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How Spiders Work
by Tom Harris

Love 'em or hate 'em, you've got to admit that spiders are some pretty impressive, well-equipped animals. They have a top-notch sensor array, a built-in construction set, a heavily armored body and a highly effective venom-injection system. How many other creatures can claim that?

Photo courtesy Steve & Miriam Clark
A lynx spider in the Australian Outback

These remarkable adaptations have made spiders some of the most successful carnivores in history. In their 400 million years of existence, they've spread over every continent and mastered nearly every environment on Earth. Today, there are about 40,000 known spider species, and potentially thousands more we haven't discovered yet. This is pretty astounding when you consider that there are only about 4,000 different species in the entire mammal kingdom.

In this edition of HowStuffWorks, we'll find out how these unique animals spin webs, attack prey and walk straight up walls. We'll also look at some particularly interesting spider species, including spiders that swim, spiders that jump from branch to branch and spiders that can kill a person.

Spider Basics
Spiders look a lot like insects, but they're actually part of an entirely different class of animals, called Arachnida. Spiders make up the order Araneae within this class, which also includes mites, ticks and scorpions.

Click on the buttons at the bottom of the image to see how spiders fit into animal taxonomy.

While spiders vary considerably in size, shape and behavior, nearly all species share a basic set of characteristics:

  • They have eight legs, made up of seven segments each.
  • They feed primarily on insects.
  • They can inject venom into their prey.
  • They can produce silk.
  • They have a pair of small appendages on the head, called pedipalps.
  • Their bodies are divided into two sections, the cephalothorax and the abdomen, joined by the thin pedicel. The cephalothorax -- a fused head and thorax -- distinguishes spiders from insects, which have a separate head, thorax and abdomen.

Spiders have the same basic bodily systems as people, but they don't work in the same way and they're arranged differently in the body. The cephalothorax contains the brain, stomach, eyes and mouth, and the abdomen contains the heart, digestive tract, reproductive organs and lungs.

Photo courtesy Ed Nieuwenhuys

Spiders have two different types of respiratory systems -- trachea and book lungs (most species have both, but some have one or the other). Compared to human lungs, these respiratory systems are incredibly simple. Trachea are just long tubes that run from a slit in the exoskeleton through the body. Air flows in, oxygen diffuses into the blood and carbon dioxide diffuses into the air. The spider's normal movement provides all of the necessary energy to push air in and out. Book lungs are a series of very thin, leaf-like structures (like the pages in a book). The inside of each leaf is filled with blood, and the outside is exposed to air. As in the trachea, the book lungs exchange oxygen and carbon dioxide through diffusion.

The spider's blood, called hemolymph, circulates oxygen, nutrients and hormones to the different organs in the body. Unlike humans, spiders have an open circulatory system. The spider's simple heart -- a tube surrounded by a muscle, with a one-way valve on each end -- pumps blood into the body cavity, all around the spider's organs. Organs get oxygen because they're soaking in blood.

One of the most amazing things about spiders is how much they can accomplish with such a small brain. The spider's central nervous system is made up of two relatively simple ganglia, or nerve cell clusters, connected to nerves leading to the spider's various muscles and sensory systems. The simple instructions encoded in these nerve cells give spiders all the information they need to undertake complex tasks, such as building webs and attacking prey. Some species even exhibit learning behavior. If something isn't working -- a web in a particular spot, for example -- the spider will give up the activity and try something new.

Photo courtesy MicroAngela
Electron microscope image of one of a spider's sensitive hairs
While most spiders have multiple pairs of eyes, vision is a secondary sense in the vast majority of species. Most spiders primarily interact with the world through tactile sensation. They are covered in highly sensitive hairs that pick up even low-level vibrations in whatever the spider is standing on (the ground, floor, leaf or web, for example). Many spiders have additional hairs, called trichobothria, which pick up vibrations in the air (sound). Most spiders also have a sense of taste and smell, which play a role in feeding and reproduction.

Many spiders have special adaptations that allow them to walk easily along relatively smooth or vertical surfaces. The end of each leg is covered with thick brushes of hair, and the end of each hair is covered in tiny microscopic "feet." All the tiny feet grip the small bumps on whatever the spider is walking on, allowing the spider to move easily over most terrain (the smooth surface of the bathtub being a notable exception for many species).

Spiders have thick brushes of hair, each ending in tiny microscopic feet that let it walk up vertical surfaces.

