SPIDERS
Spider are creatures with eight legs and sensory hairs on their bodies. Many but not all produce silk and webs. There are peorhaps 150,000 different species of spider of which about 35,000 have been identified. They are found almost everywhere: rain forests, deserts, houses, even 100,000 feet in the sky, drifting in the wind from thread-like "parachutes” and floating in the open sea.
The first spiders walked the earth about 400 million years ago. They are more closely related to scorpions and horseshoe crabs than insects. In the early days they used silk mainly to construct hiding places.
Web weaving is thought to have evolved around 135 million years ago in the Cretaceous period when dinosaurs roamed the earth. A segment of spider web preserved in a 110-million-year-old piece of amber shows that spiders were weaving perfect orb-style webs way back then, earlier than previously thought. The amber sample contained 26 web strands grasping a mite, a fly, a wasp leg and a beetle as well as drops of web glue. Enough clues have been found on the web threads to show they were part of a classic orb web made of concentric circles joined by radiating "spokes."
According to a Greek legend spiders were creating when a seamstress named Arachne challenged the goddess Athena to a contest and then felt ashamed of her boldness and hung herself. Athena felt sorry for her and brought her back to life as a spider and made her noose into a web. Her name was also given to the scientist who study spiders (arachnologists), the fear of them (arachanaphobia) and the class of animals that includes them (Arachnida).
Websites and Resources: Arachnology Homepage with Lists of Resources arachnology.be/Arachnology ; Wikipedia article on Arachnids Wikipedia ; Spider Pictures venomous-spiders.nanders.dk/spiderpictures ; Spider Identification Chart U.S. Spiders termite.com/spider-identification ; Spiderz Rule spiderzrule.com ; Wikipedia article oon Spiders Wikipedia ; Britannica on Arachnids britannica.com ;
Websites and Resources on Insects: Insect.org insects.org ; Insect Images.org insectimages.org ; BBC Insects bbc.co.uk/nature/life/Insect ; Insect and Arachnid entomology.umn.edu/cues/4015/morpology ; Wikipedia article Wikipedia ; Virtual Insect home.comcast.net ; National Geographic on Bugs National Geographic ; Smithsonian bug info si.edu/Encyclopedia_SI/nmnh/buginfo ; Entomology for Beginners bijlmakers.com/entomology/begin ; BugGuide bugguide.net ;
Websites and Resources on Animals: ARKive arkive.org Animal Info animalinfo.org ; Animal Picture Archives (do a Search for the Animal Species You Want) animalpicturesarchive ; BBC Animals Finder bbc.co.uk/nature/animals ; Animal Diversity Web animaldiversity.ummz.umich.edu ; International Field Guides media.library.uiuc.edu ; animals.com animals.com/tags/animals-z ; Encyclopedia of Life eol.org ; World Wildlife Fund (WWF) worldwildlife.org ; National Geographic National Geographic ; Animal Planet animal.discovery.com ; Wikipedia article on Animals Wikipedia ; Animals.com animals.com ; Endangered Animals iucnredlist.org ; Endangered Species Resource List ucblibraries.colorado.edu ; Biodiversity Heritage Library biodiversitylibrary.org
Scorpions, Centipedes and Spiders
Scorpions and spiders are members of the Arachnid class of animals. They have eight legs and a hard exoskeleton and molt like insects . Many species are carnivores that feed primarily on insects. Many produce poisons and have hair on their legs that can detect sounds, sensation, objects and even taste.
Members of the Arachnid class of animals emerged about 400 million years ago and are more closely related to horseshoe crabs than insects.
Insects, centipedes, millipedes, arachnids (including spiders and scorpions) and crustaceans belong to the phylum of arthropods. Arthropods account for three fourths of all known animals. All have exoskeletons made of chitin; a body divided into segments and protected by cuticle; jointed legs arranged in pairs; an open circulatory system with organs bathed in a liquid called hemolymph that is pumped around the body by the heart; and a nervous system comprised of paired nerve chords.
Insects have three pairs of legs, spiders and scorpions have four, crabs and shrimps have five and centipedes and millipedes have many.
