal fertilization|internal but indirect, in other words the sperm is not inserted into the female's body by the male's genitals but by an intermediate stage. Unlike many land-living arthropod s, male spiders do not produce ready-made spermatophore s (packages of sperm) but spin small sperm webs on to which they ejaculate and then transfer the sperm to syringe -like structures on the tips of their pedipalp s. When a male detects signs of a female nearby he checks whether she is of the same species and whether she is ready to mate; for example in species that produce webs or "safety ropes", the male can identify the species and sex of these objects by "smell".
Spiders generally use elaborate courtship rituals to prevent the large females from eating the small males before fertilization, except where the male is so much smaller that he is not worth eating. In web-weaving species precise patterns of vibrations in the web are a major part of the rituals, while patterns of touches on the female's body are important in many spiders that hunt actively, and may "hypnotize" the female. Gestures and dances by the male are important for jumping spider s, which have excellent eyesight. If courtship is successful, the male injects his sperm from the pedipalps into the female's genital opening, known as the epigyne , on the underside of her abdomen. Female's reproductive tracts vary from simple tubes to systems that include seminal receptacles in which females store sperm and release it when they are ready.

Males of the genus Tidarren amputate one of their palps before maturation and enter adult life with one palp only. The palps are 20% of male's body mass in this species, and detaching one of the two improves mobility. In the Yemen i species Tidarren argo , the remaining palp is then torn off by the female. The separated palp remains attached to the female's epigynum for about four hours and apparently continues to function independently. In the meantime the female feeds on the palpless male. In over 60% of cases the female of the Australian Latrodectus hasselti|redback spider kills and eats the male after it inserts its second palp into the female's genital opening; in fact the males co-operate by trying to impale themselves on the females' fangs. Observation shows that most male redbacks never get an opportunity to mate, and the "lucky" ones increase the likely number of offspring by ensuring that the females are well-fed. However males of most species survive a few matings, limited mainly by their short life spans. Some even live for a while in their mates' webs.

Females lay up to 3,000 egg (biology)|eggs in one or more silk egg sacs, which maintain a fairly constant humidity level. In some species the females die afterwards, but females of other species protect the sacs by attaching them to their webs, hiding them in nests, carrying them in the chelicerae or attaching them to the Spinneret (spider)|spinnerets and dragging them along.

Baby spiders pass all their larva l stages inside the egg and hatch as spiderlings, very small and sexually immature but similar in shape to adults. Some spiders care for their young, for example a wolf spider 's brood cling to rough bristles on the mother's back, and females of some species respond to the "begging" behaviour of their young by giving them their prey, provided it is no longer struggling, or even regurgitation (digestion)|regurgitate food.

Like other arthropod s, spiders have to molt to grow as their cuticle ("skin") cannot stretch. In some species males mate with newly molted females, which are too weak to be dangerous to the males. Most spiders live for only one to two years, although some tarantula s can live in captivity for over 20 years.

Size

Spiders occur in a large range of sizes. The smallest, Patu digua from Colombia, are less than in body length. The largest and heaviest spiders occur among tarantulas , which can have body lengths up to 90& nbsp;mm (3.5& nbsp;in) and leg spans up to 250& nbsp;mm (10& nbsp;in). Spiders and their Kin , Herbert W. Levi and Lorna R. Levi, Golden Press, p. 20 and p. 44

Coloration

Only three classes of pigment ( ommochrome s, bilin (biochemistry)|bilins and guanine ) have been identified in spiders, although other pigments have been detected but not yet characterized. Melanin s, carotenoid s and pterin s, very common in other animals, are apparently absent. In some species the exocuticle of the legs and prosoma is modified by a tanning process, resulting in brown coloration.
Bilins are found, for example, in Micrommata virescens , resulting in its green color. Guanine is responsible for the white markings of the European garden spider Araneus diadematus . It is in many species accumulated in specialized cells called guanocyte s. In genera such as Tetragnatha , Leucauge , Argyrodes or Theridiosoma , guanine creates their silvery appearance. While guanine is originally an end-product of protein metabolism, its excretion can be blocked in spiders, leading to an increase in its storage. Structural coloration|Structural colors occur in some species, which are the result of the diffraction, scattering or interference of light, for example by modified setae or scales. The white prosoma of Argiope (spider)|Argiope results from hairs reflecting the light, Lycosa and Josa both have areas of modified cuticle that act as light reflectors.

