Spiders are an order of about 52,000 species, nearly all venomous and almost none dangerous to people, and some of them solve problems that would be respectable in an animal with a thousand times more neurons.
Spiders are the subject on this site where the gap between public belief and evidence is widest. Almost all are venomous and almost none can harm a person; the fear is close to universal and the risk is close to nil. What the research actually shows is stranger and more interesting than the folklore. Jumping spiders of the genus Portia hunt other spiders by plucking their webs, working through vibration patterns by trial and error until one draws the resident out, and will take long detours to prey they can no longer see. They do this with a nervous system of roughly a hundred thousand neurons — about a millionth of a human brain.
In-depth record · 87% complete · reviewed 2026-08-09
What this page covers
The order Araneae — around 52,000 described species. This record covers spiders in general, with the cognition material drawn almost entirely from jumping spiders (Salticidae).
Often confused with: Opiliones (harvestmen — not spiders; one body section, no silk, no venom); Acari (mites and ticks — also arachnids, also not spiders); Solifugae (camel spiders — arachnids, but not spiders)
Quick facts
Order
Araneae — about 52,000 described species
Venom
Nearly all species are venomous; a very small number are medically significant to humans
Prey consumed
Estimated 400–800 million tonnes a year globally — a model, not a measurement
Silk
Not all spiders build webs; many hunt actively and use silk only for shelter or egg sacs
A spider has two body sections, eight legs, and silk glands at the rear of the abdomen. Harvestmen — the long-legged animals often called daddy longlegs — have one body section, make no silk and have no venom at all. Mites, ticks and camel spiders are arachnids but not spiders. The distinction matters because a great deal of alarming spider folklore is actually about animals that are not spiders.
Two body sections joined by a narrow waist; harvestmen have one.
Eight legs, in four pairs — insects have six.
Silk produced from spinnerets at the rear of the abdomen.
Venom in nearly all species, delivered through fangs.
Words used here
Arachnid
The wider class containing spiders, scorpions, mites, ticks and harvestmen. All spiders are arachnids; most arachnids are not spiders.
A jumping spider will take a long detour to prey it can no longer see
Well supported
Good evidence backs this, though some details remain open.
Portia jumping spiders shown unreachable prey select, from alternative routes, the one that leads to it, and complete detours during which the target is out of sight.
Who this applies to
Portia jumping spiders, a specialised spider-hunting genusDo not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Portia fimbriata, Portia
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
Reproduced across several detour designs by the same research group over decades. Confidence is held at moderate because samples are small, the work is concentrated in one laboratory tradition, and the most convenient synthesis of it is a review chapter rather than a primary report.
How far it can be extended
Explicitly not generalised. Portia is an unusual predator that hunts other spiders on their own webs; most spiders show nothing comparable.
Caveats
What the spider retains during the detour is inferred from behaviour, never measured.
Trial-and-error and planning are genuinely hard to separate from outcome data alone.
A brain of roughly a hundred thousand neurons does this — which is the interesting part, and also a reason to be careful about the word "planning".
Still unanswered
How is a goal held in a nervous system this small?
Nelson and Jackson, 2011 · Spider Behaviour: Flexibility and Versatility (Cambridge University Press)
Review chapter synthesising the detour experiments, in which spiders chose the correct route with the goal out of sight.
The detour is what makes this result hard to dismiss. A spider that walks away from prey, loses sight of it entirely, follows a route around an obstacle and arrives at the right place is holding onto something during the journey. Nobody knows what, or how, and that is a genuinely open question rather than a rhetorical one.
A web is not only a trap — it is a vibration sensor the spider can tune
Emerging evidence
Real findings exist, but too few or too recent to be settled.
Orb web silk transmits vibration across a wide frequency band with transmission properties that depend strongly on tension. A spider can alter those properties through the tension it applies to threads, making the web a mechanical filter whose characteristics are adjustable rather than fixed.
Who this applies to
orb-weaving spiders, characterised in one species
Studied in
Nephila edulis
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence
The mechanical characterisation is solid and directly measured. Whether spiders adjust tension purposefully in use, as opposed to tension varying with posture, has not been demonstrated behaviourally.
How far it can be extended
Silk mechanics and the dependence of transmission on tension are properties of the material and the geometry, so they apply wherever an orb web is built.
Caveats
Laboratory mechanics, not behavioural demonstration of deliberate tuning.
One orb-weaver; sheet-web and cobweb architectures differ enormously.
Tension changes as a by-product of posture, which is hard to distinguish from purposeful tuning.
