Spiders surround us. The Smithsonian Institute says that “typical temperate habitats may support up to 800 individual spiders per square meter (10.8 square feet).” Yet, if you think about it, how many times have you been bitten? Several times? Well, probably not; experts say spiders seldom bite and are typically blamed for bites from other insects.
Spiders are beneficial
Spiders aren’t interested in humans — they’re on the hunt for other critters, and do us a big favor: National Geographic says a single spider eats about 2,000 insects a year. Using the figures above, that translates into 1,600,000 insects per square meter! Imagine the value of spiders on a worldwide scale.
Spiders help reduce populations of insect pests in our yards and farmlands, and in our homes, as well. They benefit other wildlife, too, because they’re food for birds, reptiles, small mammals and other organisms. Spider silk is one of the strongest, most elastic natural fibers and has given rise to the synthesized silk used for parachutes and other products.
Spiders are animals in the taxonomic class Arachnida (Ah-RACK-nee-duh) and the order Araneae (Ah-RAIN-ee-ee). Arachnida is derived from the Greek arakhne, for spider. Spiders comprise the largest order in Arachnida, with 109 families. There are 10 other orders in that class, which includes mites, ticks, harvestmen, and scorpions, among others.
There are at least 43,650 identified species of spiders in the world. They inhabit every continent except Antarctica and nearly every kind of terrestrial habitat. About 3,400 species inhabit North America.
Spiders have been around for eons, but there aren’t many fossil records because their bodies are soft and easily deteriorate. Those that do exist are usually preserved in amber (hardened tree sap).
Nephila is the longest-lived modern spider genus found so far, dating back 165 million years (the Middle Jurassic). The world’s smallest spider is believed to be the male Patu digua, only 0.02 inches (.37 mm), about the size of a pinhead. The world’s largest spider is the Goliath Birdeater Tarantula, Theraphosa blondi, which has a leg span of up to 10 inches (25.4 cm) and may weigh more than 6 ounces (170 g).
It’s commonly thought that spiders are insects, but they’re classified separately. Several characteristics distinguish the two groups apart. The most noticeable are:
- Spiders have two body sections; insects have three.
- Spiders have eight legs; insects have six.
- Spiders don’t have antenna; insects do.
Spiders come in different shapes, sizes and colors — short, long, round, oblong, thin, fat, spiky, hairy, smooth, bright, dull, and more. Those shown below are just a partial representation of the variety and sometimes-beautiful coloration of spiders.
The front section of a spider is called the cephalothorax (SEFF-uh-low-THORAX). The back section is the opisthosoma (oh-PIS-tho-SOMA), or abdomen.
The cephalothorax (also called the prosoma in some groups) is composed of the spider’s head and the thorax, which are fused together. It contains the brain, central nervous system, esophagus, part of the digestive system, venom glands, and nerve cells. The spider’s eyes and mouth are at the front, along with the pedipalps, chelicerae, and fangs. Attached to the cephalothorax are the spider’s eight legs, four on each side.
The cephalothorax is quite hard compared to the abdomen. There is no neck between the two sections (except for assassin spiders). Instead, they’re connected by a thin, cylindrical “waist” called a pedicel. A bundle of nerve fibers and the foregut pass through it to the abdomen. The pedicel is flexible, which allows the abdomen to move in all directions as spiders make their webs. Spiders are commonly thought to have no internal skeleton, but they do have a small one made up of cartilage-like material.
The top and bottom of the cephalothorax are covered by “shells.” The top one is called the carapace, and the bottom one is the sternum. The plates of the shell are made of protein and chitin, which produces a tough material called cuticle. Below the cuticle is a layer of epithelial cells containing pigment granules which give spiders their colors.
The opisthosoma, or abdomen, is soft and pliable, which allows it to expand when filled with lots of food or when holding eggs. It houses the heart, book lung, silk glands, digestive system, and ovaries. At the very end are four to six spinnerets and the anus.
Each leg has seven segments: the coxa, trochanter, femur, patella, tibia, metatarsus and at the end, the tarsus. At the tip of each tarsus are claws. Some spiders have two claws. Web-spinning spiders have three and use the center one for grasping silk threads.
