All about spiders: basics, body, behavior

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This page is Part 1 of 2.  These links will get you quickly to your topic of interest on either page.
Part 1Why beneficial •  Background  •  Physical characteristics  •  Intelligence  •  Fangs  •  Senses  •  Digestive System  •  Other Systems 
Part 2:  •  Silk, Spinnerets  •  Webs  •  Reproduction  •  Lifespan, Foods, Predators

Introduction

It’s one thing when an “itsy bitsy spider climbs 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 spiders, are known to jump at the sudden sight of one. 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.”
And, 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. As you read all about spiders, you’ll discover there are many reasons to respect them. 

They 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 spiders have bitten you? Several? Experts say probably not, as spiders seldom bite and are typically blamed for bites from other insects. 

Spiders are beneficial

  • They aren’t interested in humans–they’re on the hunt for tiny critters, and that’s a huge benefit to us: National Geographic says a single spider eats about 2,000 insects a year. Imagine the value of spiders on a worldwide scale. Spiders help reduce populations of insect pests in our homes, as well. 
  • They benefit other wildlife, too, because they’re food for birds, reptiles, small mammals and other organisms.
  •  Their silk is one of the strongest, most elastic natural fibers and has given rise to the synthesized silk used for parachutes and other products.

Background

Spiders comprise the largest order in the taxonomic class Arachnida (a word derived from the Greek arakhne, meaning spider). Arachnida (Ah-RACK-nee-duh) includes other familiar members, such as mites, ticks, harvestmen, and scorpions, but they belong to different families.

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.

Origin

Spiders have been around for eons, but there aren’t many fossil records because their bodies are soft and easily deteriorate. Those that exist are usually preserved in amber (hardened tree sap). Spiders in the genus Nephila are the longest-lived modern spiders, they’ve existed since the Middle Jurassic, 165 million years ago. The world’s smallest spider today is probably the male Patu digua, only 0.02 inches (0.37 mm), about the size of a pinhead. The 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).

Physical characteristics

It’s commonly thought that spiders are insects, but they’re classified separately. Several features 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. These are just a handful of their wide variety and, in some cases, beauty:

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

Spiders come in many shapes, sizes, and colors. (Termininja / Wiki; CC BY-SA 3.0)



Body sections

Spiders have two main body parts: Cephalothorax (front section) and Opisthosoma (abdomen). Their skeleton is on the outside of their body (exoskeleton), instead of inside, like a human’s. It’s commonly thought they have no internal skeleton whatsoever, but they do have a small one made of cartilage-like material.

Drawing of spider anatomy including labels for the various body parts.

Spider anatomy. (James Henry Emerton, as derived by Peter Coxhead / wiki; cc by-sa 4.0)



Cephalothorax

The cephalothorax (SEFF-uh-low-THORAX), or prosoma, is composed of the spider’s head and thorax, which are fused. 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 other body parts called pedipalps, chelicerae, and fangs. Attached to the cephalothorax are the spider’s eight legs, four on each side.

“Shells” cover the top and bottom of the cephalothorax, which make it hard. The top shell is called the carapace, and the bottom is the sternum. The shells are made of protein and chitin. Chitin is a polysaccharide that produces a tough material called cuticle (like those on our fingernails). Underlying the cuticle is a layer of epithelial cells containing granules of pigment which give spiders their colors.

Legs

Spiders have eight legs. Each has seven segments: the coxa, trochanter, femur, patella, tibia, metatarsus and at the end, the tarsus. At the tip of each tarsus are claws. Web-spinning spiders have three of them and use the center one for grasping silk threads. Other spiders have two claws.

Brushes of hair cover the tip of each leg, and the end of each hair has microscopic gripping “feet” that help the spider grasp surfaces, even slick ones. Some surfaces defeat them, though–glass and porcelain, for example. (You’ve probably found a spider trapped in your bathtub or sink.)

Have you ever wondered what happens if a spider loses a leg? Well, if it’s young and still has at least one more molt to go, the leg will regenerate.

Brain and intelligence

Their brain’s most crucial role is processing and responding to sensory information. It takes up a disproportionately large area of the spider’s body–eighty percent or more in some species. (Uh, oh, who expected intelligence?!)

Yes, research is showing that they’re surprisingly smart! Studies show that orb-weaver spiders can count. They keep track of how many silk-wrapped prey they’ve stored in their web. Remove one or more, and the spider will search for them in proportion to the number taken. And, 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

Their mouth is at the front of the head where you’d expect it. Two fang-tipped appendages, called chelicera (kuh-LISS-er-uh), are in front of it. Spiders use their chelicerae to catch and hold prey while injecting venom through the fangs. The chelicerae usually have finely serrated inner edges which can be used to saw up their victim.