Still unanswered
Do spiders adjust web tension for a specific sensing task?
How much information about prey type is recoverable from web vibration alone?
The material properties of the silk that make this possible.
A web is usually described as a trap, and it is at least as much an instrument. Silk carries vibration across a wide band of frequencies, and how it carries them depends sharply on tension — which the spider controls through how tightly it holds and strings the threads.
That reframes the animal sitting motionless at the hub. It is not waiting passively for something to arrive; it is holding an external structure that collects mechanical information, and it can change the structure’s properties. Whether it tunes deliberately for a task, or tension simply varies with posture, is not settled — and the distinction matters more than the framing debate around it.
Some researchers go further and call the web part of the spider’s cognitive system: a small brain does not need to remember much if the structure it sits in remembers for it. NatureHQ records that as a philosophical framing rather than an additional empirical claim, because calling the web part of the mind adds no prediction to the observation that web design affects later behaviour.
Words used here
Extended cognition
The position that a cognitive system can include structures outside the body. A framing about where to draw a boundary, not a testable finding.
High-resolution vision in an animal a few millimetres across.
A jumping spider keeps its eyes still and moves the retina behind them, scanning like a scanner
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
The principal eyes of jumping spiders have a narrow, elongated, high-acuity retina with a field of view of only a few degrees. Muscles move the retina laterally, vertically and rotationally behind a fixed lens, sweeping the high-resolution strip across the visual scene.
Who this applies to
jumping spiders
Studied in
Metaphidippus, Salticidae
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Directly observed optically — the retina is visible through the cornea — and consistent across the family.
How far it can be extended
The moving-retina arrangement is characteristic of the Salticidae and has been observed across genera.
Caveats
This describes the two principal eyes only; the other six work quite differently and handle motion detection.
Acuity comparisons between spider and vertebrate eyes are approximate.
How the scanned strips are combined into a percept is not established.
Still unanswered
How does a spider integrate a scene it has only ever seen in narrow strips?
How much of jumping spider cognition is constrained by this serial way of seeing?
Stubbs and Stubbs, 2016 · Proceedings of the National Academy of Sciences
A proposed mechanism for depth perception in the same eyes.
How we know
Watching a retina move behind a fixed eye
How does an animal a few millimetres long get vision sharp enough to stalk prey and judge a jump?
Jumping spider eyes have an unusual property that makes this tractable: the retina of the large forward-facing pair can be seen through the cornea, so it can be watched directly while the animal is presented with visual targets. Land examined the structure optically and tracked the retinal movements that followed.
What happened
The principal eyes carry a long, narrow, boomerang-shaped retina of very high acuity but a field of view only a few degrees wide. Muscles move it laterally, vertically and rotationally behind the fixed lens, sweeping that narrow high-resolution strip across the scene.
What it shows
A genuinely different solution to seeing well with a small eye. Rather than building a wide retina, the spider builds a narrow excellent one and scans it — trading instantaneous field of view for resolution, and assembling the scene over time.
What it does not show
It describes the two principal eyes only; the other six work differently and handle wide-field motion. It also does not explain how strips scanned at different moments are combined into a usable percept, which remains open. Acuity comparisons with vertebrate eyes are approximate.
The controls — what makes this evidence rather than a story
Targets were presented in known positions, so retinal movement could be related to where the object actually was.
The retina was observed in the intact living animal rather than reconstructed from sections.
Retinal structure and its movement were characterised together, so acuity and scanning could be related.
A vertebrate builds a wide retina and moves the whole eye. A jumping spider does the opposite: the lens is fixed in the head and the retina behind it is moved by muscles, sweeping a narrow strip of very high acuity across the scene like a flatbed scanner.
That buys resolution comparable to a pigeon’s in an eye smaller than a pinhead, and it costs something specific. At any instant the spider is seeing a sliver a few degrees wide; the scene is assembled over time rather than taken in at a glance. The six secondary eyes handle motion detection and the wide field, and hand targets to the principal pair.
This is what makes Portia’s detour behaviour so striking. A spider that plans a route out of sight of its prey — climbing away from the target, losing sight of it, and arriving from above — has to have taken in enough of the scene, strip by strip, to hold it in mind while acting on it.
Jumping spiders are not typical. Most spiders have eight eyes and see badly with all of them — enough to tell light from dark and to register a shadow moving overhead, and not enough to identify anything. An orb-weaver sitting in the middle of its web is, for practical purposes, working blind and reading the world through vibration in the silk. The eight eyes are not eight versions of the same thing: they are two developmental sets, a forward-facing principal pair and three pairs of secondary eyes pointing sideways and back, and different families have enlarged, reduced or lost different ones. Net-casting spiders have two enormous rear-facing eyes for hunting at night; cave spiders have almost none.