Spider Characteristics and Behavior
In addition to their eight legs, spiders have eight eyes and some species have four lungs. The fact they have lungs enables to reach large sizes than insects whose size is limited because they don’t have lungs (See Insects). Some, like wolf spiders and jumping spiders, have excellent eyes and hunt primarily by sight. But most spiders have very weak eyes are rely on their sense of touch and their sensory hairs to gain information about their environment..
Almost all species of spider are hairy, but their hair is not like human hair. Spider hair is actually part of the spider's body — extensions of the exoskeleton call senate. Some of the follicles are actually sense organs for touching, tasting, hearing, and are used for detecting prey or mates.
Hair follicles on their legs and abdomens are the most important sense organs for most spider species. Spider hairs resemble short, hollow pegs. They are moist inside and enable spiders to taste the surface they are touching. Tiny, ringed pits help them to smell prey, enemies and potential mates. Long hairs, extremely sensitive to air movement, alert spiders to — the merest twitch of a nearby insect." Water spiders are able to float on the water because their hair trap air. Some spiders have hairs that spiderlings cling when they ride on their mothers back.
Spiders are very solitary creatures. They don't socialize except at mating time and "don't express affection the way dogs and cats do." They spends most of their life hunting, eating, reproducing and escaping predators. Most of everything they do is genetically programmed and instinctual.
Some spiders masquerade as foul tasting ants to deter predators. Other are very good at playing dead. Some spiders can leap 25 times their body length to seize prey.
Feeding Spiders
Spiders eat about 15 percent of the body weight every day. They are mostly carnivores that feed on insects. Some eat larger prey such as small frogs. Spiders help humans by eating insects that attack crops. According to one calculation two million spiders can be fund on one acre of farmland and they consumer billions of insect a year.
Spiders don't necessarily need to eat all that often. Most species can go for a month or more without food and tarantulas can live as long as year. Female spiders are usually the most active hunters because they need more food to feed their young.
Spiders employ different hunting methods. Many use webs to catch prey. Many also use venom to paralyze but necessarily their prey. If a victim is too large the spider can immobilize it with layers of silk. Biting spiders sometimes paralyze prey with a bite to a certain nerve and then wrap it in silk and stored live until it can be consumed or given away as mating present. Some small spiders work as a group to catch large prey but this kind of behavior is rare.
All spiders are fluid feeders. They can not eat solid food. They digest their food externally by vomiting digestive juices onto their prey or injecting it with flesh-softening secretions and then suck up the liquified food using a strawlike appendage.
Jumping spiders often prey on mosquitos. Adults take about an hour and half to feed on a mosquito. Juveniles feed by attaching their fangs to the thorax of the mosquito and then draw blood from the insect’s abdomen.
Spiders are also feed on by many creatures such as birds, reptiles and large insects. Some species of wasp paralyze spiders and lay their eggs inside them. When eggs hatch, the larvae slowly eat the fresh tissue of the spider dies form being eaten alive. See Wasps
Spider Breeding
A male spiders do not have a penis or similar devise to implant sperm directly into a female. Instead he spins a small silken napkin, deposits sperm on it from his genital pore, sucks the sperm into feeler-like organs on his head and squirts the sperm from these into the female's genital pore.
Females are often significantly larger than the males and often try to eat them. Males usually signal they female somehow they are not prey, deliver the sperm; and try to escape before the females eats them. Some males signal the female visually. Others send special rhythmic vibrations along their webs. Other present gifts of insects.
The males are wired by the instincts to mate and little else. Females on the other hand try to get as big and fat as they can so they as produce as many eggs as possible. Female spiders lay anywhere from 20 to 100 eggs. When the offspring they often ride on their mother’s back. As the young grow larger they shed there skins about a dozen times until the reach maturity.