The end of each leg is covered in brushes of hair, and the end of each hair has microscopic gripping “feet” that help the spider grip surfaces, even slick ones, as they walk. There are some surfaces that spiders have trouble scaling, however — glass and porcelain, for example; you’ve probably found them trapped in bathtubs or sinks. If a young spider loses a leg, it can be regenerated.
Brain and intelligence
The spider’s brain takes up a disproportionately large area of its body — 80 percent or more with some species. Its most role is processing and responding to sensory information. Research is showing that spiders are surprisingly smart. Maybe this should have been obvious all along, considering how intricately they design their webs.
Orb-weaver spiders can count. They keep track of how many silk-wrapped prey they have stored in their web and if one or more of them are experimentally stolen the spider will search for them in proportion to the number taken. Jumping spiders plot intricate routes to reach and ambush their prey — scientists say this is on a par with the intelligence of larger animals.
Mouth and fangs
Located at the front of the head is the spider’s mouth. Near it are two appendages called chelicera (kuh-LISS-er-uh), with a fang at the tip of each one. Spiders use their chelicera to catch and hold prey while injecting venom into it with their fangs. The chelicerae generally have finely serrated inner edges which can be used to saw up their victim.
Fangs are positioned in one of two ways. Those of spiders in the infraorder Mygalomorphae point down and open and close vertically, in parallel. The fangs of spiders in the infraorder Araneomorphae move transversely in a pincer-type action. Most spiders have fangs too small to pierce human skin.
Spiders can move their fangs and also fold them up when not in use. Only spiders in the small family Uloboridae lack venom.
The pedipalps are two appendages at the front of the cephalothorax. Located between the chelicerae and first pair of legs, they have six segments and are usually shorter than the spider’s legs (although they’re sometimes mistaken for legs). Spiders use them like arms to hold prey. Males also use them for mating. You can often identify an adult male by the bulbous tips of his pedipalps, which females lack.
Spiders use their vision primarily for detecting motion. Most have four pairs, positioned symmetrically. A few families have three pairs and a few primitive species, such as a group called cave spiders that live in the dark, have very little or no eyesight. Like humans, spiders have a curved cornea and a lens, but their vision has a fixed focus and isn’t as quite as good. Scientists believe at least some species of spiders have color vision.
Different families have different placement, which helps in identifying species. Notice the location of the spiders’ eyes in the two examples shown below.
The primary eyes, always located in the center, have the best vision. The other, or secondary, eyes are less acute, but no less important — they provide periphery views and aid in seeing in dim light. Secondary eyes have a tapetum (tap-EET-um), which is a light-reflecting layer — shine a strong flashlight on spiders in the dark, and you’ll see these eyes, even if you can’t discern any other part of the body.
Jumping spiders (family Salticidae) probably have the best eyesight of all. Here’s an informative and entertaining video about spider eyesight, that features these spiders, by Bob TheSpiderHunter:
Hearing, taste, smell
Spiders don’t have ears, and they can’t hear in the way humans do. They “hear” with hairs on their body, which are sensitive to touch and vibration. Each hair is like a single, independent ear. The most sensitive are the hairs on the legs, called trichobothria (TRICK-oh-BAH-three-uh), which sense the tiniest movement of air, including sound waves. They’re so sensitive they can feel air movement down to one ten-billionth of a meter (the width of an atom).
Hairs also “taste” by sensing the chemical composition of what they touch. (“Hmm, is that a human, a delicious moth or the girly spider of my dreams?” Spiders don’t take action unless they detect prey or a mate.) It’s thought that spiders are tasting their prey when they use their pedipalps to manipulate it.
Spiders smell with hairs and sensory organs on their legs. They also have small cavities called tarsal organs located at the end of each leg which seem to be sensors for pheromones and changes in temperature and humidity.
Spiders communicate through vibrations, touch, hearing, and pheromones. Recent research has discovered a wolf spider species, Gladicosa gulosa, that creates airborne sounds by vibrating leaves or other surrounding structures as a means of communication between males and females. Listen to the wolf spider
Spiders are opportunistic eaters. They gorge on food, usually insects, when they can, and may go for long periods of time without eating if food is scarce. Spiders that actively hunt for prey probably eat more regularly than those who lay in wait on their webs or flowers.