A spider’s brain does not fit in its head. In small species the central nervous system spills into the body cavity and, in the smallest, down into the legs — up to about 80% of the cephalothorax in a spiderling can be nervous tissue.
Words used here
Principal eyes
The large forward-facing pair. High acuity, narrow field, and the only ones with a movable retina.
Secondary eyes
The other six. Wide-field motion detectors that direct the principal eyes towards anything worth examining.
Spiders take off using the atmosphere’s electric field, not only the wind
Emerging evidence
Real findings exist, but too few or too recent to be settled.
Money spiders in a sealed chamber with no airflow adopted tiptoe posture, released silk and became airborne when vertical electric fields comparable to atmospheric potential gradients were applied, and ceased when the field was switched off. Trichobothria — fine sensory hairs — deflect measurably in such fields.
Who this applies to
money spiders, tested in a laboratory chamber
Studied in
Erigone
You may have heard
“Spiders float away on the wind”
Partly right and incomplete for two centuries. Spiders take off on still days, from surfaces where wind could not lift them, and the silk strands splay apart instead of tangling — all of which the wind explanation could not account for and mutual electrostatic repulsion does.
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence
The zero-airflow condition is decisive — a spider becoming airborne with no wind cannot be being lifted by wind — and behaviour tracked the field being switched on and off. It remains one genus in a chamber, and how much real-world ballooning is electrostatic rather than aerodynamic is open.
How far it can be extended
Ballooning occurs across many small spider families and the sensory hairs concerned are widespread, so electrostatic detection is expected more broadly. It has been demonstrated in one genus.
Caveats
Wind still matters; this shows it is not the only mechanism, not that it is irrelevant.
One genus of very small spiders in a controlled chamber.
The relative contribution of the two mechanisms outdoors is unknown.
Still unanswered
How much ballooning in the wild is electrostatically initiated?
Do spiders select conditions using the field, or merely respond to it?
Ballooning elicited in still air by an applied electric field, with sensory hair deflection measured.
How we know
Making a spider fly with no wind at all
Spiders balloon on days with no wind, from surfaces wind could not lift them off. If it is not wind, what is it?
The difficulty with testing an electrostatic explanation outdoors is that wind is always available as an alternative. So the experiment removed it: money spiders were placed in a sealed chamber with verified zero airflow, and vertical electric fields comparable to the atmosphere’s own potential gradient were switched on and off. Separately, the spiders’ trichobothria — fine sensory hairs on the legs — were filmed at high magnification to see whether they physically responded to the field.
What happened
Spiders adopted tiptoe posture, released silk and became airborne when the field was on, and stopped when it was off. Their sensory hairs deflected measurably in the field.
What it shows
Spiders detect atmospheric electric fields and use them to initiate ballooning. It also resolves two long-standing oddities: why ballooning happens on still days, and why the silk strands splay apart instead of tangling — like charges repel.
What it does not show
It does not show that wind is irrelevant; wind clearly contributes, and how the two divide the work outdoors is unknown. One genus of very small spiders, in a chamber that is not the atmosphere, with field strengths chosen to be realistic rather than measured on the day of a real launch.
The controls — what makes this evidence rather than a story
Zero airflow, confirmed in the chamber, so wind could not contribute at all.
Fields switched on and off with the same animals, each spider acting as its own control.
Hair deflection measured directly, linking behaviour to a plausible sensory route rather than assuming one.
Spiders disperse by climbing to a high point, raising the abdomen — a posture called tiptoe — releasing silk and letting go. It has been attributed to wind since the eighteenth century, and three things never fitted: spiders take off on completely still days, they launch from surfaces where wind could not lift them, and the silk strands splay apart instead of tangling together.
All three follow from electrostatics. The atmosphere carries a vertical potential gradient; silk leaving a spider picks up charge, and like charges repel, which both splays the strands and provides lift. Money spiders in a sealed chamber with no airflow at all adopted tiptoe and became airborne when a field was switched on, and stopped when it was switched off.
Wind has not been ruled out — it clearly contributes, and how the two mechanisms divide the work outdoors is unresolved. What has been ruled out is that wind is the whole explanation.
Words used here
Ballooning
Dispersal by releasing silk and becoming airborne. Spiders have been caught in atmospheric sampling kilometres up.