Ecology and behavior

Non-predatory feeding

Although spiders are generally regarded as predatory, the jumping spider Bagheera kiplingi gets over 90% of its food from fairly solid plant material produced by acacia s as part of a Mutualism (biology)|mutually beneficial relationship with a species of ant .

Juveniles of some spiders in the families Anyphaenidae , Corinnidae , Clubionidae , Thomisidae and Salticidae feed on plant nectar . Laboratory studies show that they do so deliberately and over extended periods, and periodically clean themselves while feeding. These spiders also prefer sugar solutions to plain water, which indicates that they are seeking nutrients. Since many spiders are nocturnal, the extent of nectar consumption by spiders may have been under-estimated. Nectar contains amino acid s, lipid s, vitamin s and minerals in addition to sugars, and studies have shown that other spider species live longer when nectar is available. Feeding on nectar avoids the risks of struggles with prey, and the costs of producing venom and digestive enzymes.

Various species are known to feed on dead arthropods (scavenging), web silk, and their own shed exoskeletons. Pollen caught in webs may also be eaten, and studies have shown that young spiders have a better chance of survival if they have the opportunity to eat pollen. In captivity, several spider species are also known to feed on banana s, marmalade , milk , egg yolk and sausages .

Methods of capturing prey

The best-known method of prey capture is by means of sticky webs. Varying placement of webs allows different species of spider to trap different insects in the same area, for example flat horizontal webs trap insects that fly up from vegetation underneath while flat vertical webs trap insects in horizontal flight. Web-building spiders have poor vision, but are extremely sensitive to vibrations.

Females of the water spider Argyroneta aquatica build underwater "diving bell" webs which they fill with air and use for digesting prey, molting, mating and raising offspring. They live almost entirely within the bells, darting out to catch prey animals that touch the bell or the threads that anchor it. A few spiders use the surfaces of lakes and ponds as "webs", detecting trapped insects by the vibrations that these cause while struggling.

Net-casting spiders weave only small webs but then manipulate them to trap prey. Those of the genus Hyptiotes and the family Theridiosomatidae stretch their webs and then release them when prey strike them, but do not actively move their webs. Those of the family Deinopidae weave even smaller webs, hold them outstretched between their first two pairs of legs, and lunge and push the webs as much as twice their own body length to trap prey, and this move may increase the webs' area by a factor of up to ten. Experiments have shown that Deinopis spinosus has two different techniques for trapping prey: backwards strikes to catch flying insects, whose vibrations it detects; and forward strikes to catch ground-walking prey that it sees. These two techniques have also been observed in other deinopids. Walking insects form most of the prey of most deinopids, but one population of Deinopis subrufa appears to live mainly on Tipulidae|tipulid flies that they catch with the backwards strike.

Mature female bolas spider s of the genus Mastophora build "webs" that consist of only a single "trapeze line", which they patrol. They also construct a bolas made of a single thread, tipped with a large ball of very wet sticky silk. They emit chemicals that resemble the pheromone s of moth s, and then swing the bolas at the moths. Although they miss on about 50% of strikes, they catch about the same weight of insects per night as web-weaving spiders of similar size. The spiders eat the bolas if they have not made a kill in about 30 minutes, rest for a while, and then make new bolas. Juveniles and adult males are much smaller and do not make bolas. Instead they release different pheromones that attract moth fly|moth flies , and catch them with their front pairs of legs.

The primitive Liphistiidae , the " trapdoor spider s" (family Ctenizidae) and many tarantula s are ambush predator s that lurk in burrows, often closed by trapdoors and often surrounded by networks of silk threads that alert these spiders to the presence of prey. Other ambush predators do without such aids, including many crab spiders, and a few species that prey on bee s, which see ultraviolet , can adjust their ultraviolet reflectance to match the flowers in which they are lurking. Wolf spider s, jumping spider s, fishing spider s and some crab spider s capture prey by chasing it, and rely mainly on vision to locate prey.

