Spider (arthropod) - biology. I INTRODUCTION Spider (arthropod), any of a large group of invertebrates (animals without backbones) that have spinning glands used to produce silken threads and webs. There are about 40,000 species of spiders. Spiders are found worldwide, except for in the oceans, and they live in all habitats and at most elevations. Spiders, along with daddy longlegs, mites, ticks, and scorpions, belong to a group of animals known as arachnids. Spiders are not insects. Spiders have a different body structure than insects. The bodies of spiders divide into two parts (a fused head and thorax and an abdomen), and spiders have eight legs and eight eyes; they lack antennae and wings. In contrast, the bodies of insects form three parts (head, thorax, and abdomen), and insects have six legs, two eyes, two antennae, and, typically, four wings. All spiders are carnivorous (feeding mainly on the flesh of other animals). Spiders eat insects and sometimes other arthropods (invertebrate animals with jointed limbs, segmented bodies, and hard shells known as exoskeletons), including other spiders. Scientists often divide spiders into two types: web spiders and ground spiders. Web spiders produce webs to capture prey, while ground spiders hunt prey directly without using a web. Almost all spiders use poison glands to kill or paralyze their prey or to defend themselves. Contrary to popular belief, most spider bites are not dangerous to humans. Of the 40,000 species of spiders, only about 30 species produce bites that may cause illness. Spiders rarely attack humans unless they feel threatened, and if they do bite, the wound is rarely serious. There is absolutely no reason to kill any spider or to call an exterminator if you have spiders in your house. As an old English saying goes, "If you want to live and thrive, let a spider run alive!" II EXTERNAL BODY Spiders range in size from less than 1.0 mm (0.04 in) to more than 10 cm (4 in) in length, with a leg span of up to 20 cm (8 in). A spider's body is divided into two parts: the front portion, called the prosoma or cephalothorax, and the rear portion, called the opisthosoma or abdomen. A narrow stalk called the pedicel connects these two parts. A hard shell called an exoskeleton covers the entire body of a spider. The exoskeleton is made of cuticle, a material composed of a combination of protein and tough fibers called chitin. The cuticle forms thin layers stacked on top of one another, an arrangement that improves the strength and elasticity of the exoskeleton. The spider's cuticle provides attachment sites for many muscles, and it also prevents desiccation (loss of body water). The cephalothorax cuticle is strong and stiff, while the cuticle of the abdomen is soft and extensible. As a spider grows, it sheds or molts its exoskeleton and grows a new one to cover its larger body. A Cephalothorax Structures The cephalothorax contains a number of structures and appendages: one pair of biting mouthparts known as chelicerae; a pair of poison glands; one pair of short, leglike appendages called pedipalps or palps; and four pairs of legs. The spider's eight eyes are also located on the cephalothorax. A1 Mouthparts When a spider catches prey it uses a pair of jointed appendages known as the chelicerae, located in front of the mouth opening. Chelicerae resemble tiny pocketknives. Each chelicera has a sharp fang that swings out of its resting position to stab into the victim. Near the tip of the fang is a duct opening that comes from a poison gland. The fang acts like a hypodermic needle--it ejects venom from the poison gland and delivers it into the prey. Spiders also use chelicerae as multipurpose tools. They have been called the "hands" of the spider. Spiders can use their chelicerae to perform tasks such as digging burrows in the soil and transporting small prey. A2 Poison Glands Most spiders have a pair of poison glands that lie within the cephalothorax. Each bulblike poison gland produces and stores toxin. A muscle spirals around the gland. When this muscle contracts, it squeezes poison from the gland through a duct into the fangs of the chelicerae, which then pass the poison into the prey. A3 Palps and Legs Behind the chelicerae are a pair of palps, segmented limbs that are used in feeding and as feelers. Male spiders also use palps to transfer sperm to females during mating. Adjacent to the palps are four pairs of long, hairy legs. Unlike human hair, each spider hair found on the legs acts as a sensory organ, sensitive to touch and vibration. Each leg is made up of seven jointed segments, called the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. More than 30 muscles control the movement of each leg. In addition, some joints of the leg move by the hydraulic action of body fluid. The tips of the legs have two or three small claws that are used for climbing or grasping the spider's silk thread. Many ground spiders have specialized adhesive hairs beneath their claws, known as claw tufts or scopulae. These claw tufts enable the spiders to walk surefootedly on smooth, vertical surfaces--even upside down on glass. A4 Sensory Organs Most spiders are active at night, and as a result they use their other senses more than they use their eyesight, which is not well developed. In addition to the thousands of hairs found on the palps and legs that are highly sensitive to touch and vibrations, spiders also have hairs on their feet that they use to taste things. Most spiders have four pairs of simple eyes (eyes with a single lens) that are located on the