Spider Silk
Spider silk which is one of the most amazing and strongest materials on the face of the earth. Woven from "pleated protein sheets surrounded by springy coils of amino acids," it is twenty times stronger than steel yet more elastic than nylon and three times harder to break than Kevlar. It can be dry or sticky or as thin as a million of an inch and can absorb more energy before it breaks than any other material. The formula for spider silk was developed in the age of the dinosaurs and has remained virtually unchanged ever since. [Source: Richard Connif. National Geographic, August 2001]
Spider silk thread, which are generally about three to five micrometers across, are produced in the silk glands in the rear of the abdomen from a highly concentrated solution that is half water, half proteins. Most spiders have three pairs of showerhead-like spinnerets (stubby appendages that pull the silk out of the glands) that are covered it hundreds of silk-releasing spigots (external opening of the silk glands). Spiders cut the threads with sudden yanks from their back of the hind legs.
An individual spider can produce different kinds of silk that can emerge sticky, wooly, wet or dry. The spinnerets controls the thickness and texture by combing multiple strands into a single, solid thread. The silk which is squeezed out like toothpaste by special muscles and turns from liquid into thread not because it dries but because tension reorients the protein molecule from folded rod shapes to chains of linked logs.
Spider silk is biodegradable. It is made with water as a solvent rather than harsh organic chemicals. Its tensile strength is far superior to steel by weight and near as string ast steel by volume. But what makes it particularly strong is its elasticity. Kevlar stop bullets with strength, spider silk stops them by stretching.
One of these secrets behind the strength of spider silk is the fact that single thread is comprised of thousands of filaments each only a few nanometers across, too small to be seen in a regular microscope.. With some many strands if a few break so what. Interspersed with the filaments are fluid-filled channels that help disperse the force making one filament less likely to break. Another secret to the strength of spider silk is the way the proteins that make up the silk are bonded into longs chains and spun in a sophisticated way be spider’s spinnerets. The protein molecules line up in the direction of the flow and formed into liquid crystals, then stretched long and thin before the water removed to make it hard. Scientists have built and patented a device that mimics some of these processes.
Uses of Spider Silk
Each species of spider produces its own kind of silk. Some produce different silks for different purposes: building webs, making eggs sacs, lining nests, weaving tents for offspring and producing safety lines used in escapes. The toughest threads are often composed of dragline silk, which is used to make the framework of spider webs and to leap from houses or trees.
Some spiders spin silk to wrap their prey or keep their eggs safe. Other kinds of spiders make silk that can attract victims or warn off larger animals that might damage the web. Many of species of spiders hangs from a single strand of silk that is cemented at a surface at irregular intervals, in the same way a rock climber attaches his rope. The strand can be adjusted in midair if it is to short to absorb the fall.
The golden silk spider has seven pairs of glands that each produce a different kind of silk: one for wrapping prey, another from wrapping the spider’s eggs, two kinds of sticky capture silk used to make the circles on a web.
Bola spiders hurl weighted single filaments at its prey like a lasso or bola. Some spiders lasso passing insects with a sticky droplet at the end of a silken thread. The scaffold spider constructs a traps that snags its victim with glue and hoists its into the air. Others climb to high perch and send out strand of silk that is picked up the wind and carried up 100,000 feet in the air and 200 miles overland. Some birds steal spider silk and use it in the construction of their nests.
Spider Webs
Classic orb webs are comprised of: 1) a spoke-like framework made of strong dry threads a few thousands of a millimeter thick; and 2) a spiral that looks like concentric circles made of thinner, highly elastic, glue-covered silk for trapping prey. The web must be string enough to hold prey and support the weight of the spider; elastic enough to flex in the wind or when a flying insect hits it; and springy enough to bounce back before the sticky threads become a tangle of goo.
Some spiders use 600 spinning nozzles, which can weave seven different kind of silk into highly resilient configurations. Web building is genetically programmed and not learned, with each species making its own unique pattern. About a third of all spider make orb webs. Another third weave sheet webs, cobwebs and other kinds of webs. A typical orb web takes about an hour and half to make and requires 100 feet of silk. Webs rarely remain intact for more than 24 hours. Some spiders make webs and reweave them up to five times a day.
Most spider webs are spun by females. Males usually give up the practice when they reach maturity in part because they don’t much food. Females need the food that webs provide so they can make eggs. Orb webs are usually spun in flyways with the aim of catching flying insects. The first threads to be laid down are a dryline made in the shape of a star that act like scaffolding. After the spider makes the frame and “spokes” then an auxiliary spiral and finally a spiral of sticky threads is produces.