Some jumping spiders of the genus Portia (spider)|Portia hunt other spiders in ways that seem intelligent, outflanking their victims or luring them from their webs. Laboratory studies show that Portia s instinctive tactics are only starting points for a trial-and-error approach from which these spiders learn very quickly how to overcome new prey species. However they seem to be relatively slow "thinkers", which is not surprising, as their brains are vastly smaller than those of mammalian predators.

Ant mimicry|Ant-mimicking spiders face several challenges: they generally develop slimmer abdomens and false "waists" in the cephalothorax to mimic the three distinct regions (tagmata) of an ant's body; they wave the first pair of legs in form to their heads to mimic antenna (biology)|antennae , which spiders lack, and to conceal the fact that they have eight legs rather than six; they develop large color patches round one pair of eyes to disguise the fact that they generally have eight simple eyes, while ants have two compound eyes; they cover their bodies with reflective hairs to resemble the shiny bodies of ants. In some spider species, males and females mimic different ant species, as female spiders are usually much larger than males. Ant-mimicking spiders also modify their behavior to resemble that of the target species of ant; for example, many adopt a zig-zag pattern of movement, ant-mimicking jumping spiders avoid jumping, and spiders of the genus Synemosyna walk on the outer edges of leaves in the same way as Pseudomyrmex . Ant-mimicry in many spiders and other arthropods may be for protection from predators that hunt by sight, including birds, lizards and spiders. However, several ant-mimicking spiders prey either on ants or on the ants' " livestock ", such as aphid s. When at rest, the ant-mimicking crab spider Amyciaea does not closely resemble Oecophylla , but while hunting it imitates the behavior of a dying ant to attract worker ants. After a kill, some ant-mimicking spiders hold their victims between themselves and large groups of ants to avoid being attacked.

Defense

There is strong evidence that spiders' coloration is camouflage that helps them to evade their major predators, birds and parasitic wasp s, both of which have good color vision. Many spider species are colored so as to merge with their most common backgrounds, and some have disruptive coloration , stripes and blotches that break up their outlines. In a few species, such as the Hawaiian happy-face spider, Theridion grallator , several coloration schemes are present in a ratio that appears to remain constant, and this may make it more difficult for predators to recognize the species. Most spiders are insufficiently dangerous or unpleasant-tasting for Aposematism|warning coloration to offer much benefit. However a few species with powerful venoms, large jaws or irritant hairs have patches of warning colors, and some actively display these colors when threatened.

Many of the family Theraphosidae , which includes tarantula s and baboon spider s, have urticating hair s on their abdomens and use their legs to flick them at attackers. These hairs are fine seta e (bristles) with fragile bases and a row of barbs on the tip. The barbs cause intense irritation but there is no evidence that they carry any kind of venom. A few defend themselves against wasps by including networks of very robust threads in their webs, giving the spider time to flee while the wasps are struggling with the obstacles. The golden wheeling spider, Wheel spider|Carparachne aureoflava , of the Namibian desert escapes parasitic wasps by flipping onto its side and Cartwheel (gymnastics)|cartwheeling down sand dune s.

Social spiders

A few species of spiders that build webs live together in large colonies and show social behavior, although not as complex as in Eusociality|social insects . Anelosimus Anelosimus eximius|eximius (in the family (biology)|family Theridiidae ) can form colonies of up to 50,000 individuals. The genus Anelosimus has a strong tendency towards sociality: all known American species are social, and species in Madagascar are at least somewhat social. Members of other species in the same family but several different genera have convergent evolution|independently developed social behavior. For example, although Theridion nigroannulatum belongs to a genus with no other social species, T. nigroannulatum build colonies that may contain several thousand individuals that co-operate in prey capture and share food. Other communal spiders include several Philoponella species (family Uloboridae ), Agelena consociata (family Agelenidae ) and Mallos (spider)|Mallos Mallos gregalis|gregalis (family Dictynidae ). Social predatory spiders need to defend their prey against kleptoparasite s ("thieves"), and larger colonies are more successful in this. The herbivorous spider Bagheera kiplingi lives in small colonies which help to protect eggs and spiderlings. Even widow spider s (genus Latrodectus ), which are notoriously cannibalistic, have formed small colonies in captivity, sharing webs and feeding together.