To many arachniphobes, large, hairy tarantulas are as bad as it gets. But in fact, tarantulas, a loose term for large, hairy spiders in the mygalomorphae suborder, are relatively harmless to humans. Most spend their time in the brush or in holes, waiting for their prey to wander along. They do not spin webs, and they aren't venomous to humans. Their fangs can puncture the skin, however, and some species can shoot barbed hairs from their back.

Spider Structure and Growth
Like people, spiders move by contracting muscles attached to a skeleton. But Instead of an internal skeleton covered in flesh, spiders have have an exoskeleton -- a stiff support structure on the outside of the body. Exoskeleton segments are connected together with joints so the spider can move them back and forth.

Muscles attached on the inside of the exoskeleton contract to move the legs inward, but spiders don't have any muscles to extend the legs back out again. Instead, they have to force bodily fluids (mainly blood) into the legs to push them outward. If a spider loses too much body water, it can't generate the necessary hydraulic pressure to push its legs out. This is why you sometimes see spiders on their backs with their legs curled up.

Photo courtesy Ed Nieuwenhuys
Aculepeira armida, an orb web spider

The exoskeleton is made of several layers of cuticle, a composite material containing various proteins and chitin, a long-chain polysaccharide (sugar). The chitin and protein molecules are arranged in long chains, in successive layers, like the grain in plywood.

This structure makes cuticle extremely strong, as well as highly effective at keeping the spider from drying out, but the material does have one serious drawback. While it's flexible enough for movement, it can't expand like human bones and tissue -- in other words, it can't grow. In order to increase its size, the spider has to form a new, larger cuticle exoskeleton and shed its old one (this is called molting).

Molting occurs frequently when a spider is young, and some spiders may continue to molt throughout their life. At the appropriate time, hormones tell the spider's body to absorb some of the lower cuticle layer in the exoskeleton and begin secreting cuticle material to form the new exoskeleton. The new exoskeleton is typically folded to some extent, so it can expand once the spider sheds the older one. The spider also secretes a molting fluid between the old exoskeleton and the new one. Once the new exoskeleton is finished, the spider absorbs the molting fluid. This creates a gap between the two exoskeletons, which makes it easier to separate them.

Micaria romana: While these spiders have eight legs and two body segments, like all other spiders, they superficially resemble their primary prey, ants.

To shed the old exoskeleton, the spider has to bust out from the inside. It increases its heart rate to pump a lot of hemolymph (the spider's blood) from the abdomen into the cephalothorax. The pressure expands the cephalothorax, which pushes on the old exoskeleton until it cracks. The spider flexes its muscles until the old exoskeleton falls away.

Typically, the spider does most of its growing immediately after losing the old exoskeleton, while the new exoskeleton is highly flexible. The new exoskeleton is also very soft in this stage, making the spider particularly vulnerable to attack. Many species will lower themselves on a silk line during the molting process, so they're out of reach of predators while the cuticle material hardens.

Jumping Spiders

Photo courtesy Steve & Miriam Clark

Jumping spiders, as you might expect, have the ability to jump great distances -- as far as 50 times their own length. They don't have particularly strong muscles in their legs; they actually spring forward using hydraulic pressure. A powerful muscle in the cephalothorax squeezes fluids from the body into the legs to make them expand.

With more than 5,000 species around the world, jumping spiders are one of the more common spider varieties around. They're characterized by large eyes, which help them spot potential prey at a good distance. In contrast to web-spinning spiders, most jumping spiders hunt sort of like cats, stalking their prey and then springing on them at high speed.

Spinning Silk
The main thing that distinguishes spiders from the rest of the animal kingdom is their ability to spin silk, an extremely strong fiber. A few insects produce similar material (silk worms, for example), but nothing comes close to the spinning capabilities of spiders. Most species build their entire lives around this unique ability.

Scientists don't know exactly how spiders form silk, but they do have a basic idea of the spinning process. Spiders have special glands that secrete silk proteins (made up of chains of amino acids), which are dissolved in a water-based solution. The spider pushes the liquid solution through long ducts, leading to microscopic spigots on the spider's spinnerets. Spiders typically have two or three spinneret pairs, located at the rear of the abdomen.

Photo courtesy MicroAngela
Electron microscope image of a spider's silk spigots

Each spigot has a valve that controls the thickness and speed of the extruded material. As the spigots pull the fibroin protein molecules out of the ducts and extrude them into the air, the molecules are stretched out and linked together to form long strands. The spinnerets wind these strands together to form the sturdy silk fiber.