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

The fangs of the Black Wishbone Spider, Aname atra, are positioned vertically. (Steven Clark / Wiki; cc by-sa 2.5)



Spiders have fangs that operate in one of two ways. Those of spiders in the infraorder Mygalomorphae point down and open and close vertically, in parallel. Spiders in the infraorder Araneomorphae have fangs that move transversely in a pincer-type action. Spiders can move their fangs and also fold them up. Only spiders in the small family Uloboridae lack venom. (Most spiders have fangs too small to pierce human skin.) 

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

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



Pedipalps

The pedipalps are two segmented appendages at the front of the cephalothorax. They’re located between the chelicerae and the first pair of legs and are usually shorter than the legs (although it can be easy to mistake them). 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.

Senses

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, so it isn’t quite as good. Scientists believe at least some species have color vision.

Different families have different eye patterns, which helps to identify species. Notice the location of the spiders’ eyes in the two examples shown below. 

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)


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)



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 help see in dim light. Secondary eyes have a tapetum (tap-EET-um), which is a light-reflecting layer, so here’s a fun activity for you and your kids: On a summer night, use a strong flashlight to look around on shrubs or plants; 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 it, starring these spiders: Bob TheSpiderHunter

Hearing, taste, smell

Spiders don’t have ears, and they don’t hear in the way humans do. Instead, 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 of them are the long hairs, called trichobothria (TRICK-oh-BAH-three-uh), on their legs–they can feel air movement down to one ten-billionth of a meter (the width of an atom).

The hairs also “taste” by sensing the chemical composition of what they touch. (“Hmm, is that a human, a delectable mouth-watering moth or the girly spider of my dreams”?). Spiders don’t take action unless they detect prey or a mate. Experts think they might be tasting their prey when they use their hairy pedipalps to manipulate it.

As for their sense of smell, once again, hairs play a role, but there’s more. In addition to the sensitive hairs on their legs are small cavities at the ends called tarsal organs. They seem to be sensors for pheromones and changes in temperature and humidity.

Communication

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   

Opisthosoma (abdomen) and pedicel

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.

The abdomen is connected to the cephalothorax (front body part) by a thin, waist-like tube called the pedicel. A bundle of nerve fibers from the brain, as well as the “foregut,” pass through it. The pedicel is flexible, which allows the abdomen to move in all directions as spiders make their webs.

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

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



 Eating

Spiders are opportunistic eaters. They gorge on food, usually insects, when they can, and may go for long periods without eating if food is scarce. Those that actively hunt for prey probably eat more regularly than spiders that lay in wait on their webs or flowers.

 Their mouth is shaped like a short, tiny drinking straw and sucks rather than chews. They use their chelicerae (sometimes called jaws) to grasp and hold prey while injecting venom. To immobilize and carry their prey, some spiders wrap it in silk, turning it over and over to cover it completely. Sometimes the unfortunate victim is stored away to eat later on.

Spiders eat a liquid diet, so they must turn their prey into that. Here’s how it happens (it’s gross but interesting):

Digestive system

A tube leads from the mouth through a foregut and into a muscular reservoir in the spider’s 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 repeatedly until the prey is dissolved and entirely 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. From there, a series of pouches, called Caeca (SEE-kuh), which store digestive fluids, range outward. The abdomen of a well-fed spider may be near to bursting with caeca filled with liquefied food. Sometimes caeca even extend down into the spider’s legs. Such a spider can go a long time without eating.

Alimentary system

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-cor-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.

Circulatory system

The spider’s circulatory system is an “open system,” wherein the heart pumps blood through arteries that empty directly into the body cavity. The blood, called hemolymph, 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.)

Heart

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 a spider’s heart rate is. One study reports that it ranges from thirty to two hundred beats a minute, depending on the species and extent of its activity. Another places the heart rate in a range of nine to 125 beats per minute. Results from various studies on bigger spiders (tarantulas, for example, which have bigger hearts), shows a range of thirty to seventy beats per minute.

Lungs and breathing

Spiders don’t actively breathe. Air enters, and carbon dioxide is removed passively as the heart beats. There are two kinds of breathing systems among spiders: Some have a network of tracheae that connect to an outside opening (spiracle) in the spider’s body, located near the spinnerets. But most spiders have a “book lung.”

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

Book lung. (The Spider Book, by John Henry Comstock, 1912 / Wiki; PD)



Surrounded by blood, the book lung is thin, hollow and has plates stacked like pages in a book. Connected to it is a slit-like opening in the spider’s underside, where oxygen enters, and carbon dioxide exits. Some spiders have two pairs of book lungs. Others, such as orb-weavers and wolf spiders, have one book lung and trachea. There are still others that have only trachea.

On Page 2:  spinnerets, silk, webs, reproduction, spiderlings, lifespan, food, predators, defenses    
*Top photo: Jumping spider. (Macrotiff / PIxabay; PD)

More reading:

General anatomy of insects   
 How to tell if a spider is a not a Brown Recluse 

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