In a study on how drugs affect the mind certain illegal drugs were injected into spiders to see how their webs would turn out. When the spider was given "speed" the web was tangled and irregular. With tranquilizers and marijuana the webs were smaller. Under the influence of LSD the web was more symmetrical.
Spider Silk and Humans
Scientist are trying to produce synthetic materials similar to spider silk for use in space ships, stealth bombers, optical instruments and other purposes. Doctors are examining spider silk to make sutures one-tenth the diameter of current sutures as well as possible artificial ligaments and tendons. Spider silk is too elastic however for bullet proof-vests. It would stop the bullet but not until it stretched to the other side of your body.
Why isn't spider silk produced like silkworm silk? The advantage of silkworm silk is that can be unraveled in one continuous strand and the spider's aggressive and cannibalistic nature makes raising them in numbers difficult.
One Canadian company spins silk similar to that produced by spiders from the milk of genetically engineered goats that look like normal goats. Spider-silk genes were implanted into goat embryos that developed into adults and passed the genetic trait onto their offspring. The advance was made possible because goat udders have similar structure to spider silk-making organs. Spider-silk proteins have also been clone into tobacco and potato plants, which secrete them on their leaves.
Eating and Studying Spiders
Spiders are eaten in many places. It has been said that Nephilia spider tastes like a potato. See Cambodia
Spiderlogist capture their specimens with a plastic straw-like tube. When the scientist see a spider, he or she sucks on the straw. A "strategically placed nylon swatch" captures the prey and keeps it from zipping down the scientists's throat. Corn starch is sprayed onto webs so that scientist can observe their structure and shape with breaking the threads.
SpiderPharm is a company that raises black widows brown recluses, tarantulas and other poisonous spiders for their venom. Roderick MacKinnon, the winner of the 2003 Nobel Prize in chemistry, uses tarantula and scorpion venom to study potassium ion channels in cells.
Spider expert Fritz Vollrath occasionally takes spiders to a sychotron in Grenoble, France and places the spider on a little platform and hooks its thread to tiny reels that pull the thread out of the spiders body .at different speeds. The synchoton produces high energy X-rays that can be analyzed to give insights into how the silk and silk making glands respond to different stimuli.
Spider expert: Jonathon Coddington of the Smithsonian’s Museum of Natural History.
Venomous Spiders
There are 200 kinds of poisonous spider. Some spider venoms make hole in cell membranes causing the cells to die. Most are designed to paralyze prey rather than kill it. That way prey remains fresh until it is eaten. Almost all spider have venoms which affect their prey, but is rarely harmful to humans. The fact that some spider venom is harmful to humans is a coincidence. Among these are the venoms produced by tarantulas, black widows and brown recluses.
A black widow bite can cause severe pain and muscle spasms. Brown recluse venom degrades tissue and produces a gangrene-like wound. Funnel spider venom leads to trembling, increased blood pressure and vomiting. Tarantula venom can inhibit the movement in signal-carrying ions in cell channels, resulting in numbing, paralysis, convulsions, high blood pressure, cardiac arrhythmia.
The black widow most is the poisonous spider. It can be identified by a red hourglass marking on it belly. Its venom is more potent that of a rattlesnake. Still death occurs to only about four people a years in part because the amounts of venom injects are relatively low. Symptoms include chills, breathing difficulty and muscle cramps. An antivenin has been developed Contrary to the popular myth the larger female doesn't necessarily bite the head off the smaller male after mating. Like all species of spider, if hungry, females will sometimes eat their own kind.
Most victims of black widow or tarantula bites are advised to wash the wound with soap, place ice on it and take aspirin or acetaminophen. Brown spider bites, which attack tissue and cause necrotic lesions, require a trip to hospital
The atrax, native to Australia and New Zealand, and sometimes called the Sydney funnel web spider, is said to be "very large, very poisonous, very aggressive; lunges at prey or in self defense...Bite: painful...may include muscle spasm, profuse sweating, high blood pressure, fluid in lungs, coma."
Image Sources:
Text Sources: Mostly National Geographic articles. Also “Life on Earth” by David Attenborough (Princeton University Press), New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
Last updated March 2011