Linda Crampton is a writer and former science teacher with an honors degree in biology. She enjoys writing about science and nature.
Spiders are fascinating animals. People seem to have a "love them or hate them" attitude towards the creatures. Their long, scurrying legs and the potentially dangerous bites of some spiders may scare people. On the other hand, it's interesting to watch the animals build their web or hunt for food. In addition, the silk that they produce is a wonder of nature, and their ability to destroy harmful insects is very useful.
Spiders are hunters and help us by feeding on some of the insects that damage crops, harm animals, and cause disease. They produce several types of silk, an amazingly strong substance that has potentially important applications. While it's true that some spiders are venomous and may even be deadly, researchers are discovering that the venom of certain species may have medical or agricultural uses.
According to Guinness World Records, spiders range in size from the tiny Patu marplesi, which is about 0.43 mm (0.017 inches) in length, to the goliath birdeater, which may have a legspan of up to 28 cm (11 inches). There is some controversy about the latter accolade, however. The giant huntsman spider reportedly has a legspan of up to 30 cm (12 inches).
Some reports say that camel spiders are gigantic animals. The animals aren't spiders, despite their name, though they are related to the animals. They are said to have a maximum length of only six inches. Claims that huge specimens have been found are unsubstantiated. Some of the photos of the animals are misleading with respect to their size.
Parts of a Spider's Body
The two projections around the mouth of a spider are called pedipalps. They are sensory and manipulatory structures that are also involved in reproduction.
Differences Between Spiders and Insects
Spiders belong to the phylum Arthropoda, the class Arachnida, and the order Araneae. Camel spiders are also classified in the class Arachnida, but they belong to the order Solifugae. The study of arachnids is known as arachnology. A person who studies the animals is known as an arachnologist.
Spiders and insects belong to the same phylum (the Arthropoda) but to different classes. Some major differences between the two kinds of arthropods are listed below.
- Spiders have eight legs while insects have six.
- Spiders have two body parts (cephalothorax and abdomen) while insects have three (head, thorax, and abdomen).
- Insects have compound eyes and simple ones.
- Spiders have only simple eyes.
- Unlike insects, spiders have no antennae.
Arachnids and the Story of Arachne
The word "arachnid" comes from the Ancient Greek myth about Arachne, a girl who loved to weave and was famous for her skill. She boasted that she could create better fabric than Athena, the goddess of wisdom. Athena was furious with this claim.
Athena and Arachne competed in a weaving contest. Although Arachne created a beautiful fabric, the fabric created by the goddess was even better. Arachne fell into a state of deep despair and no longer wanted to live. Out of pity, the goddess turned her into a spider so that she could continue to weave.
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- Most spiders have eight eyes. Some have fewer. The number of eyes is always even.
- Sinopoda scurion is the first spider known to have no eyes. The species was discovered in a cave in Laos in 2012.
- The hairs on a spider’s body have important sensory functions. Special hair-like structures on the legs known as trichobothria detect sound vibrations and air currents.
- The animals have other hairs on their legs that detect odours as well as ones that are sensitive to touch.
- Small slits on the body surface detect physical deformation of the outer covering of a spider. The slits are known as lyriform slit sense organs or simply as slit sense organs.
Digestion and Circulation
- Spiders eat liquid food. They secrete digestive enzymes into their food to liquefy it before they swallow it.
- The arachnids are said to have a “sucking” stomach. The structure helps to draw the liquid food into the digestive tract.
- The animals also have light blue blood. Technically, the fluid in the animal's circulatory system is called hemolymph, not blood.
- Human blood contains a pigment called hemoglobin that transports oxygen around the body. Hemoglobin contains iron and is red when it's attached to oxygen. Spiders have hemocyanin instead of hemoglobin. Hemocyanin contains copper instead of iron and is blue when it's attached to oxygen.
- Hemoglobin is present inside our red blood cells. Hemocyanin is located in the liquid part of a spider's hemolymph.
- A spider's heart is tubular and is located on the back of its abdomen.
- The animals have an open circulatory system. They do have some hemolymph vessels, but in most of the body the hemolymph surrounds the organs instead of being confined to vessels.
- Pressure created by moving hemolymph helps the arachnids to move their legs and also helps them to shed their outer layer (the exoskeleton) during molting.
Jumping spiders are often beautiful. They are small animals that have two unusually large eyes and good vision. They are excellent hunters and leap onto their prey. The arachnids belong to the family Salticidae.
Structures called Malpighian tubules are attached to the digestive tract. They absorb waste from the hemolymph. The name of the tubules is capitalized because they’re named after a scientist name Marcello Malpighi (1628—1694), a doctor who studied the anatomy of animals.
The Malpighian tubules send waste in the form of uric acid to a sac called the stercoral pocket, which is shown in the illustration above. The uric acid is eventually combined with waste from the digestive tract and eliminated from the body through the anus.
