All about spiders

On this page: History, physical description, Senses, Communication
See part 2 for: Spinnerets, Silk, Webs and web-building, Life span, Food sources and Predator
Scared of spiders? It’s one thing when an “itsy bitsy spider climb(s) up the waterspout” and quite another if it crawls up an arm! Who among us doesn’t jump? Or scream! Even entomologists, who study all kinds of bugs, are known to jump at the sudden sight of a spider. Spiders are among the top five most-feared animals and researchers aren’t sure just why. Whatever the reason, the result is that spiders, as the late comedian Rodney Dangerfield might have put it, “don’t get no respect.” That’s too bad, because spiders are beneficial animals that are mostly harmless to humans. Their nature is to run from us, and they aren’t aggressive, unless provoked. Most can’t even bite through human skin.

We’re surrounded by spiders. 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 actually been bitten? 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. That would mean 1,600,000 insects per 10.8 square feet are removed by spiders! Imagine that 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, as 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 synthesized silk that’s 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 deteriorate easily. Those that do exist have usually been preserved in amber (hardened tree sap).

Fossil of spider clinging to captured prey, trapped in yellow amber.

Fossil of spider and captured prey in amber (Oregon State Univ. / Wiki; cc by-sa 2.0)

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 .37 millimeters (0.015-inch, 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 and may weigh more than 6 ounces.

Physical characteristics
It’s commonly thought that spiders are insects. Actually, they’re classified separately, because there are many characteristics that distinguish the two groups from each other. The most noticeable are:

  • Spiders have eight legs, insects have six.
  • Spiders don’t have antennae, insects do.
  • Spiders have two body sections, insects have three.

Spiders come in different shapes, sizes and coloration — short, long, round, oblong, thin, fat, spiky, hairy, smooth, bright, dull, and more. But, all can be easily identified as spiders because of their two body parts — the head region, called the prosoma, or cephalothorax (SEFF-uh-low-THORAX), and the abdomen, or opisthosom.

Composite image of nine different spiders, showing their diversity in appearance and colors.

Composite image of nine different spiders, showing their diversity in appearance and colors.

(Termininja / Wiki; cc by-sa 3.0)

The prosoma is quite hard compared to the abdomen, which is very soft. There is no neck between the two regions (with the exception of assassin spiders). Instead, they’re connected by a thin, cylindrical “waist” called a pedicel, through which a bundle of nerve fibers pass from the prosoma into the abdomen. The pedicel is flexible, which allows the abdomen to easily move in all directions as spiders make their webs. The top and bottom of the prosoma are covered by “shells.”

The top one is called the carapace and the bottom one is the sternum. The plates are made of protein and chitin, which produces a tough material called cuticle. Below the cuticle is a layer of epithelial cells which contain pigment granules — these give spiders their colors.

Spiders are commonly thought to have no internal skeleton, but they do have a small one made up of cartilage-like material.

Prosoma (Cephalothorax)
The prosoma contains the brain, central nervous system, esophagus, part of the digestive system, venom glands and nerve cells. The spider’s eyes and mouth are located at the front, along with the pedipalps, chelicerae and fangs.

The spider’s brain takes up a disproportionately large area of its body — 80 percent or more with some species. Research is showing they’re 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 their prey and make an attack — scientists say that’s on a par with the intelligence of larger animals. 

Drawing in brown ink of a spider, showing its three body sections, the legs, prosoma and abdomen.

(1) legs (2) prosoma (3) abdomen (CDC / Wiki)

The spider’s legs are attached to the prosoma, four on each side. 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 can’t scale, however — glass, for instance, and you’ve probably found them trapped in bathtubs or sinks! While still young, if a spider loses a leg, it can regenerate it.

Close up of the large fangs of the Black Wishbone Spider, showing their vertical position.

Fangs of the Black Wishbone Spider and others in infraorder Mygalomorphae are positioned vertically.

Located near the mouth are two appendages called chelicera (kuh-LISS-er-uh). A fang is 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 prey. Spiders can move their fangs and also fold them up when not in use. Only spiders in the small family Uloboridae lack venom. The fangs of most spiders are so tiny they can’t pierce human skin.

Close up of the large fangs of a spider in the infraorder Araneomorphae, showing their transverse position.

Fangs of spiders in infraorder Araneomorphae move transversely (Liji Jinaraj / Flickr) (CC Lic.)

Fangs are positioned in one of two ways. Those of spiders in the infraorder Mygalomorphae point straight down and open and close vertically, in parallel. The fangs of spiders in the infraorder Araneomorphae move transversely in a pincer-type action.  How to tell if a spider is a not a Brown Recluse 

The pedipalps (or palps) are another pair of appendages at the front of the prosoma. 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. They’re also used by males for mating. You can often identify an adult male by his bulbous palp tips, which females lack.

The spider’s brain has some capacity for learning from past experiences, but its most important role is processing and responding to sensory information.

Close up of a crab spider, showing the positioning of its eight eyes

Eye pattern of a crab spider. (USGS Bee Inventory and Monitoring Lab / Wiki; cc by 2.0)

Most spiders have four pairs of eyes, and they’re 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 good. They use their vision mostly for detecting motion.

The largest eyes on a spider’s head are in the center. Called the primary eyes, they always have the best vision. The other eyes, 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.

Close up showing the four large front eyes of a species of spider called a jumping spider.

Eye pattern of a jumping spider. (Lukas Jonaitis / wiki; cc by 2.0)

A family of spiders called jumping spiders (Salticidae) are probably those with the best eyesight.

Scientists believe at least some kinds of spiders have color vision. The arrangement of eyes differs between families, which helps in identifying species.

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 isn’t simply a single part that’s grouped with all the others to form one large “ear.” Instead, each 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.) Spiders also use their pedipalps to “smell” and taste. It’s thought that spiders are tasting their prey when they use their palps to manipulate it. Another way they sense their surroundings is with small cavities located at the end of each leg. Called tarsal organs, they seem to be sensors for pheromones and changes in temperature and humidity.

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

Eating, digestive system

Color illustration showing the external and internal anatomy, with descriptive labels, of a female two-lunged spider.

Anatomy of a female two-lunged spider, derived from The Spider Book, by John Henry Comstock, 1912 / Wiki

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 on 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 completely cover it.

Spiders eat a liquid diet. So, their meal must be turned into a liquid before it can be consumed. Here’s how that’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 prosoma and squeezes through the pedicel into the abdomen.)

Ranging outward from the midgut are 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 is how spiders can 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!

Opisthosoma (abdomen)
The 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.

The abdomen is attached to the prosoma by a hollow, cylindrical “waist,” called a pedicel. Nerve fibers and part of the foregut pass through it. It’s very flexible, which allows spiders to move their abdomen in all directions.

Blood system
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. It’s surrounded by a membranous bag called the pericardium. 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.

Black-and-white drawing of a spider's book lung, with descriptive labels.

Book lung (The Spider Book, by John Henry Comstock, 1912, / Wiki


Spiders don’t breathe in an active way. 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, it’s 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 into the book lung 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. Still others have only a trachea.

More reading about spiders and insects:

All about spiders – part 2: Spinnerets, Silk, Webs and web-building, Life span, Food sources and Predators
General anatomy of insects