A spider’s mouth is shaped like a short, tiny drinking straw and sucks rather than chews. The mouth is surrounded by two appendages (chelicerae) with fangs at the tips. Sometimes referred to as jaws, the chelicerae are used to grasp and hold prey while injecting venom into it. To immobilize and carry their prey, some spiders wrap it in silk, turning it over and over to cover it completely.
Spiders eat a liquid diet, so their meal must be turned into a liquid before it can be consumed. Here’s how it’s done: A tube leads from the mouth through a foregut and into a muscular reservoir in the abdomen, called the sucking stomach.
The sucking stomach contains digestive enzymes and, with some assistance from the pharynx, is capable of pumping its contents both forward and backward. Spiders regurgitate these enzymes onto their prey to dissolve it and then suck the liquefied food back into their bodies. They perform this action several times until the prey is dissolved and consumed. Tiny hairs surrounding the spider’s mouth filter out solid particles as the spider sucks, but some minuscule solids do still get through.
From the sucking stomach, the food moves into the midgut. The midgut begins in the cephalothorax and squeezes through the pedicel to the abdomen. Ranging outward from the midgut is a series of pouches called caeca (SEE-kuh). These are glands that store digestive fluids. The abdomen of a well-fed spider may be nearly bursting with caeca filled with liquefied food. Sometimes caeca even extend down into the spider’s legs. This allows spiders to go for long periods without eating.
Spiders don’t produce urine, but they do produce uric acid from “kidneys,” called malpighian (mal-PIG-ee-un) tubules. The uric acid is semi-solid and combines with solid waste in a pouch called the stercoral (STER-corr-all) pocket, then moves to the hindgut and out through the anus. Look around on a spider’s web, and you’ll see small white or colored spots — spider poo.
The spider’s circulatory system is called an open system, where blood is pumped from the heart through arteries, which empty directly into the body cavity. The blood is called hemolymph. It contains hemocyanin, a copper-based protein that causes it to turn blue-green when oxygenated. (A human’s blood is red because of an iron-based protein.)
The heart is a tube with small holes in it called ostia. A membranous bag called the pericardium surrounds it. It isn’t entirely clear what the heart rate is. One study reports that the heart rate of spiders ranges from 30 to 200 beats a minute, depending on the species and extent of its activity. Another study reports resting heart rates in a range from 9 to 125 beats per minute. Results from various studies on bigger spiders (tarantulas, for example, which have bigger hearts), shows a range of 30 to 70 beats per minute.
Lungs and breathing
Spiders don’t actively breathe. Instead, air is taken in, and carbon dioxide is removed passively as the heart beats. Spiders have two kinds of breathing systems: Some have a network of tubes called trachea that are connected to an outside opening (spiracle) in the spider’s body, located near the spinnerets. But most spiders have a “book lung.”
Surrounded by blood, the book lung is thin, hollow and has plates stacked like pages in a book. It’s connected to a slit-like opening in the spider’s underside, where oxygen is drawn in, and carbon dioxide is expelled. Some spiders have two pairs of book lungs. Others, such as orb weavers and wolf spiders, have one book lung and a trachea. There are still others that have only a trachea.
Spinnerets and silk
Spinnerets are the organs through which a spider produces silk threads. They’re located at the rear of the abdomen, on the underside. Spiders normally have three pairs, but some species have only one pair and others as many as four. Seven types of silk glands have been identified, but no spider has all seven. The spinnerets move independently but in a coordinated way.
Silk glands produce proteins and other organic molecules which flow in liquid form through microscopic tubes to the spinnerets. As the mixture moves along, a process occurs which removes water from the proteins and adds hydrogen. The result is an acidic fiber which solidifies as it exits into the air through the spinnerets.
Each silk gland produces silk used for a different purpose: attachment silk; dragline and web frame silk; silk for wrapping, sperm web, and egg cocoons; and sticky silk. Spiders can use the threads singly or in combination. Silk threads are about one-millionth of an inch thick, but spiders can use muscles and valves in the necks of their spinnerets, singly or in combination, to thicken them.
Spiders use their silk in many ways, and different spiders use silk for different purposes. Not all spiders build webs, but all spiders do produce silk and use it for draglines. All males use silk to spin a sperm web before mating, and all females spin a silk cocoon around their eggs. Some spiders use silk to build shelters. Spiders are thrifty with their silk. Webs begin to degrade after a few days, so often spiders eat them and re-use the protein.