Photo courtesy Ed Nieuwenhuys
Argiope bruennich, an orb web weaver, spinning silk
Most spiders have multiple silk glands, which secrete different types of silk material optimized for different purposes. By winding different silk varieties together in varying proportions, spiders can form a wide range of fiber material. Spiders can also vary fiber consistency by adjusting the spigots to form smaller or larger strands.

Some silk fibers have multiple layers -- for example, an inner core surrounded by an outer tube. Silk can also be coated with various substances suited for different purposes. Spiders might coat fiber in a sticky substance, for example, or a water-proof material.

Spider silk is incredibly strong and flexible. Some varieties are five times as strong as an equal mass of steel, and twice as strong as an equal mass of Kevlar. This has attracted the attention of scientists in a number of fields, but up until recently, humans haven't been able to get much out of this natural resource. It's simply too hard to extract silk from spiders, and each spider has only a small amount of it.

Photo courtesy Steve & Miriam Clark
A female golden orb spider in its web

This may change in the near future. Researchers at a company called Nexia Biotechnologies have genetically modified goats using silk-producing genes from spiders. The hope is that a small number of goats will be able to produce a large amount of silk material in their milk. Engineers will be able to put this material to work in aircraft, bullet-proof vests and artificial limbs, among other things (check out this page for more information).

Actually forming silk material is only the first half of a silk-spinning process. In the next section, we'll find out how spiders put silk fiber to work.

Using Silk
While all spider species spin silk, they do a number of different things with the fiber once they produce it. Not all species spin webs, and many don't use their silk in hunting at all. Just like human building materials, spider silk has a wide range of applications.

Photo courtesy Ed Nieuwenhuys
Purse web spiders coat a hole in the ground with silk and lie in wait for passing prey.

One of the most common silk uses is the dragline. As some spiders move from place to place, they lay out a thin, dry thread behind them. Just like a mountain climber, the spider uses the thread as a safety line. If it gets in trouble, it can quickly backtrack on the line to get to safety.

Another common silk-use is nursery building. In most species, female spiders will spin a thick, protective cocoon for their developing eggs and sometimes the spiderlings once they've hatched. Some species will leave the cocoon unattended while the young spiders develop, and others, such as wolf spiders, will carry the cocoons around with them.

Photo courtesy Steve & Miriam Clark
A redback spider with her silk-spun egg sac

The most familiar use of silk, of course, is web building. Web structure varies a good deal from species to species. Some spiders build totally disorganized cob webs, some form long funnels out of silk sheets and some work as a colony to form huge masses of silk sheets around plant life. Some spiders, such as the net-casting spider, will form a small web between their legs and quickly wrap up any insect that gets caught.

The best-known web is the orb web, the intricate design spun by most garden spiders. This web is one of the most remarkable structures in nature, and its construction is among the most incredible animal activities you'll ever see. We'll examine this process in the next section.

Diving Spiders
Diving water spiders, or Argyroneta aquatica, are one of the most amazing animals on the planet. These spiders breathe air just like other arthropods, but they live most of their lives underwater. The trick is the spider's diving bell.

To build the diving bell, the spider first forms a web platform underwater, typically connected to a plant. Then it carries air bubbles down under the web, on the underside of its body. It releases the air bubbles under the web to gradually build a stable underwater air pocket -- the web keeps the air bubbles from rising to the surface. The spider ventures out to catch fish, tadpoles and other pond animals, and brings the prey back to the diving bell to eat it. It also mates, lays eggs and raises spiderlings in the bell.

The Orb Web
An orb-spinning spider puts its elegant traps together pretty quickly, proceeding easily from step to step according to the instruction manual preprogrammed into its brain. The diagram below shows the major steps.

Every web begins with a single thread, which forms the basis of the rest of the structure. To establish this bridge, the spider climbs to a suitable starting point (up a tree branch, for example) and releases a length of thread into the wind. With any luck, the free end of the thread will catch onto another branch. If the spider feels that the thread has caught onto something, it cinches up the silk and attaches the thread to the starting point.

It walks across the thread, releasing a looser thread below the first one. It attaches this thread on both ends and climbs to its center. The looser strand sags downward, forming a V shape. The spider lowers itself from this point, to form a Y-shape. This forms the core support structure of the web.

Photo courtesy Steve & Miriam Clark
A female redback spider in her web

The spider easily grips the thin threads with special serrated claws, a smooth hook and a series of barbed hairs on the end of its legs. As it walks along the initial structural threads, it lays more frame threads between various anchor points. Then it starts laying out radius threads from the center of the web to the frames. The spider does not coat the frame and radius threads with sticky material, since it needs to walk across them to get around the web.

