Spider
Spiders, the order known to scientists as Araneae, may kill between 400 and 800 million tons of prey every year. That estimate comes from a worldwide spider population of roughly 25 million tons. These animals live on every continent except Antarctica and have settled in nearly every land habitat. Taxonomists have recorded 53,680 species. Most people meet a spider as a small shape in a corner and feel a flicker of dread. Yet spiders breathe air, walk on eight limbs, and make a material that engineers still struggle to match. How does a body with no antennae, and almost no leg muscles for straightening its limbs, become so successful? How does a creature with a tiny brain solve problems? And why have spiders crept into myths on several continents?
Each leg of a spider is built from seven parts, named from the body outward as the coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The tarsus ends in a claw with either two or three points, depending on the family. Spiders have almost no extensor muscles in their legs. The only ones sit in the hip joints, so blood pressure does the work of straightening the limbs, at up to eight times the resting level. A spider with a punctured cephalothorax cannot extend its legs, which is why the legs of dead spiders curl up. Jumping spiders exploit this system. By suddenly raising the pressure in the third or fourth pair of legs, they can leap up to 50 times their own length.
Bristles called scopulae let many active hunters walk up vertical glass and across ceilings. The tufts sit between the paired claws, and each bristle splits at its tip into as many as 1,000 branches. The grip appears to come from contact with extremely thin layers of water on a surface. Whatever the surface, a walking or running spider keeps at least four legs planted on it.
The size range is startling. The smallest spider, Patu digua of Colombia, measures less than 0.37 mm in body length. At the other end, tarantulas can reach a body length of 90 mm and a leg span of 250 mm.
The body itself is divided into just two parts. The front part, the cephalothorax or prosoma, carries the legs, the eyes and a nervous system fused into a single mass. In all but the primitive Mesothelae, every ganglion behind the esophagus has merged, so no nerve centers remain in the abdomen. The rear part is the soft, egg-shaped abdomen, which holds the heart and the breathing organs. Some scientists question both names, because there is no fossil or embryological evidence that spiders ever had a separate thorax. In front of the mouth, spiders carry only a pair of chelicerae, each with two sections and a fang that is generally venomous. The fangs fold away when idle, and thick beards on the upper sections filter solid lumps from food. A narrow, cylindrical waist called the pedicel joins the two body parts, and it lets the abdomen swing freely while the spider spins silk.
Four pairs of eyes sit on the top-front of a spider's cephalothorax, arranged in patterns that differ from family to family. The large pair at the front can form images, even though similar pigment-cup eyes in most arthropods only sense the direction of light. These are the only eyes with muscles, so they alone can move the retina. The remaining pairs are immobile. In many spiders they gather light bounced back from a reflective layer, which is why wolf spiders can be found at night when a torch beam glints from their eyes. Jumping spiders lack that layer in their secondary eyes.
The sharpest vision belongs to jumping spiders, whose acuity exceeds that of dragonflies by a factor of ten. Dragonflies have by far the best sight among insects. The jumping spider achieves this with a series of telephoto-style lenses, a retina of four layers, and the ability to swivel its eyes and merge images from different stages of a scan. The price is speed, because all that scanning is slow.
Not every spider has eight eyes. Periegops suterii has six, members of the family Caponiidae can have as few as two, and cave dwellers may have none at all, as with the Kauaʻi cave wolf spider. Holothele maddeni keeps vestigial eyes that cannot see.
For web builders, touch and chemistry matter far more than sight. An adult Araneus may carry up to 1,000 chemosensitive bristles, mostly on the tarsi of the first pair of legs. Males have more of them on their pedipalps than females do, and they respond to the sex pheromones that females release. The jumping spider Evarcha culicivora goes further. It uses the scent of vertebrate blood, picked up from blood-filled mosquitoes it has caught, to attract the opposite sex. Slit-shaped sensors in the joints detect force and vibration, and spiders lack any balance sensor, so they use their eyes to tell which way is up. Some giant webs even act as extended, reconfigurable ears.
Up to six types of gland feed the spinnerets at the tip of a spider's abdomen, and each spinneret carries many spigots connected to single glands. Most spiders have three pairs of these short, movable appendages, though the number ranges from one to four pairs. Silk begins as a liquid. It hardens not through contact with air but because the act of pulling it out changes the internal structure of its protein. Its tensile strength is similar to nylon, chitin, collagen and cellulose, yet it stretches much further before breaking. Spitting spiders even make silk in modified venom glands, and a few families that lost their venom entirely now kill prey with silk.
Some spiders carry a cribellum, a spinneret with up to 40,000 spigots, each producing one very fine fiber. A comblike row of bristles called the calamistrum pulls the fibers into a woolly thread that snags the bristles of insects. The earliest spiders had cribella, and their silk was the first capable of catching insects. Later spiders evolved silk coated in sticky droplets, and most modern groups have dropped the cribellum.