Web types

There is no consistent relationship between the classification of spiders and the types of web they build: species in the same genus|genera may build very similar or significantly different webs. Nor is there much correspondence between spiders' classification and the chemical composition of their silks. Convergent evolution in web construction, in other words use of similar techniques by remotely related species, is rampant. Non-orb web designs and the spinning behaviors that produce them have received very little attention from arachnologist s, despite the fact that the majority of spiders build non-orb webs. The basic radial-then-spiral sequence visible in orb webs and the sense of direction required to build them may have been inherited from the common ancestors of most spider groups. It used to be thought that the sticky orb web was an evolutionary innovation resulting in the diversification of the Orbiculariae . Now, however, it appears that non-orb spiders are a sub-group that evolved from orb-web spiders, and non-orb spiders have over 40% more species and are four times as abundant as orb-web spiders. Their greater success may be because sphecid wasp s, which are often the dominant predators on spiders, much prefer to attack spiders that have flat webs.

Orb webs

About half the potential prey that hit orb webs escape. A web has to perform three functions: intercepting the prey (intersection); absorbing its momentum without breaking (stopping); and trapping the prey by entangling it or sticking to it (retention). No single design is best for all prey. For example: wider spacing of lines will increase the web's area and hence its ability to intercept prey, but reduce its stopping power and retention; closer spacing, larger sticky droplets and thicker lines would improve retention, but would make it easier for potential prey to see and avoid the web, at least during the day. However there are no consistent differences between orb webs built for use during the day and those built for use at night. In fact there is no simple relationship between orb web design features and the prey they capture, as each orb-weaving species takes a wide range of prey.

The hubs of orb webs, where the spiders lurk, are usually above the center as the spiders can move downwards faster than upwards. If there is an obvious direction in which the spider can retreat to avoid its own predators, the hub is usually offset towards that direction.

Horizontal orb webs are fairly common, despite being less effective at intercepting and retaining prey and more vulnerable to damage by rain and falling debris. Various researchers have suggested that horizontal webs offer compensating advantages, such as: reduced vulnerability to wind damage; reduced visibility to prey flying upwards, because of the back-lighting from the sky; enabling oscillation s to catch insects in slow horizontal flight. However there is no single explanation for the common use of horizontal orb webs.

Spiders often attach highly visible silk bands called decorations or stabilimenta to their webs. Field research suggests that webs with more decorative bands captured more prey per hour. However a laboratory study showed that spiders reduce the building of these decorations if they sense the presence of predators.

There are several unusual variants of orb web, many of them convergently evolved, including: attachment of lines to the surface of water, possibly to trap insects in or on the surface; webs with twigs through their centers, possibly to hide the spiders from predators; "ladder-like" webs that appear most effective in catching moth s. However the significance of many variations is unclear.

In 1973, Skylab 3 took two orb-web spiders into space to test their web-spinning capabilities in zero gravity. At first both produced rather sloppy webs, but they adapted quickly.

Tangleweb spiders (cobweb spiders)

Members of the family Theridiidae weave irregular, tangled, three-dimensional webs, popularly known as cobwebs.There seems to be an evolutionary trend towards a reduction in the amount of sticky silk used, leading to its total absence in some species. The construction of cobwebs is less stereotyped than that of orb-webs, and may take several days.

Other types of webs

The Linyphiidae generally make horizontal but uneven sheets, with tangles of stopping threads above. Insects that hit the stopping threads fall on to the sheet or are shaken on to it by the spider, and are held by sticky threads on the sheet until the spider can attack from below.

Evolution

Fossil record

Although the fossil record of spiders is considered poor, almost 1000 species have been described from fossils. Because spiders' bodies are quite soft, the vast majority of fossil spiders have been found preserved in amber . The oldest known amber that contains fossil arthropods dates from in the Early Cretaceous period. In addition to preserving spiders' anatomy in very fine detail, pieces of amber show spiders mating, killing prey, producing silk and possibly caring for their young. In a few cases amber has preserved spiders' egg sacs and webs, occasionally with prey attached; the oldest fossil web found so far is 100& nbsp;million years old. Earlier spider fossils come from a few lagerstätte n, places where conditions were exceptionally suited to preserving fairly soft tissues.