Some spiders use book lungs to breathe, some use tracheae, and some use both systems. A book lung is located in a cavity that has an opening connecting it to the outside world. The lung consists of layers or sheets of tissue that contain hemolymph. Oxygen passes into the sheets to be distributed around the spider's body. Carbon dioxide waste moves in the opposite direction. Tracheae are tubes that transport oxygen from openings on the surface of the spider through its body. As in the case of book lungs, carbon dioxide is moved in the opposite direction.
- The male spider deposits his sperm on a special web that he creates, which is known as a sperm web.
- He then picks up the sperm with the tips of his pedipalps.
- Once the pedipalps are loaded, the male inserts the sperm into the female’s body.
- The female stores the sperm in her seminal receptacles until her eggs are ready to be fertilized.
- Once the eggs are fertilized, they are released from the female’s body into an egg sac. The female makes this sac from silk.
- When the eggs hatch, tiny spiderlings are released.
- The females of some species abandon the air sacs in a hidden place and leave them to their fate. Some guard the eggs until they hatch. Others carry the egg sacs around until they produce spiderlings.
- The females of some wolf spider species carry the spiderlings on their back until they are mature enough to leave, as shown in the video below.
Silk Production and Properties
Spiders are famous for their ability to produce silk. The animals have two or three pairs of spinnerets on the underside of their abdomen. Each spinneret contains spigots that release liquid silk from a special gland. The liquid contains silk proteins that are dissolved in water. The silk solidifies immediately after it leaves a spinneret.
Spider silk has some interesting characteristics. It's five times stronger than a steel wire of the same diameter. It's also very elastic. A strand of the type of silk that is used to capture prey can be stretched to two to four times its original length without breaking.
Researchers are hoping to eventually make artificial spider silk. In order to do this, they need to learn more about the production of the natural substance.
Uses of Spider Silk in Nature
Several types of silk exist, each having slightly different properties from the others. The material has essential functions in the life of a spider. The arachnids use silk to:
- build a sticky web to trap prey (if the species makes a web for this purpose)
- make a dragline to connect the spider to its web
- wrap up prey so that it can't escape
- make shelters
- make a sperm web
- make egg sacs
In addition, young spiders that have recently hatched use silk to help them move to a new habitat. The spiderlings climb to the top of a high object, stick the end of their abdomen into the air, and release one or more strands of silk from their spinnerets. The silk is often caught by air currents, enabling the spiderlings to drift to a new habitat. The process is known as ballooning.
Human Use of the Silk
Humans have been fascinated by the strength and elasticity of spider silk for a long time. The material has been used in a minor way as a fishing line or fishing net and as a wound dressing. It's also been used in the crosshairs of microscopes and other optical instruments. The problem is that a single spider produces only a small amount of silk, which has prevented large-scale applications for the material.
In 2010, scientists in the United States found a way to incorporate the genes for making dragline silk (the strongest kind) into goats. The goats produced the material in their milk. The silk didn't have all the properties of the material made by spiders, however.
In the rest of the article, I describe some facts about specific types of spiders. The video below shows a trapdoor spider catching its prey. The action begins at about the 1:49 mark. The wait is well worthwhile since the arachnid captures more than one beetle.
An Interesting Hunting Mechanism
Trapdoor spiders build burrows that they line with silk. They also construct a trapdoor for their burrow. The door is made of a combination of plant material, soil, and silk and resembles cork in appearance. It's attached to the burrow by a silken hinge. When the door is closed, the arachnid's burrow is camouflaged.
The spider creates lines of silk radiating from the outside of its trapdoor, which act as trip lines. It waits in its burrow, holding on to the closed door with its claws, until it feels vibrations created by an animal disturbing the trip lines. The spider then leaps out of the burrow and grabs its prey.
Mirror Spiders or Thwaitesia spp.
Mirror spiders belong to the genus Thwaitesia and live in tropical climates. They are beautiful arachnids that have shiny, silver patches on their abdomen. The patches have a coloured border and remind people of mirrors, which gives the animals their name. The animals are also known as sequined spiders. In at least some species, the sizes of the shiny patches varies according to the spider's "mood".
Nicky Bay is a macro photographer who has taken some wonderful photos of mirror spiders. He's observed that the silver patches on the arachnids shrink when the animals appear to be agitated or threatened. When the spiders relax, the patches expand and cover almost the whole abdomen, producing a reflective and beautiful surface.
Diving Bell or Water Spiders
The diving bell spider, or Argyroneta aquatica, is the only spider that is known to spend its whole life underwater. Like its relatives, it breathes air. It makes a bell from silk and fills it with air that it traps on the hairs of its abdomen and legs when it visits the water surface.