Spider silk is stronger than steel on a weight basis. If you made a steel rod and compared it to a rod of silk of the same weight, the silk rod would be stronger. Silk is amazingly elastic, too, able to stretch up to many times its original length. The thread of the orb-weaver spider Stegodyphus sarasinorum, an Asian species, is said to stretch up to 20 times its original length.
Not all spiders build webs, but those that do know instinctively how to do it. Webs may be relatively simple or very elaborate. Different species create differently patterned webs. Some spiders attach a formless tangle of silk threads to something (for example, the corner of a ceiling — the cobwebs we see up there are tangled webs that have accumulated dust and dirt).
Sheet web spiders create sheets of silk between tree branches, on shrubs and other suitable places — you may have noticed some on your lawn, where they last until they’re mowed away. Some webs are triangular, some funnel-shaped. The webs of bola spiders (in the genus Mastophora) are a single strand of silk, and there are other shapes, too. With some exceptions, a web can help in identifying a spider.
Webs are used for capturing prey. When an insect gets caught in the web’s sticky strands the spider rushes to it and uses its fangs to inject venom to kill it. Some spiders wrap their prey completely in silk to immobilize it while it dies. Webs are also used for courtship rituals and for securing egg sacs.
The most noticeable webs in our yards are probably the circular ones made by orb-weaving spiders (families Araneidae, Tetragnathidae, Uloboridae). Like other spiders, they’re methodical about spinning their webs and do it using only the tips of their legs.
How an orb-weaver builds a web
Orb-weavers spin their webs at night. From a suitable starting point, such as the branch of a tree, they release a thread of strong silk from a spinneret into the wind. If it doesn’t snag on something, the spider may pull it back and eat it to recycle the protein it contains before trying again.
Once it catches on something, the spider pulls the thread tight and anchors it. Then, it walks back and forth, reinforcing the line by adding more threads. Called the bridge line; it will support the weight of the entire web.
Next, the spider strings a loose thread from the bridge that gravity pulls down into a U-shape. It then moves to the bottom of the U and drops a thread from it that pulls the U into a Y-shape. After that, the spider produces a bottom line to attach to the tail of the Y thread, and this completes the frame. The completed Y forms the first three radial threads of the web. Then more radial threads are strung. Finally, threads are added in an “auxiliary spiral” from the center outward.
So far, the threads aren’t sticky. But, now, a sticky thread is strung between the radial lines, with the spider eating up the auxiliary spiral as it lays the sticky spiral.
The final construction consists of a non-sticky framework and radial lines, plus sticky circular lines. The spider then sits motionless in the center, waiting to feel vibrations on sticky silk — an announcement that prey is trapped. The spider moves on its web by stepping on non-sticky radial threads and the framework, while avoiding the sticky circular threads.
When daylight comes, the spider slips away to a resting spot. If the web doesn’t get too damaged, the spider may use it again and again, even patching it up a bit. Some spiders eat their web each morning, except for the framework, and re-spin it every night.
Orb-weavers don’t generally get stuck to their webs. That’s because they know where to step; they move their hind legs at an angle that somehow minimizes the effect of the stickiness, and they have tiny barbs on the bristles of their feet that keep them from sliding into sticky silk. In spite of those advantages, accidents do happen. When spiders get stuck, they seem to have a special oily coating that makes them less likely to stick. They don’t get trapped permanently.
Animation: how an orb-weaver constructs a web
Males actively search for females. Courtship differs, depending on the species. Males of some species “dance” or wave their pedipalps. Male gift-giving spiders, Paratrechalea ornata, catch prey and wrap it up as a gift of food for females (but, if they’re hungry they may cheat and eat the victim, then present its wrapped carcass as a gift!) Mating may take only seconds or many hours, depending on the species. It’s often said that females eat males after mating; however, that doesn’t commonly happen. Males wander off to die a few days later, having accomplished their life’s purpose.
Spiders reproduce in an unusual way. The female’s organs for producing eggs and receiving sperm are in her abdomen. The male’s sperm-producing organs, two testes, also are in his abdomen, but he lacks a penis.