After building all the radius threads, the spider lays more non-stick silk to form an auxiliary spiral, extending from the center of the web to the outer edge of the web. The spider then spirals in on the web, laying out sticky thread and using the auxiliary spiral as a reference. The spider eats up the auxiliary spiral as it lays out the sticky spiral, resulting in a web with non-sticky radius threads, for getting around, and a sticky spiral for catching bugs.

Photo courtesy Ed Nieuwenhuys
Argiope bruennichi, an orb web spider

The spider sits in the middle of its web, monitoring the radius threads for vibrations. If an insect gets caught in any part of the web, the spider will feel the motion through the radius threads and make its way to the vibration source. In this way, the web extends the spider's sensory system over a much wider area. The spider might also leave the web, to retreat to a separate nest, while monitoring the web via a connected signal line.

Web-spinning spiders have an innate ability to tell the difference between vibrations from insect prey and vibrations from other sources (a leaf falling into the web, for example). Many species can also distinguish the characteristic vibrations of dangerous insects, such as wasps, from their preferred prey.

Photo courtesy Ed Nieuwenhuys
An orb web spider with wrapped prey

When the orb web has deteriorated and is no longer useful, many spider species will destroy it, eating up all the threads so it can recycle the raw silk material. Spiders may leave the heavy bridge thread so that they can easily rebuild the web at a later point.

In the next section, we'll find out what spiders do when they catch their prey, whether in a web or on the ground.

Spiders are predators, above all else, so hunting and killing is where they really shine. In the bug world, spiders are fairly fearsome animals -- they're the tiny equivalent of wolves, lions or sharks.

Photo courtesy Ed Nieuwenhuys
A orb web spider feeding on a fly

Different spiders employ different hunting strategies. As we saw in the last section, some species build intricate webs to ensnare passing insects. Other spiders, such as the various wolf spiders, seek their prey out on the ground. Trap door spiders dig holes, cover them up with dirt doors hinged with spider silk, and lie in wait for passing prey. Similarly, some spiders hide inside flowers to catch feeding insects by surprise.

Photo courtesy Ed Nieuwenhuys

Photo courtesy Ed Nieuwenhuys
Two examples of crab spiders camouflaging themselves to catch insects off guard: Misumena vatia, the yellow crab spider above, can change its coloration over a couple of days to match flowers and other surroundings.

Whatever their hunting strategy, the vast majority of spiders follow the same basic killing and feeding procedure. The spider's primary weapon is its chelicerae, a pair of jointed jaws in front of the mouth. Each jaw has two major parts: the basal segment, the bulk of the jaw, and the sharp fang housed inside of it.

Normally, the fang is retracted inside the basal segment. When the spider catches its prey, it swings the fangs out into the animal's body. The fangs work something like hypodermic needles. They have a small hole in the tip and a hollow duct inside. The duct leads to the venom gland, either inside the basal segment or farther back in the cephalothorax. When the spider pierces its prey with the fang, it squeezes out the venom, injecting the animal with enough neurotoxin to paralyze or kill. This makes it safe for the spider to feed on its prey, without the risk of a struggle.

In the mygalomorph spider suborder, which includes the various tarantulas, the chelicerae are positioned so that the fangs swing forward into the prey, like an axe. In the dominant araneomorph suborder, the chelicerae swing in toward each other, like a pincer. For the mygalomorph system to work effectively, the prey has to be on ground or another solid surface -- the spider has to sandwich the prey between something else and the fangs. The araneomorph system works whether or not the prey is on solid ground -- the chelicerae simply push against each other.

Photo courtesy Steve & Miriam Clark

Photo courtesy Steve & Miriam Clark
Huntsmen spiders, liquefying and feeding on their prey

After paralyzing its prey, some spiders may wrap it up in silk to make it easier to transport back to the nest. Some species actually cover the prey in silk before injecting the venom, making it easier to attack. A female spider may carry wrapped prey back to its young spiderlings, and a male may bring the wrapped prey to a female as a courtship gift.

Most spiders don't eat their prey whole; instead, they expel digestive enzymes onto or into the animal to liquefy it. Some spiders use their fangs to inject the digestive fluid directly into the animal. This sort of spider liquefies the animal's insides, leaving the exoskeleton more or less intact. Then it sucks the liquefied remains into its stomach through hairs on its chelicerae and mouth, which act as a filter. Other species chew their prey up with serrated "teeth" on the chelicerae before vomiting digestive fluid on the body and sucking in the liquid remains.