The females spend most of their life inside their bell. They take their prey to the bell to digest it. They also molt, mate, and lay their eggs inside the bell. Males build a diving bell, too, but they don't spend as much time inside it. In addition, their construction is smaller than the female's.
The Venom: A Neurotoxin or a Cytotoxin
Almost all spiders produce venom to subdue their prey, but only some make a venom that is dangerous to humans. All spiders have fangs, too, but many of these fangs are too weak to penetrate human skin.
The venom is released from the animal's venom gland and sent down a duct in a fang. A hole at the tip of the fang releases the venom as the arachnid bites. The substance is a neurotoxin, which damages nerves, or a cytotoxin, which destroys cells. Cytotoxic venoms are also called necrotic ones.
Two venomous spiders of interest are the Brazilian wandering spider and the black widow. Both produce neurotoxins. The brown recluse spider produces a cytotoxin.
The Brazilian Wandering Spider
The Brazilian wandering spider (Phoneutria nigriventer) is often considered to be the most venomous spider in the world. An antivenom to fight the harmful effects of the animal's bite is available, however. The effects are sometimes relatively mild, but the venom has the potential to cause serious symptoms.
The spider is native to South America. It doesn't build a web. Instead, it patrols the jungle floor at night as it looks for food. During the day, it hides in a secluded place, such as under a log or a rock or inside a termite mound. It also has the habit of hiding in banana plants, which gives it the alternate name of banana spider. Unfortunately, the animal may enter homes and hide in clothing or shoes.
The animal's bite can be very painful. The venom is a neurotoxin that interferes with the action of calcium in the body. Calcium is needed for muscle contraction. If enough venom is injected or if the antivenom isn't obtained quickly, the venom can cause paralysis and stop the breathing process. Though this effect is certainly possible, relatively few people seem to have died from the spider’s bite.
The Brazilian wandering spider has the nickname "Viagra spider". It's venom is being studied in relation to impotence problems. Unfortunately, the effects of the venom in this respect can be painful and last for hours. In tiny quantities, however, the substance or a synthetic derivative might one day be useful when prescribed by a doctor.
The Black Widow
Widow spiders belong to the genus Latrodectus. The animals were given their common name because researchers noticed that the females of some species ate the male after mating. Widow spiders live in many different countries. Black widows are found in parts of the United States and Canada.
The female black widow has a red or orange, hourglass-shaped area on the underside of her abdomen, as shown in the photo above. The spider's venom contains a toxin called latrotoxin. The female's venom is much more potent than the male's.
The bite of a black widow spider can cause lactrodectism. Symptoms may include stomach cramps, muscle pain and spasms, headache, nausea, vomiting, sweating, and a rapid heartbeat. The symptoms may last from several days to several weeks.
Black widow spiders aren't aggressive and bite in self defence. Sometimes humans are bitten when they disturb the animal's habitat without noticing its presence. The bite may send venom into the victim's bloodstream. Even dangerous spiders can give "dry" bites (ones in which little or or no venom is released), though.
The death rate from black widow bites is very low, but it isn't zero. Someone who is bitten by the animal needs to get medical help right away in order to treat any symptoms that appear and prevent the development of serious effects.
The Brown Recluse
The brown recluse spider (Loxosceles reclusa) ranges from dark yellow to brown in colour. There is a dark brown, violin-shaped mark on the back of its cephalothorax. The narrow neck of the violin points towards the animal's abdomen. This feature can't be used to definitively identify the animal because other spiders also bear the mark. The mark combined with the unusual eyes does identify the species, however. The spider has three pairs of eyes. One pair is located in the centre of the back of the head and the other two are located on either side of the central one. Most spiders have eight eyes instead of six.
Like black widows, brown recluse spiders are shy animals. They may bite if they are disturbed, however. Often the bite causes no serious problems, but sometime necrosis, or tissue death, occurs. Death from the bite is very rare but does occur. Medical aid should always be sought after a brown recluse bite.
About 40,000 species of spiders have been discovered and named. There are probably many more that haven't yet been found. They live in many different habitats and are widespread around the world. Though their basic features are the same, the different species have unique and interesting characteristics.
I'm always happy when I find a spider to observe. I think that the creatures are fascinating animals that are worth studying. I'm lucky that there are no dangerous species living near my home, though. If there were, I might not be so eager to watch them.
- Under the spell of spiders from Smithsonian Education
- Spider myths (and information) from Burke Museum
- Facts about spider silk from Bristol University in the UK
- California trapdoor spider information from the Catalina Island Conservancy
- The diving bell and the water spider from the phys.org news service
- Envenomation by wandering spiders from Springer Nature (Abstract)
- Spider venom and erectile dysfunction from WebMD
- Effects of a black widow spider bite from WebMD
- Brown recluse facts from the University of Kentucky
- Information about venomous spiders from the CDC (Centers for Disease Control and Prevention)
- World’s first eyeless spider discovered from phys.org