Not to worry, though, as males have a workaround for that: They extrude sperm through a furrow in the underside of their abdomen onto a small web they construct, called a “sperm web.” The sperm is then drawn, probably by capillary action, into an organ called an embolus, located at the tip of the pedipalps. Only then does mating proceed — the male inserts the tip of his pedipalp into the female’s external genitalia, called the epigyne (EP-ee-jyne). The epigyne is a small area of hardened tissue located on the underside of her abdomen, near the book lung.
In most groups, the male inserts his left pedipalp into the left opening of the female’s epigyne and his right pedipalp into the right opening. In more advanced species, the female’s epigyne and the male’s pedipalp are so species-specific that, like a key and a lock, spiders of different species don’t match up and can’t mate. This makes the epigyne the most certain characteristic for identifying species. (Some spiders, including more primitive groups, are less complicated.)
Sperm moves from the epigyne into storage receptacles called spermathecae, where it will stay until the female is ready to use it. As eggs pass from her ovaries to leave her body, she releases some or all of the stored sperm to fertilize them. Females can store enough sperm from a single copulation to last their entire life. The expelled eggs, which often look like drops of liquid, are deposited onto a silk pad and then wrapped in multiple layers of silk. The wrapped sacs are round or disk-shaped.
Females of some species stick their egg sacs to a surface and abandon them. Others guard them, lay them on their web, or carry them around between their chelicerae or attached to their spinnerets.
A single egg sac may contain hundreds or thousands of eggs. If a female constructs several sacs, each one will contain fewer eggs. In many species, the female dies after laying her last egg sac. Others, however, live one or two years and take care of their offspring for a period after they hatch.
Most eggs hatch when the weather is warm. In spring and summer, eggs may hatch in two to four weeks. If eggs hatch in the fall or winter, the spiderlings stay in the sac until spring.
Spiderlings are usually on their own from the time they leave their egg sac, but mothers of some species feed their young. Wolf spiderlings ride around on their mother’s back for about 10 days.
Most spiderlings leave their birthplace by “ballooning” away: They climb up a tall object, such as a branch, tree or fence post, and let the breeze pull silk from their spinnerets to use as a “dragline.” A newly hatched spider weighs so little that as the dragline lengthens it eventually pulls the tiny body up and away. At some point the airborne spider catches on something and from there begins its independent life.
As they grow, baby spiders outgrow their skin and must shed (molt) it several times. The bigger the spider grows, the more times it will molt.
A full life span for most spiders is a year or two, depending on the species. But, most become food for predators or die in some other way before they reach maturity. There are some exceptions. Spiders in the infraorder Mygalomorphae, which tend to be big and hairy (tarantulas, for instance), can live years longer. Some take a decade or more to reach maturity. Captive tarantula males live only one or two years, but females can live 25 years!
Whether they’re captive or not, male spiders have shorter lives than females. One reason is that they move about in search of mates and face more attention from predators. Males that don’t come to a tragic end tend to die soon after mating.
Spiders feed mostly on insects, but also on other spiders. Large spiders may catch small animals, such as lizards, frogs, baby snakes and baby rodents. Large orb weavers will feed on small birds or bats caught in their web. One exception to an all-meat diet is the male crab spider Misumena vatia, known to feed on pollen while sitting on flowers as they lay in wait for insects. There are four typical ways that spiders catch prey. Depending on the species, they:
- Live in a silk-lined burrow and leap out to catch prey.
- Lay in wait to ambush prey on plants, flowers, the ground, under stones, on tree bark and other suitable places.
- Actively search for prey.
- Spin a web to entrap prey.
Spiders are preyed on by birds, frogs, toads, snakes, lizards, ticks and wasps. In some countries, monkeys prey on them. Daddy Longlegs (which look like spiders, but are not) prey on spiders. Spiders have to watch out for other spiders, too. Roundworms and mites parasitize spiders. Humans accidentally or intentionally kill them.
A spider’s best defense is to hide. Many are well-camouflaged and often hide in plain sight. Or, they may run into a crevice, under bark, under a rock or anywhere else a predator may not be able to enter. Some trapdoor spiders construct burrows with hinged doors made of silk, soil, and vegetation — when threatened, they can run inside and tightly close the door. Golden orb-weavers of Asia coat their webs in a chemical that repels many insect predators. Wolf spiders will bite in defense. Tarantulas and others in the family Theraphosidae have urticating (stinging) hairs they flick at predators.