Venomous to Humans
A very small percentage of spiders are venomous to humans. Venomous means the spiders may inject humans with dangerous poison (generally called venom in this context), whereas poisonous means the spiders would be harmful if eaten. The effects of spider venom vary depending on the species, age and sex of the spider, and on the age and health of the bitten person.

Neurotoxins in the venom may affect the human nervous system, causing dizziness, difficulty breathing, nausea, blurred vision and muscle rigidity, among other things. The venom may also kill tissue surrounding the bite. Generally, if a bite victim gets medical attention, they'll suffer minimal damage. If left untreated, a spider bite can kill, though this is very rare.

In North America, the most famous dangerous spiders are the black widow and the brown recluse. Both spiders can potentially kill, but the danger is slight for healthy adults. The spiders are both reclusive by nature, and will only bite if they feel threatened. Check out this site for more information on dangerous spiders.

Spider Sex
The male spider's primary objective in life is to impregnate one or more female spiders before other males can. As it turns out, this is no easy task in most species.

The first obstacle is actually finding a female spider. Most spider species are completely solitary animals, meaning they live and feed on their own, and they are generally spread out over a wide area, making an available female relatively scarce. The male spider has the daunting task of tracking down a sexually mature, receptive female in the area before other males can get there.

Photo courtesy Steve & Miriam Clark

Photo courtesy Ed Nieuwenhuys
Males and females of the same spider species often look totally different. Above, a tiny male golden orb spider (follow the yellow arrow) climbs on a giant female. Below is a female (green) and male (brown) of the crab spider species Micrommata virescens.

In most species, the female makes it easier on the males by "advertising" herself with pheromones, communicative chemicals. Many female ground spiders will secrete a pheromone on their drag line, the silk thread they leave trailing behind them. When males of the same species come across the dragline, they smell the pheromone with the chemical sensors on their front legs and follow the dragline to the female.

Web-spinning females may release pheromones directly into the air or coat their webs with pheromones, to make a natural "chemical antenna." Males may also stake out developing, sexually immature female spiders, so they can be the first to mate after the spider's final molt.

Once the male locates a female, it has to contend with any other males in the area. In species where the female spins a pheromone-coated web, the male's first order of business is to destroy the web to cut off the signal attracting any other males. If other males are present, the spiders in most species will fight it out for the right to copulate with the female.

After taking care of any other male contenders, the spider's next task is to deal with the female spider itself. Male spiders are generally much smaller than females in their species, making them easy prey. The male has to signal to the female that it is a spider of the same species, not food or a potential predator, and that it intends to copulate. This is courtship.

Photo courtesy Ed Nieuwenhuys
An Araneus diadematus male approaches a female.

Courtship varies considerably among different species. Many web-building spiders will use vibration as a means of courtship communication. The male may strum a unique signal on a thread connected to the female's web to identify itself and get across its intentions. Many spiders with better eyesight, such as various wolf spiders and jumping spiders, will "dance" to court the female.

Once the female recognizes the male's courtship behavior, she will position herself for sex, signaling to the male that she is receptive, or she will make it clear that she is not receptive (by shaking her web, for example, or just crawling away). If the male is desperate to mate, because all the females in the area will soon lay their eggs, he may proceed anyway, with full understanding that the female might kill him.

Both the male and female reproductive organs are at the rear of the abdomen, but spiders don't mate by coupling these organs. Instead, the male deposits some sperm onto a small web and picks it up on the end of his pedipalps. When the female is in position, the male deposits the sperm in the female's genital opening. The female stores the sperm in receptacles near the ovaries. When she is ready to lay her eggs, months down the road in some species, she uses the sperm to fertilize them. Some spiders may lay hundreds, even thousands of eggs in one shot.

As we saw earlier, some spider species will encase their eggs in a silken pouch and abandon them, and others will stay with them until the babies hatch. Many wolf spider species carry their hatched spiderlings around on their back until the spiders are mature enough to take care of themselves.

Photo courtesy Ed Nieuwenhuys
This crab spider keeps its spiderlings safe in a silk sac.

Photo courtesy Steve & Miriam Clark
A wolf spider carries its young spiderlings on its back.

The spiderlings continue molting, growing larger until they reach sexual maturity. Then the entire cycle begins again -- males seek out females, and females lay eggs. Most spiders have a fairly short lifespan, ranging from a few months to a couple of years. But some spiders, such as various female tarantulas, can live as long as 20 years. These spiders lay eggs many times throughout their life, and they molt annually, mainly to replace damaged body parts.

For much more information about spiders and other interesting animals, check out the links on the next page.

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