Trichobothria
Fine sensory hairs on a spider’s legs, sensitive enough to deflect in an electric field or to air movement from a passing insect.
Hunting spiders are not a separate kind of spider — they are web-builders that stopped
Well supported
Good evidence backs this, though some details remain open.
Molecular phylogeny with web architecture mapped onto it indicates a single origin for the orb web, with much subsequent spider diversity consisting of lineages that modified or abandoned web-building. Several of the most species-rich groups, including jumping spiders, are active hunters descended from web-building ancestors.
Who this applies to
spiders as an order
Studied in
Araneae
You may have heard
“There are web spiders and hunting spiders”
True as a description of what they do now and misleading about what they are. Web-building is the older condition, and the hunters — including jumping spiders and wolf spiders — descend from ancestors that built webs and gave them up. None of them gave up silk.
Why we rate it this way, and what the caveats are
Well supportedModerate confidence
A well-sampled molecular phylogeny with behaviour mapped onto it. Confidence is moderate because reconstructing ancestral behaviour is inference from living species, and whether webs in distantly related groups are homologous is disputed.
How far it can be extended
Based on a molecular phylogeny sampling across spider families, though web homology across distant lineages remains contested.
Caveats
Ancestral behaviour is reconstructed, not observed.
Whether webs in distantly related groups share an origin is contested.
Sampling of the enormous non-orb-weaving diversity remains incomplete.
Still unanswered
What drove the repeated abandonment of web-building?
Is the single origin of the orb web robust to better sampling?
The silk that persists in hunting lineages for draglines, egg cases and ballooning.
It is natural to treat web-builders and hunters as two equivalent strategies. Phylogeny says otherwise: web-building is the older condition, the orb web appears to have a single origin, and much of the spider diversity people encounter consists of lineages that modified or abandoned it.
Jumping spiders, wolf spiders and crab spiders are all descendants of web-builders that gave the web up. None of them gave up silk — it persists as draglines, egg cases, retreats and the ballooning thread. What changed is that the silk stopped being a trap and the animal started doing the catching.
Not all webs are orbs. Sheet webs, tangle webs, funnel webs and the single sticky thread a bolas spider swings at moths are all distinct architectures, and lumping them together is the commonest error in writing about spiders.
Words used here
Orb web
The familiar wheel-shaped web. One architecture among many, and apparently a single evolutionary origin.
Dragline
The safety line a spider trails constantly. It is why a disturbed spider drops and hangs rather than falling.
A spider is an arachnid rather than an insect: eight legs, two body sections instead of three, no antennae and no wings. It is unambiguously an animal, and the confusion with insects is worth clearing up because almost everything else follows from the differences.
No ears. Spiders sense vibration through slit sensilla in the legs, and some jumping spiders respond to airborne sound through leg hairs — hearing without an ear.
Chemical sense through the legs and pedipalps rather than a nose; a spider tastes the surface it stands on.
Breathing through book lungs, tracheae, or both, depending on the group. There is no diaphragm and no active pumping.
Legs extended hydraulically by pumping fluid rather than by extensor muscles, which is why a dead spider curls up — the pressure has gone.
A digestive system that works outside the body: enzymes are injected or poured onto prey and the liquefied result is sucked back.
External digestion explains a number of small mysteries at once. Spiders produce very little solid waste, because very little solid ever goes in. They cannot eat anything they cannot liquefy. And the discarded remains under a web are an emptied shell rather than leftovers.
Young spiders can regenerate a lost leg at the next moult, and a spider that has stopped moulting cannot. Most spiders live about a year; some tarantulas live for decades, with females far outliving males in almost every group where both have been measured.
Spiders drink. Water is taken from droplets, damp surfaces and dew on webs, and a spider in a dry building is often looking for it — which is one of the reasons they turn up in baths.
Words used here
Book lung
A breathing organ of stacked plates like the pages of a book, in a cavity on the underside of the abdomen.
Slit sensillum
A vibration sensor in the exoskeleton, mostly on the legs. How a spider "hears" its web.
Moult
Shedding the exoskeleton to grow. The only occasion on which a lost leg can be regrown.
The male usually walks away. That is the part the story leaves out.
Most male spiders survive mating and leave
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Sexual cannibalism is documented in a minority of spider species, principally among orb-weavers and widow spiders, and even in those species it does not occur at every mating. In most spiders courtship concludes with the male departing, and in several species males mate repeatedly. Where cannibalism is frequent it is associated with extreme size dimorphism and with male strategies that make it advantageous to the male.