Spiders that never build capture webs still depend on silk. They wrap sperm and fertilized eggs in it, trail it as a safety rope, line nests with it, and, in some species, let their young ride it like parachutes. The material is so light, strong and elastic that researchers have inserted spider silk genes into mammals and plants to see whether they could become silk factories. Attempts are also being made to produce it in goats' milk and in plant leaves.
The orb web is the best studied design, and it must intercept, stop and hold prey. Even so, about half of the prey that hit one escape. The hub, where the spider waits, usually sits above the center, because spiders move downward faster than upward. Spiders also add bright silk bands called stabilimenta, and field research found that webs with more bands caught more prey per hour. A laboratory study showed that spiders build fewer of them when they sense predators. The orb web once looked like the key to spider success, but non-orb spiders turn out to have over 40% more species and four times the abundance. One suggested reason is that sphecid wasps, the dominant predators of spiders, much prefer to attack spiders on flat webs. Cobwebs, built by the family Theridiidae, are tangled and three-dimensional, and some species use no sticky silk at all.
Orb weavers have also been tested in orbit. In 1973, Skylab 3 carried two orb-web spiders into space, and both spun sloppy webs at first before adapting quickly. In late 2020, reports of newer experiments showed that a light source could orient spiders enough to build normally shaped webs in zero gravity.
Female bolas spiders of the genus Mastophora build no capture web at all, only a single trapeze line that they patrol. From it they swing a thread tipped with a large ball of very wet, sticky silk. They release chemicals resembling moth pheromones, then swing the ball at the moths that arrive. They miss about half the time, yet they catch roughly the same weight of insects per night as web weavers of similar size. If a spider makes no kill within about 30 minutes, it eats the bolas, rests, and makes a new one.
Net-casting spiders of the family Deinopidae hold a tiny web between their first two pairs of legs and lunge with it, up to twice their body length, which can enlarge the web's area by a factor of up to ten. Deinopis spinosus strikes backward at flying insects it detects by vibration, and forward at walking prey that it sees. The water spider Argyroneta aquatica lives almost entirely inside an underwater diving bell of silk that it fills with air. Ambushers wait instead. Trapdoor spiders of the family Ctenizidae lurk in burrows, and some crab spiders that prey on bees can adjust their ultraviolet reflectance to match the flowers where they hide.
Some hunters seem to think. Jumping spiders of the genus Portia stalk other spiders by outflanking them or luring them from their webs. Their instinctive tactics are only starting points, and trial and error lets them learn to defeat new prey species quickly. They appear to be slow thinkers, which is unsurprising given brains vastly smaller than those of mammalian predators. Ant mimics go to even greater lengths. They develop false waists, wave their front legs like antennae, and adopt a zig-zag walk. The crab spider Amyciaea imitates a dying ant to draw in worker ants.
Defense relies on camouflage against birds and parasitic wasps, which see color well. Tarantulas and baboon spiders of the family Theraphosidae flick irritating hairs from their abdomens at attackers. The golden wheeling spider, Carparachne aureoflava, escapes wasps in the Namibian desert by flipping onto its side and cartwheeling down sand dunes.
Spiders cannot swallow solids, because their guts are too narrow. Some pump digestive enzymes into the prey and suck up the liquefied tissue, leaving an empty husk. Others grind the prey to pulp with their chelicerae and the bases of their pedipalps while flooding it with enzymes. Not every spider hunts. The jumping spider Bagheera kiplingi gets over 90% of its food from Beltian bodies, a plant material that acacias produce in a partnership with ants. Scientists described the species as herbivorous in 2008. Juveniles of several families also drink plant nectar, and studies show some species live longer when it is available.
Males of the genus Tidarren amputate one of their palps before reaching maturity. The palps make up 20% of the male's body mass, and losing one improves mobility. In the Yemeni species Tidarren argo, the female then tears off the remaining palp. It stays attached to her epigynum for about four hours and apparently keeps working on its own, while she feeds on the male.
The Australian redback spider goes further. In over 60% of cases the female kills and eats the male after he inserts his second palp, and the males cooperate by trying to impale themselves on her fangs. Observation suggests most male redbacks never get to mate at all. Most other species are less lethal, and males survive a few matings, limited mainly by short lives.
Male spiders do not insert sperm directly. They spin a small sperm web, deposit sperm on it, and draw it into palpal bulbs at the tips of their pedipalps. Elaborate courtship keeps the larger females from eating the males before fertilization. Web builders signal with precise patterns of vibration. Hunting species may stroke the female in ways that seem to hypnotize her. Jumping spiders, with their excellent eyesight, rely on gestures and dances. The only known direct fertilization is in the Israeli spider Harpactea sadistica, where the male pierces the female's body wall and injects sperm straight into her ovaries.
