Amazing Spider Facts D. Indumathi, The Institute of Mathematical Sciences, Chennai There are approximately 38,000 known species of spiders. Scientists believe there are probably as many more to be discovered. In fact, s piders are found on every continent except Antarctica. An estimated 1 million spiders live in one acre of land. The number might be closer to 3 million in the tropics. It is estimated that a human is never more than 10 feet away from a spider—ever. Spiders are vital to a healthy ecosystem. They eat harmful insects, pollinate plants, and recycle dead animal and plants back into the earth. They are also a valuable food source for many small mammals, birds, and fish. Spiders : How they eat Spiders eat insects by filling them with digestive juices. Spiders do not have teeth, so they cannot chew their food. Instead, they inject digestive juices into the innards of their meal. Then the spider sucks it up as liquid food. Most spiders’ fangs are like pincers that move sideways toward each other to bite. Others, such as bird-eating spiders, have long fangs that point straight down. Spiders eat more insects than birds and bats combined. The Bagheera kiplingi is the world’s only (mostly) vegetarian spider. Spider Anatomy Spiders are not insects. They are arachnids, along with scorpions, mites, harvestmen, and ticks. All arachnids have eight legs and two main body parts (a cephalothorax and an abdomen). In contrast, insects have six legs and three main body parts (a head, a thorax, and an abdomen). Most spiders have eight eyes and are very near sighted. Spiders also have tiny hairs on their legs to help them hear and smell. A spider has no bones. Rather, it has an exoskeleton, which is like a hard suit of armor that protects its body. While humans have muscles on the outside of their skeleton, spiders have muscles on the inside. Because an exoskeleton does not grow, a spider molts. Typically, a spider molts about 10 times throughout its life. Some spiders, such as house spiders, are able to run up walls because their feet are covered in tiny hairs that grip the surface. They can’t get out of a bathtub, however, because the surface is too slippery. Other spiders, such as garden spiders, cannot crawl up walls because their legs end in claws, which help them grip threads of silk instead. Spiders have blue blood. In humans, oxygen is bound to hemoglobin, a molecule that contains iron and gives blood its red color. In spiders, oxygen is bound to hemocyanin, a molecule that contains copper rather than iron. Most spiders live alone, meeting other spiders only to mate. A few species of spiders are social and live in groups. For example, in Africa, the web of social spiders such as Stegodyphus colonies can cover whole trees. In India, webs may cover trees for several miles. Spider Webs All spiders spin silk, but not all spiders spin webs. Abandoned spider webs are called “cobwebs.” Spiders have between two and six spinnerets at the back of their abdomen. Each one is like a tiny showerhead that has hundreds of holes, all producing liquid silk. The silk that comes out of the spider’s spinneret is liquid, but it hardens as soon as it comes in contact with air. Some spiders have up to seven types of silk glands, each creating a different type of silk—such as smooth, sticky, dry, or stretchy. The silk in a spider’s web is five times stronger than a strand of steel that is the same thickness. A web made of strands of spider silk as thick as a pencil could stop a Boeing 747 jumbo jet in flight. Scientists still cannot replicate the strength and elasticity of a spider’s silk. In tropical regions, net-throwing spiders make a small silken web that they throw over their prey. Web-weaving spiders have two or three claws at the tip of each leg that they use to swing from strand to strand without getting stuck in the sticky part of their web. Additionally, a spider’s body has a special oily substance that keeps it from getting stuck in its own web. When a spider travels, it always has four legs touching the ground and four legs off the ground at any given moment. Spider bites The venom of the black widow spider attacks nerves by blocking their signals to the muscles, which causes the muscles to contract repeatedly and often painfully. Black widow bites can also cause other nerve-related problems, such as high blood pressure, restlessness, and severe facial spasms. The effects of a spider bite vary according to several factors, including the amount of venom injected and the size and age of the person who was bitten. Children and elderly people are especially susceptible. The most deadly spiders in the world include the black widow, funnel web, and brown recluse spiders. One of the most feared spiders in the world, the tarantula, actually has surprisingly weak venom and a bite that feels more like a wasp sting. Only the bite of the female black widow is dangerous; the male is much smaller than the female, and males and juveniles are harmless to humans. Only the female has the telltale red hourglass shape on its underside; the male has yellow and red bands and spots on the abdomen. The most venomous spider in the world is the Brazilian Wandering Spider, or the banana spider. This aggressive spider wanders the forest floors of Central and South America looking for food. Just a small amount of venom is enough to kill a human. The brown recluse gets its name from its color and its "shy" nature. The bite of the brown recluse spider, which is found in the southeastern United States, is particularly dangerous because its bite is initially painless. A person may be bitten without realizing it, but after awhile the skin starts to swell and become incredibly painful. A bite could kill a person if not treated. The funnel web spider is an aggressive spider that attacks and bites people. Its poison has been known to kill in just 15 minutes. Fortunately there is an antivenom, and deaths from this spider are now rare. Spider varieties The world’s biggest spider is the goliath spider (Theraphosa blondi). It can grow up to 11 inches wide, and its fangs are up to one inch long. It hunts frogs, lizards, mice, and even small snakes and young birds. The world’s smallest spider is the Patu marplesi. It is so small that 10 of them could fit on the end of a pencil. Giant trapdoor spiders are considered living fossils because they are similar to spiders that lived over 300 million years ago. They are found in southeastern Asia, China, and Japan and are over 4 inches across, including their legs. Spider Web Facts Spider webs contains vitamin K, which assists in reducing bleeding Hundreds of years ago, people put spider webs on their wounds because they believed it would help stop the bleeding. Scientists now know that the silk contains vitamin K, which helps reduce bleeding. Spiders are the only group of animals to build webs. Over millions of years, webs have evolved into a variety of kinds, such as sheets, tangles, ladders, and the elegant orb web. When most people think of a web, they think of an orb web. Scientists in the United States Defense Department are trying to copy gold orb weaver silk in order to use it for bulletproof vests. The Darwin bark spider creates the strongest material made by a living organism. Their giant webs can span rivers, streams, and even lakes and is 10 times stronger than Kevlar. Spiders have inspired scientists to make space robots. For example, the “Spidernaut” is a mechanical spider that is designed to crawl over the outside of a spacecraft to carry out repairs. Its weight is spread evenly over its eight legs to avoid damaging the surface of the spacecraft. Scientists have also designed miniature pieces of equipment with parts that move just like a spider’s leg. Spider Physics Spider legs use hydraulic pressure to move. A spider’s muscles pull its legs inward, but cannot extend its legs out again. Instead, it must pump a watery liquid into its legs to push them out. A dead spider’s legs are curled up because there is no fluid to extend the legs again. Spider webs are not passive traps. Instead, because of electrically conductive glue spread across their surface, webs spring towards their prey. Scientists also found that the glue spirals on the web distort Earth’s electric field within a few millimeters of the web. Fun Fact There is a spider in Hawaii that looks like it's smiling. Dubbed the "Hawaiian happy-face spider," the cheerful-looking spider is under the threat of extinction. Adapted from Karin Lehnardt, senior writer, , with pictures from the net.