Who this applies to
spiders generally, with cannibalism concentrated in particular families
Studied in
Araneae, Araneidae, Latrodectus
You may have heard
“Female spiders eat the male after mating”
It happens, in a minority of species, and has been generalised to the order. In most spiders the male walks away — and in the best-known case that does fit the story, the redback, the male throws himself into position rather than being caught, which makes it his tactic rather than her ambush.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Mating behaviour has been observed directly across many families, and the concentration of cannibalism in a few lineages is a consistent finding.
How far it can be extended
The distribution of the behaviour across families is well documented; frequency within a species varies with condition and context.
Caveats
Laboratory arenas restrict escape and may inflate observed cannibalism rates.
In some species — notably the redback — the male actively somersaults into the female’s fangs, which is a male strategy rather than a female one.
Frequency varies with female hunger, male size and whether the female has mated before.
Still unanswered
What maintains cannibalism at intermediate frequencies rather than at zero or at every mating?
Mating systems, courtship, and the distribution and frequency of sexual cannibalism.
Spider mating is odd in a way that has nothing to do with cannibalism. The male has no penis and no direct connection between his testes and the organs he mates with. He spins a small sperm web, deposits sperm onto it, and draws the sperm up into his pedipalps — the two short appendages at the front that look like boxing gloves on a mature male. Those palps are the delivery device, loaded separately beforehand, and mating is the act of inserting one into the female’s epigyne.
Getting close enough to do that is the dangerous part, and courtship is mostly a problem of identification: the male has to signal that he is a mate rather than prey, to an animal that detects the world by vibration and is holding a web tuned to catch things his size. So he plucks the web in a species-specific rhythm, or in the jumping spiders — which see well — performs a visual display of waving legs and, in some species, iridescent abdominal flaps. Courtship can last hours. It is a negotiation, and it usually works.
The female stores the sperm rather than using it immediately, sometimes for months, and fertilises the eggs as she lays them. They go into a silk egg sac — a few dozen eggs in some species, over a thousand in others — which she may guard, carry attached to her spinnerets, or abandon depending on the group. Wolf spiders carry the sac and then carry the hatched young on their backs for days.
Time to hatch: typically two to four weeks in temperate species, though eggs laid in autumn frequently overwinter and hatch in spring.
Spiderlings emerge as miniature adults — there is no larval stage and no metamorphosis.
Growth happens only at a moult, five to ten times for a small species and far more for a tarantula.
Most temperate spiders live about a year, timed so that the adults die and the eggs overwinter.
Female tarantulas live twenty to thirty years and sometimes longer; the males of the same species die within a year or two of maturing.
That last asymmetry runs through the whole order. Males stop moulting when they mature, which fixes their size and ends any capacity to regenerate a lost leg, and then they leave the web and spend the rest of their lives searching. Females keep growing. The reason a spider found wandering across a floor in autumn is usually male is that wandering is what the last weeks of a male spider’s life consist of.
Playing dead is a real defence in several families, and there is no fixed duration to it. A spider stays motionless until the vibration it is responding to stops — seconds if the disturbance passes, many minutes if it does not.
Words used here
Pedipalp
The short leg-like appendage beside the fangs. In a mature male it is enlarged and used to transfer sperm.
Egg sac
A silk parcel of eggs. Guarded, carried or abandoned depending on the species.
Spiderling
A newly hatched spider. Structurally an adult in miniature — spiders do not metamorphose.
Almost every spider is a predator of live animals, overwhelmingly insects and other spiders. Ants and cockroaches are both eaten — ants by specialists that can handle formic acid and by many generalists that cannot and avoid them, cockroaches by anything large enough, which in a house usually means a huntsman or a large orb-weaver. Bigger spiders take proportionately bigger prey: fishing spiders catch small fish and tadpoles, and a few large tropical species take frogs, lizards and occasionally birds. Exactly one spider is known to be largely vegetarian, a Central American jumping spider that lives on the protein bodies of acacias, and it is famous precisely because it is the exception.
The eating itself is the strange part, and it explains several things at once. A spider has no jaws that chew and a gut opening too narrow to pass solids. It injects digestive enzymes into the prey, or floods them over it, waits while the tissue liquefies, and sucks the result back through a muscular pumping stomach. Digestion happens outside the body, and what is left is an emptied husk rather than a carcass.
Very little solid waste is produced, because very little solid is ever taken in.
Anything that cannot be liquefied cannot be eaten, which sets a real limit on prey type.
A spider can survive weeks and sometimes months without a meal — metabolism is low and there is no cost of keeping warm.