Females lay up to 3,000 eggs in silk sacs that keep a fairly constant humidity. Some species die afterward, while others guard the sacs in nests or drag them along. A wolf spider's brood clings to rough bristles on the mother's back. Females of some species give their begging young prey that has stopped struggling, or even regurgitate food. In one exceptional case, mothers of the jumping spider Toxeus magnus produce a milk-like substance and feed their offspring until they are sexually mature.
Spiders must molt to grow, because their cuticle cannot stretch, and in some species males mate with newly molted females that are too weak to be dangerous. Most spiders live only one to two years. Tarantulas can survive over 20 years in captivity, and an Australian female trapdoor spider was documented living 43 years in the wild before a parasitic wasp killed her.
A minority of species live in groups. Anelosimus eximius can form colonies of up to 50,000 individuals, and every known American species of its genus is social. Theridion nigroannulatum colonies hold several thousand spiders that cooperate in prey capture and share food. Even widow spiders, notorious cannibals, have formed small colonies in captivity, sharing webs and feeding together.
Palaeotarbus jerami, a trigonotarbid from the late Silurian period, is the oldest known arachnid that lived only on land. It had a triangular cephalothorax, a segmented abdomen, eight legs and a pair of pedipalps. It was not a spider. Attercopus fimbriunguis, from the Devonian, carries the earliest known silk-producing spigots, and its discoverers hailed it as a spider. Its spigots may have sat on the underside of the abdomen rather than on mobile spinnerets, so it probably used silk for nests or egg cases rather than webs.
Spider fossils are scarce because the bodies are soft, yet almost 1,000 fossil species have been described. Most come from amber, which preserves anatomy in fine detail and has captured spiders mating, killing prey and producing silk. The oldest fossil web found so far is 100 million years old. Mongolarachne jurassica, found at Daohuogo in Inner Mongolia, China, was the largest known fossil spider as of 2011, with a body almost 25 mm long.
True spiders appear in the Carboniferous, including members of the primitive Mesothelae. One of them, Palaeothele montceauensis, had five spinnerets. The main modern groups, Mygalomorphae and Araneomorphae, first show up in the Triassic. Some Triassic mygalomorphs belong to the family Hexathelidae, which today includes the Sydney funnel-web spider. A 2020 study using a molecular clock, calibrated with 27 chelicerate fossils, suggests spiders diverged from other chelicerates between 375 and 328 million years ago.
Classifying the living spiders has been contentious, with over 20 different schemes proposed since 1900. Arachnologists currently count 139 families and 4,502 genera. The two suborders are Mesothelae and Opisthothelae, and the latter splits into Mygalomorphae and Araneomorphae. The living Mesothelae are only the family Liphistiidae, which has about 196 species in Southeast Asia, China and Japan. They dig silk-lined burrows with thin trapdoors. Species of the genus Liphistius run silk tripwires from their tunnels, while Heptathela spiders rely on built-in vibration sensors and have no venom glands.
Mygalomorphs, with about 3,879 species, include tarantulas and Australasian funnel-web spiders. They are heavily built and cannot make the piriform silk that lets other spiders glue strands together, which makes web building harder. They rarely balloon on air currents, so their populations form clumps. Araneomorphae, or true spiders, number about 49,605 species and make up over 90% of the total. Their fangs cross in a pinching action, unlike the nearly parallel fangs of mygalomorphs. The order name Araneae comes from the Latin aranea, which borrowed the Ancient Greek word arákhnē.
Australian funnel-web spiders have been linked to 13 human deaths over 50 years, despite being judged the most dangerous spiders on clinical and venom grounds. The Brazilian wandering spider of the genus Phoneutria has also been given that title. Across the whole 20th century, about 100 deaths from spider bites were reliably reported, compared with about 1,500 from jellyfish stings. Spiders bite people only in self-defense, and few bites are worse than a mosquito bite or bee sting. Recluse spiders of the genus Loxosceles and widow spiders of the genus Latrodectus can cause serious harm, but they prefer to flee.
Many reported bites are probably something else. A review published in 2016 found that 78% of 134 medical case studies of supposed spider bites did not meet the criteria for verification. For Loxosceles and Latrodectus, the two genera with the most reported bites, over 90% of the reports could not be verified. The news adds to the confusion. A study of online newspaper articles on spider-human encounters from 2010 to 2020 found that 47% contained errors and 43% were sensationalist.
Arachnophobia is the abnormal fear of spiders, webs and spiderlike shapes, and it is one of the most common specific phobias. Some statistics show symptoms in 50% of women and 10% of men. The fear may be an exaggerated survival instinct from early humans, or a cultural habit most common in predominantly European societies.
