Amazing Spiders: Strange, Interesting, and Scary Facts
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 spiders 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 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 certain venoms 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 huge animals. The animals aren't spiders, however, despite their name, and they have a maximum length of only six inches. Claims that gigantic camel spiders have been found are unsubstantiated. In addition, 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 the pedipalps. These are sensory and manipulatory structures that are also involved in reproduction.
Differences Between Spiders and Insects
- Spiders have eight legs while insects have six.
- Spiders have two body parts (cephalothorax and abdomen) while insects have three (head, thorax and abdomen).
- Spiders have simple eyes instead of complex eyes.
- Unlike insects, spiders have no antennae.
The Story of Arachne
Spiders belong to the phylum Arthropoda, class Arachnida and order Araneae. The study of arachnids is known as arachnology. A person who studies the animals is known as an arachnologist.
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.
Internal Anatomy of a Spider
- Spiders have light blue blood. Technically, the fluid in the spider'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 contain 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.
- Spiders 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 spiders to move their legs and also helps them to shed their outer layer (the exoskeleton) during molting.
- 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 many hemolymph vessels. Oxygen passes into these vessels to be distributed around the spider's body. Carbon dioxide waste moves in the opposite direction.
- Tracheae are tubes that transport air from openings on the surface of the spider through its body.
- Spiders eat liquid food. They secrete digestive enzymes into the food to liquify it before they swallow the food.
Spiders are famous for their ability to produce silk. There are two or three pairs of spinnerets on the undersde of their abdomen. Each spinneret contains spigots that release liquid silk from a silk gland. The liquid contains silk proteins (long chains of amino acids) that are dissolved in water. 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 spider silk that is used to capture prey can be stretched to two to four times its original length without breaking.
Uses of Silk in Nature
There are several types of silk, each having slightly different properties from the other types. Silk has essential functions in the life of a spider. Spiders use silk to:
- build a sticky web to trap prey (if the species makes a web)
- 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 or bed (on which the male deposits his sperm before picking it up with his pedipalps to insert into a female's sperm receptacle)
- make egg sacs
In addition, young spiders that have recently hatched, or spiderlings, 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 a strand of silk. The strand is often caught by air currents, enabling the spiderlings to drift to a new habitat. The process is known as ballooning.
Researchers are hoping to make artificial spider silk eventually, but to do this they need to learn more about the production of the natural substance.
Human Use of Spider Silk
Humans have been fascinated by the strength and elasticity of spider silk for a long time. The silk 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 spider dragline silk (the strongest kind) into goats. The goats produced the silk in their milk. The goat milk silk didn't have all the properties of the silk made by spiders, however.
Why would scientists try to get spider silk from goats instead of from spiders? The reason is that spider "farms" are impractical because spiders in close proximity tend to kill each other. Goats are much easier to keep in captivity.
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 spider captures more than one beetle.
Hunting Mechanism of Trapdoor Spiders
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 spider'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 animals 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 spiders 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 other spiders, the diving bell spider breathes air. It makes a diving 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 female spiders spend most of their life inside a 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 bell is smaller than the female's.
A Water Spider Builds His Bell
Spider 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 spider's venom gland and sent down a duct in a fang. A hole at the tip of the fang releases the venom as the spider bites. Spider venom is a neurotoxin, which damages nerves, or a cytotoxin, which destroys cells. Cytotoxic venoms are also called necrotic venoms.
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 latter two animals are found in North America. They are the two spiders on the continent that are potentially dangerous.
The Brazilian Wandering Spider
The Brazilian wandering spider (genus Phoneutria) is often considered to be the most venomous spider in the world. There is an antivenom available to fight the venom, however.
The spider doesn't build a web. Instead, it patrols the jungle floor at night, looking 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 spider may enter homes and hide in clothing or shoes.
The animal's bite is 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. It may have medical benefits when used in small quantities, however. For example, it's being studied in relation to impotence problems due to its interesting effect on one of the male reproductive organs.
The Black Widow
Widow spiders belong to the genus Latrodectus. The spiders 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 spider often 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 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 spider's habitat without noticing the animal's presence, however. 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 spider needs to get medical help right away in order to start treating any symptoms that appear and to help protect against the risk of dying from the bite.
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 the 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.
I'm always happy when I find a spider to observe. I think that they are fascinating animals. I'm lucky that there are no dangerous spiders living near my home, though. If there were, I'm sure I wouldn't be so eager to watch them!
Reference: Venomous Spiders in the United States
Spider Bite Information from the CDC (Centers for Disease Control and Prevention)
© 2013 Linda Crampton