Water is drunk from droplets, damp surfaces and dew, and thirst is a common reason spiders appear in sinks and baths.
The scale of it is easy to underestimate. The global spider community is estimated to consume somewhere between 400 and 800 million tonnes of prey a year, an estimate built from spider biomass and feeding rates rather than from counting, and comparable in magnitude to the total meat and fish consumption of the human population. It should be read as an order-of-magnitude statement.
The world’s spiders eat a staggering amount of insects each year — but the figure is an estimate
Emerging evidence
Real findings exist, but too few or too recent to be settled.
A global extrapolation from regional spider biomass and feeding-rate data estimated annual prey consumption by all spiders at roughly 400–800 million tonnes, predominantly insects and springtails.
Who this applies to
all spiders, as a modelled global total
Studied in
Araneae
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence
A carefully constructed estimate published in a peer-reviewed journal, but built on two uncertain layers — spider density and per-spider feeding rate — with bounds the authors state plainly and popular coverage almost always drops.
How far it can be extended
The claim is explicitly about the group as a whole, because it is a global extrapolation rather than a measurement of any species.
Caveats
This is a model, not a measurement, and the range spans a factor of two.
Spider density data are sparse for tropical and canopy habitats.
The figure is usually quoted as a single number with no range and no method.
Still unanswered
How much do spider populations actually regulate insect numbers?
A spider has muscles to fold its legs and none to straighten them
Established
Specialists would state this without hedging. Multiple independent lines of evidence agree.
Spiders possess flexor muscles in the legs but lack extensor muscles at the femur–patella and tibia–metatarsus joints. Extension is achieved hydraulically by raising haemolymph pressure in the prosoma, which forces fluid into the limbs. Loss of internal pressure through death, desiccation or a fresh moult therefore leaves the legs flexed.
Who this applies to
spiders generally
Studied in
Araneae
You may have heard
“Dead spiders curl up because their legs relax”
Relaxing is exactly what makes them curl, and not for the reason implied. There is nothing to relax *into* a straight leg — extension is done by pumping fluid, so when the pump stops the flexors win by default. The curled posture is the spider’s resting shape, not a spasm.
Why we rate it this way, and what the caveats are
EstablishedHigh confidence
Directly observable anatomy, and the mechanism is confirmed by pressure measurement and by the predictable posture of a dead or dehydrated spider.
How far it can be extended
The absence of extensor muscles at those joints and the hydraulic mechanism are general features of the order.
Caveats
Some jumping power comes from the same hydraulic system, but leg muscles contribute in ways still being measured.
Pressure is generated in the prosoma, so an injury there affects every leg at once.
Details of the joints involved vary between families.
Still unanswered
How do spiders maintain the pressure needed for sustained fast running?
Leg musculature, haemolymph pressure and the mechanics of extension.
A spider climbs a window by using an interaction that operates at the scale of individual molecules. Each foot carries a dense brush of hairs called a scopula, and each of those hairs splits into hundreds of finer tips, so a single foot may make hundreds of thousands of separate contacts with the glass. Every contact contributes a tiny van der Waals attraction, and the total is enough to hold many times the spider’s weight. Nothing is sticky — there is no glue and no suction, and the spider detaches by peeling the hairs off at an angle rather than by pulling.
This is also why a spider in a bath cannot get out. Enamel is smoother than the hairs can grip and the curve is steep. The spider is not there because it came up the drain; it is there because it went in looking for water and found the one surface in the house it cannot climb.
Breathing uses no muscles at all. Depending on the group a spider has book lungs — stacked plates in an abdominal cavity, air on one side and haemolymph on the other — or a tracheal system of fine tubes carrying air directly to the tissues, or both. Gas exchange is by diffusion through openings on the underside of the abdomen. There is no chest movement and nothing that could be called a breath, which is one reason a spider can be motionless for hours at no cost.
Spider blood is not blue, though it is not red either. Haemolymph carries oxygen using haemocyanin, a copper-based pigment dissolved in the fluid, and in a living spider it is very faintly blue-tinged and mostly close to colourless. The vivid blue of illustrations is haemocyanin at concentrations spiders do not have. The same pigment, in an animal that needs far more of it, is what makes octopus blood genuinely blue — and it comes with the same limitation, since haemocyanin carries less oxygen than haemoglobin does.
This is a question NatureHQ cannot answer, and the reason is worth more than a guess would be. Two different things are being asked. Does a spider detect damage and respond to it? Almost certainly yes — that is nociception, it is close to universal in animals, and a spider withdraws from harm, favours an injured leg and will amputate one that has been damaged. Does it feel anything while doing so? That is a question about subjective experience, and there is no measurement that reaches it.