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami , from about in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalp s. The fossil was originally named Eotarbus but was renamed when it was realized that a Carboniferous arachnid had already been named Eotarbus : Attercopus fimbriunguis , from in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider. However these spigots may have been mounted on the underside of the abdomen rather than on Spinneret (spider)|spinnerets , which are modified appendage s and whose mobility is important in the building of webs. Hence Attercopus and the similar Permian arachnid Permarachne may not have been true spiders, and probably used silk for lining nests or producing egg-cases rather than for building webs. The largest known fossil spider as of 2011 is the araneid Nephila jurassica , from about , recorded from Daohuogo , Inner Mongolia in China. Its body length is almost 25 mm, (i.e., almost one inch).

Several Carboniferous spiders were members of the Mesothelae , a primitive group now represented only by the Liphistiidae .
The mesothelid Paleothele montceauensis , from the Late Carboniferous over , had five spinnerets. Although the Permian period saw rapid Adaptive radiation|diversification of flying insect s, there are very few fossil spiders from this period.

The main groups of modern spiders, Mygalomorphae and Araneomorphae , first appear in the Triassic well before . Some Triassic mygalomorphs appear to be members of the family Hexathelidae , whose modern members include the notorious Sydney funnel-web spider , and their spinnerets appear adapted for building funnel-shaped webs to catch jumping insects. Araneomorphae account for the great majority of modern spiders, including those that weave the familiar orb-shaped webs. The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families.

Family tree

It is now agreed that spiders (Araneae) are monophyletic (i.e., members of a group of organisms which form a clade , consisting of a last common ancestor and all of its descendants). There has been debate about what their closest evolutionary relatives are, and how all of these evolved from the ancestral chelicerate s, which were marine animals. The cladogram on the right is based on J.W. Shultz' analysis (2007). Other views include proposals that: scorpion s are more closely related to the extinct marine scorpion-like eurypterid s than to spiders; spiders and Amblypygi are a monophyletic group. The appearance of several multi-way branchings in the tree on the right shows that there are still uncertainties about relationships between the groups involved.

Arachnids lack some features of other chelicerates, including backward-pointing mouths and gnathobase s ("jaw bases") at the bases of their legs; both of these features are part of the ancestral arthropod feeding system. Instead they have mouths that point forwards and downwards, and all have some means of breathing air. Spiders (Araneae) are distinguished from other arachnid groups by several characteristics, including Spinneret (spider)|spinnerets and, in males, pedipalp s that are specially adapted for sperm transfer.

Taxonomy

Spiders are divided into two sub-orders, Mesothelae and Opisthothelae , of which the latter contains two infra-orders, Mygalomorphae and Araneomorphae . Over 40,000 living species (biology)|species of spiders (order Araneae) have been identified and are currently grouped into about 110 family (biology)|families and about 3,700 genus|genera by arachnology|arachnologists .

Sub-order !! Species !! Genera !! Families !! Segmented plates on top of abdomencite book !& #33; Ganglia in abdomen !& #33; Spinnerets !& #33; Striking direction of fangs
& nbsp;
Mesothelae
Mygalomorphae
Araneomorphae

Mesothelae

| author=Ono, H. | year=2002|url= http://ci.nii.ac.jp/naid/110004702016
| title=New and Remarkable Spiders of the Families Liphistiidae, Argyronetidae, Pisauridae, Theridiidae and Araneidae (Arachnida) from Japan
| journal=Bulletin of the National Science Museum (of Japan), Series A | volume=28 | issue=1 |pages= 51–60

The only living members of the primitive Mesothelae are the family Liphistiidae , found only in Southeast Asia , China , and Japan . Most of the Liphistiidae construct silk-lined burrows with thin trapdoor s, although some species of the genus Liphistius build camouflaged silk tubes with a second trapdoor as an emergency exit. Members of the genus Liphistius run silk " tripwire s" outwards from their tunnels to help them detect approaching prey, while those of genus Heptathela do not and instead rely on their built-in vibration sensors. Spiders of the genus Heptathela have no venom glands although they do have venom gland outlets on the fang tip. Full text at (60& nbsp;MB)

The extinct families Arthrolycosidae , found in Carboniferous and Permian rocks, and Arthromygalidae , so far found only in Carboniferous rocks, have been classified as members of the Mesothelae.