Some people eat spiders. Cooked tarantulas are a delicacy in Cambodia and among the Piaroa of southern Venezuela, provided the irritant bristles are removed first. Scientists, meanwhile, are studying spider venom for medicine and as a non-polluting pesticide.
Spiders have fed storytelling for centuries. Uttu, the ancient Sumerian goddess of weaving, was pictured as a spider at her web. The Roman poet Ovid tells of Arachne, a Lydian girl who challenged the goddess Athena to a weaving contest. Arachne won, but Athena destroyed her tapestry out of jealousy and, in the end, turned her into the first spider. Anansi, the trickster spider, is prominent in the folktales of West Africa and the Caribbean. The Italian tarantella is a dance meant to rid a young woman of the effects of a spider bite. Dreamcatchers depict spiderwebs, and the Moche people of ancient Peru often showed spiders in their art.
Updated
Common questions
How many spider species are there?
Taxonomists have recorded 53,680 spider species, grouped into 139 families and 4,502 genera. Araneomorphae, the true spiders, account for over 90% of them.
How do spiders extend their legs without extensor muscles?
Spiders extend their legs by hydraulic pressure, because the only extensor muscles in spider legs are in the hip joints. They can generate up to eight times their resting pressure, and jumping spiders can leap up to 50 times their own length.
Is spider silk stronger than nylon?
Spider silk has a tensile strength similar to nylon but is much more elastic, so it stretches much further before breaking. Researchers have inserted spider silk genes into mammals and plants, and are trying to produce it in goats' milk.
Are spider bites really dangerous to humans?
Only a few spider species are dangerous to people, and spiders bite humans only in self-defense. About 100 deaths from spider bites were reliably reported in the 20th century, compared with about 1,500 from jellyfish stings.
How long do spiders live?
Most spiders live one to two years, but tarantulas can live over 20 years in captivity. An Australian female trapdoor spider was documented living 43 years in the wild.
How old are spiders in the fossil record?
The main modern spider groups, Mygalomorphae and Araneomorphae, first appear in the Triassic period. A 2020 molecular clock study suggests spiders diverged from other chelicerates between 375 and 328 million years ago.
What percentage of people have arachnophobia of spiders?
Some statistics show that 50% of women and 10% of men show symptoms of arachnophobia, the abnormal fear of spiders. It may be an exaggerated survival instinct or a cultural phenomenon most common in predominantly European societies.
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- 121BookCognitive Science: An Introduction to the Study of MindFriedenberg, J. et al. — Sage — 2005
- 122JournalThe "Disgusting" Spider: The Role of Disease and Illness in the Perpetuation of Fear of SpidersDavey, G.C.L. — 1994
- 123JournalEdible arachnids and myriapods worldwide – updated list, nutritional profile and food hygiene implicationsE.M. Costa-Neto et al. — 2020-11-27
- 124BookLonely Planet CambodiaRay, N. — Lonely Planet Publications — 2002
- 125BookFierce FoodWeil, C. — Plume — 2006
- 126BookTales, Rumors, and Gossip: Exploring Contemporary Folk Literature in Grades 7–12Gail De Vos — Libraries Unlimited — 1996
- 127BookGods, Demons and Symbols of Ancient Mesopotamia: An Illustrated DictionaryJeremy Black et al. — The British Museum Press — 1992
- 128BookThe Harps that Once: Sumerian Poetry in TranslationThorkild Jacobsen — Yale University Press — 1987
- 129BookAspects of Ecphrastic Technique in Ovid's MetamorphosesElizabeth Norton — Cambridge Scholars Publishing — 2013
- 130JournalThe spinner and the poet: Arachne in Ovid's MetamorphosesByron Harries — 1990
- 131JournalEkphrasis and the Theme of Artistic Failure in Ovid's MetamorphosesEleanor Winsor Leach — January 1974
- 132A Dictionary of Classical Mythology: Summary of TransformationsM. Rosemary Wright — University of Patras
- 133BookThe Greenwood Encyclopedia of Folktales and Fairy TalesDonald Haase — Greenwood Publishing Group — 2008
- 134BookThe Book of SymbolsJana Garai — Simon & Schuster — 1973
- 135BookAmerican Anthropology: Papers from the American AnthropologistFrederica De Laguna — University of Nebraska Press — 2002
- 136Benson, Elizabeth. The Mochica: A Culture of Peru. New York: Praeger Press. 1972.
- 137Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru: Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.
- 138BookForest Spiders of South East Asia: With a Revision of the Sac and Ground SpidersChrista L. Deeleman-Reinhold — Brill Publishers — 2001
- 139BookInvertebrate ZoologyE.E. Ruppert et al. — Brooks/Cole — 2004
- 140BookSpiders of North AmericaSarah Rose — Princeton University Press — 2022