The tests that have been used to argue for pain in other invertebrates mostly have not been run on spiders. Decapods have been shown to trade off avoiding a noxious stimulus against a valuable shelter, to groom a treated site, and to alter behaviour long after a stimulus ends — evidence of something more than a reflex. Insects have produced results in both directions. For spiders the equivalent experiments are largely absent, so what exists is an inference from a nervous system that is small, differently arranged, and not obviously lacking anything required.
What can be said is that leg autotomy — the spider dropping a leg that has been grasped or injured — is under the spider’s control rather than a mechanical break, and that spiders modify behaviour after injury for extended periods. Neither observation reaches the question of experience. Both are the sort of evidence that would look like part of an answer if the rest of the work had been done.
Animal welfare
What follows practically
The honest position is uncertainty rather than a licence. Where an animal may or may not feel pain and nobody knows, the reasonable default is to avoid causing injury unnecessarily and to remove a spider rather than kill it. That is a decision about how to act under uncertainty, not a claim about what the spider experiences.
Where this applies: A general position rather than a legal one. Invertebrate welfare protection varies by country and covers cephalopods and decapods far more often than arachnids.
Words used here
Nociception
Detecting and responding to tissue damage. Present in almost all animals, and not the same as feeling pain.
Autotomy
Deliberately shedding a limb, at a designated breakage point, to escape. Controlled by the animal.
Of roughly 52,000 spider species, the number capable of causing serious harm to a healthy adult is somewhere around thirty, and they are geographically concentrated. Most reported "spider bites" in medical settings turn out on investigation to be something else — commonly bacterial infections. This is not a reassurance offered in place of evidence; it is what the clinical literature repeatedly finds.
Safety
If you are bitten
Most spider bites need nothing beyond cleaning the area. Seek medical attention if you have severe or spreading pain, muscle cramps, a spreading lesion, or symptoms beyond the bite site — and if you can do so safely, keep the spider for identification. Do not attempt to catch a spider you believe is medically significant.
Where this applies: Which species are medically significant depends entirely on region; local health services are the authority.
When to get help: Contact local emergency or poisons services for a bite with symptoms beyond the immediate area.
The ecological case for tolerating spiders indoors is strong: they consume large numbers of the insects people actually object to. The global consumption estimate below is the number usually cited for this, and it deserves its caveats.
Two everyday questions have straightforward answers that are worth stating, because the usual explanations are wrong. Spiders do not come indoors to escape the cold — house spiders in most of the temperate world live indoors permanently and are a different set of species from the ones in the garden. What changes in autumn is that mature males leave their webs and spend their remaining weeks searching for females, so they become visible. The spider crossing the carpet in September was already in the building.
A spider that appears to run at you is almost never approaching you. It is running from something, and a spider detects the world through vibration in its legs rather than by looking — so it registers that something enormous has moved nearby and bolts in a direction chosen before it has any idea where you are. Sometimes that direction is towards your feet. Jumping spiders, which do see well, are the exception in the opposite sense: they will turn and watch a person deliberately, and that is curiosity rather than threat.
The spider in the bath did not come up the drain — the U-bend is full of water. It came for the moisture and then found that enamel is the one surface in the house its foot-hairs cannot grip.
The world’s spiders eat a staggering amount of insects each year — but the figure is an estimate
Emerging evidence
Real findings exist, but too few or too recent to be settled.
A global extrapolation from regional spider biomass and feeding-rate data estimated annual prey consumption by all spiders at roughly 400–800 million tonnes, predominantly insects and springtails.
Who this applies to
all spiders, as a modelled global total
Studied in
Araneae
Why we rate it this way, and what the caveats are
Emerging evidenceModerate confidence
A carefully constructed estimate published in a peer-reviewed journal, but built on two uncertain layers — spider density and per-spider feeding rate — with bounds the authors state plainly and popular coverage almost always drops.
How far it can be extended
The claim is explicitly about the group as a whole, because it is a global extrapolation rather than a measurement of any species.
Caveats
This is a model, not a measurement, and the range spans a factor of two.
Spider density data are sparse for tropical and canopy habitats.
The figure is usually quoted as a single number with no range and no method.
Still unanswered
How much do spider populations actually regulate insect numbers?
Anatomy of the salticid principal eye showed a fixed lens with a narrow, high-acuity retina behind it that muscles sweep across the scene. Resolution comparable to a pigeon’s, in an eye smaller than a pinhead — and vision assembled over time rather than taken at a glance.