Mygalomorphae

The Mygalomorphae, which first appeared in the Triassic period, are generally heavily built and hairy, with large, robust chelicera e and fangs. Well-known examples include tarantula s, trapdoor spider s and the Australasian funnel-web spider s. Most spend the majority of their time in burrows, and some run silk tripwires out from these, but a few build webs to capture prey. However mygalomorphs cannot produce the pirifom silk that the Araneomorphae use as instant adhesive to glue silk to surfaces or to other strands of silk, and this makes web construction more difficult for mygalomorphs. Since mygalomorphs rarely "balloon" by using air currents for transport, their populations often form clumps. In addition to arthropods, mygalomorphs prey on frogs and lizards, and snails.

Araneomorphae

In addition to accounting for over 90% of spider species, the Araneomorphae, also known as the "true spiders", include orb-web spider s, the cursorial wolf spider s, and jumping spider s, as well as the only known herbivorous spider, Bagheera kiplingi . They are distinguished by having fangs that oppose each other and cross in a pinching action, in contrast to the Mygalomorphae, which have fangs that are nearly parallel in alignment.

Spiders and people

Spider bites

Most spiders will only bite humans in self-defense, and few produce worse effects than a mosquito bite or bee-sting. Most of those with medically serious bites, such as recluse spider s and widow spider s, are shy and bite only when they feel threatened, although this can easily arise by accident. Sydney funnel-web spider|Funnel web spiders ' defensive tactics are aggressive and their venom, although they rarely inject much, has resulted in 13 known human deaths. On the other hand, the Brazilian wandering spider requires very little provocation.

There were about 100 reliably reported deaths from spider bites in the 20th century, but about 1,500 from jellyfish stings. Many alleged cases of spider bites may represent incorrect diagnoses, which would make it more difficult to check the effectiveness of treatments for genuine bites.

Benefits to humans

Cooked tarantula spiders are considered a delicacy in Cambodia , and by the Piaroa Indians of southern Venezuela& nbsp;– provided the highly irritant hairs, the spiders' main defense system, are removed first.

Spider venom s may be a less polluting alternative to conventional pesticide s as they are deadly to insects but the great majority are harmless to vertebrate s. Australian Australian funnel-web spider|funnel web spiders are a promising source as most of the world's insect pests have had no opportunity to develop any Immunity (medical)|immunity to their venom, and funnel web spiders thrive in captivity and are easy to "milk". It may be possible to target specific pests by genetic engineering|engineering genes for the production of spider toxin s into virus es that infect species such as cotton bollworm s.

Possible medical uses for spider venoms are being investigated, for the treatment of cardiac arrhythmia , Alzheimer's disease , stroke s, and erectile dysfunction .

Because spider silk is both light and very strong, BioSteel|attempts are being made to produce it in goat s' milk and in the leaves of plants, by means of genetic engineering . Fine transparent spider silk fibers are used by physicists, working on optical communications, to introduce minute diffraction patterns over propagating N-slit interferometer|N-slit interferometric signals .

Arachnophobia

Arachnophobia is a specific phobia & mdash;it is the abnormal fear of spiders or anything reminiscent of spiders, such as webs or spider-like shapes. It is one of the most common specific phobias, and some statistics show that 50% of women and 10% of men show symptoms. It may be an exaggerated form of an instinct ive response that helped early humans to survive, or a cultural phenomenon that is most common in predominantly European societies.

Spiders in symbolism and culture

Spiders have been the focus of Arachnophobia|fears , stories and mythologies of various cultures for centuries. They have symbolized patience due to their hunting technique of setting webs and waiting for prey, as well as mischief and malice for the painful death their venom causes.

Web-spinning also caused the association of the spider with creation myths as they seem to have the ability to produce their own worlds. The Moche people of ancient Peru worshipped nature.Benson, Elizabeth. The Mochica: A Culture of Peru . New York: Praeger Press. 1972. They placed emphasis on animals and often depicted spiders in their art.Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru: Treasures from the Larco Museum|Museo Arqueológico Rafael Larco Herrera . New York: Thames and Hudson , 1997.

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