A spider is shown solving a problem by trial and error
Portia, hunting other spiders, was found to vary the signals it plucks on a victim’s web and to persist with whatever produced a response — generating and testing behaviour rather than running a fixed routine. The finding was hard to accommodate in an animal with a hundred thousand neurons, and it was the beginning of taking spider cognition seriously.
Silk spinning explained as liquid-crystal processing
The conversion of stored liquid protein into solid fibre was characterised as a liquid-crystalline transition driven by changes in acidity, ion exchange and water removal along a duct a few millimetres long — no heat, no pressure, no solvent, and still not reproducible industrially.
The web family tree is rebuilt, and hunters turn out to be ex-weavers
Molecular phylogeny placed the wandering hunters inside the web-building lineages rather than before them, inverting the intuitive story: the orb web came first and several groups abandoned it. Losing the web is a strategy, not a stage on the way to one.
Changes how the 1993 result reads
It does not touch the Portia result, and it changes what that result is about. If the wandering hunters are descended from web-builders, then Portia — a jumping spider that invades webs and plucks them — is not an animal that has developed a new skill but one whose lineage never lost the old sensory relationship with silk.
The web is proposed as an instrument the spider tunes
Measurement of vibration transmission showed that silk tension and web geometry determine which frequencies reach the spider, and that spiders adjust both. The web stops being a passive trap and becomes an adjustable sensor — the strongest version of the argument that some spider cognition sits outside the animal.
Changes how the 1969 result reads
The 1969 eye work made the point that a tiny animal can carry a high-performance sense organ. This makes the stranger version of the same point: the organ need not be part of the animal at all, and a spider adjusting web tension is adjusting its own hearing.
Spiders in still air, with no wind available, raised their abdomens and took off in response to an applied atmospheric-potential-gradient field, and their trichobothria were shown to deflect in it. Dispersal that had been explained entirely by wind turned out to have a second input.
A spider does not squeeze silk out — it pulls a fibre from a liquid, using water and acid
Well supported
Good evidence backs this, though some details remain open.
Silk proteins are stored as a concentrated liquid crystalline solution and converted to an insoluble fibre by acidification, ion exchange and water extraction as they pass down the spinning duct, with the fibre drawn rather than extruded under pressure.
Who this applies to
orb-weaving spiders, in which the spinning process has been characterised
Studied in
Nephila edulis, Caerostris darwini
Why we rate it this way, and what the caveats are
Well supportedHigh confidence
A direct physiological characterisation of the gland and duct, with the chemical gradient measured along its length.
How far it can be extended
The gland and duct anatomy is broadly conserved across spiders, so the mechanism is expected to be general. It has been characterised in orb-weavers.
Caveats
Spiders make up to seven silk types with very different properties; "spider silk" is not one material.
"Stronger than steel" is true only for tensile strength by weight, and toughness — energy absorbed — is the property that actually matters for a web.
Characterised in orb-weavers; other families are less studied.
Still unanswered
Can the spinning process be reproduced industrially at ambient temperature?
How do spiders switch between silk types on demand?
Measured the toughest biological material known, produced by the same ambient-temperature process.
The claim that spider silk is stronger than steel is true, in one narrow sense, and misleading in most of the ways it is used. By tensile strength per unit weight, dragline silk does exceed steel. But strength is not the property a web needs. A web has to stop a flying insect without snapping, which is a question of *toughness* — the total energy the fibre absorbs before failing — and on that measure silk is far ahead of both steel and manufactured fibres. The toughest known biological material is the dragline of Darwin’s bark spider, which builds orb webs spanning rivers up to 25 metres across.
The more remarkable fact is how it is made. Silk proteins are stored as a concentrated liquid and converted to a solid fibre as they travel down a duct a few millimetres long, by changing acidity, swapping ions and drawing out water. No heat, no pressure, no solvent — the spider pulls the fibre rather than squeezing it out. Industry cannot yet reproduce the process, which is why synthetic spider silk remains a research problem rather than a product.
"Spider silk" is also not one material. A single orb-weaver can produce up to seven types from different glands: dragline for the frame, capture spiral that is sticky and stretchy, wrapping silk for prey, and a tougher silk for the egg sac. They differ enormously in properties, and a claim about one rarely holds for another.
Words used here
Toughness
The total energy a material absorbs before breaking. Different from strength, and the property that matters for catching something in flight.
Dragline
The strongest silk a spider makes — the safety line it trails behind it and the frame of an orb web.