front of the cephalothorax. The eyes are usually grouped into two or three rows that form specific patterns in different spider families. This eye arrangement is often used to identify and classify a spider. Unlike spiders that are active at night, spiders that are active during the day, such as jumping spiders and lynx spiders, typically have good vision at close range (around 10 to 20 cm, or 4 to 8 in). Their vision easily rivals the eyesight of many insects, which have compound eyes (eyes with multiple lenses). B Spider Abdomen The spider's abdomen is soft and saclike. On the underside of the tip of the abdomen are three pairs of spinnerets. Each spinneret is studded with many fine, hairlike tubes called spigots, which produce a variety of silk threads. The spigots lead to several large silk glands inside the abdomen. Silk is formed as a liquid inside these abdominal glands. As the silk is drawn out through the spigots, protein molecules within the silk line up parallel to one another, causing the silk to harden and form strong, elastic filaments. The hardening of silk results from the drawing-out process through the spigots, not from exposure to air, as is commonly believed. Several silk threads produced by different spigots may fuse to form a stronger one. Spinnerets are actually shortened limbs. They can move to place silk strands in precise locations when the spider builds a web or wraps prey in silk. III INTERNAL ANATOMY AND FUNCTION The cephalothorax houses part of the digestive system and the central nervous system. The abdomen contains most of the spider's vital organs, including a long, tubular heart; respiratory organs; reproductive organs; and excretory organs. A Breathing Spiders use two types of breathing organs: book lungs and tracheal tubes. Narrow slits on the underside of the abdomen lead to two or four respiratory organs called book lungs. These organs are so named because they consist of alternating layers of air spaces and thin leaflets of cuticle filled with blood, making the structures resemble a slightly opened book. Oxygen from the air passes through the extremely thin cuticle leaflets directly into the blood. A small opening in front of the spinnerets, known as a spiracle, leads to tracheal tubes made of cuticle. These tiny tubes branch and spread throughout the body. Air enters the spiracle and passes through the tracheal tubes so that oxygen can travel to all the spider's body tissues. B Blood Circulation Spider blood, also known as hemolymph, contains many blood cells with oxygen-carrying pigments called hemocyanin, which give the blood a light blue color. In contrast, the primary components of human blood are red blood cells carrying the red pigment hemoglobin. Spider blood also contains many other types of blood cells that play a role comparable to that of the white blood cells of humans. Among other functions, these cells play a role in blood clotting after an injury. The spider's long, tubular heart lies toward the back side of the abdomen. When the heart contracts, it pumps blood forward into the cephalothorax and backward into the abdomen. Blood travels through closed tubes, or arteries, into spaces in the body cavity. From these spaces the blood travels to the book lungs, where it releases carbon dioxide and picks up a fresh supply of oxygen before returning to the heart. C Digestion The digestive system consists of a branched tube that extends from the mouth to the anus. In the cephalothorax, the tube enlarges to form a stomach with powerful muscles. When these muscles contract, they produce a powerful sucking action that pulls food into the midgut. Spider digestion is unusual in that it begins outside of the spider's body. When a spider captures an insect or other animal, it uses its chelicerae to pierce the prey and inject poison into the wound to paralyze or kill the animal. The spider then vomits juices containing digestive enzymes into the wound of the victim to break down and liquefy its body tissue. This liquefied tissue is then drawn through the spider's mouth and into its body by the sucking action of the stomach. Two mechanical filters in the mouth prevent solid food particles from passing into the digestive system. From the stomach, food passes into the midgut, which branches throughout the entire body. Enzymes secreted by the midgut further break down the liquefied food into nutrient molecules small enough to pass through the walls of the midgut into the blood. Nutrients can be stored for a long time in the spider's extensive digestive system, enabling many spiders to go for weeks or even months without the need to catch any prey. D Nervous System Most arthropods have a central nervous system made up of a long chain of nerve cell centers called ganglia that run throughout the body. In spiders the ganglia are concentrated in the cephalothorax, where they condense into two compact masses: the sub-esophageal ganglion and the supra-esophageal ganglion. The subesophageal ganglion directs spider locomotion. The supra-esophageal ganglion is considered the brain of the spider. Sense organs throughout the body send information to this nerve center, where information processes and complex functions begin. A spider's brain is relatively highly developed, enabling spiders to easily adapt to changes in their environment. Some scientists believe spiders can learn, and some have observed that spiders can remember where in their web they have stored captured prey; if the prey is removed, the spiders will continue searching for it in the same place for hours. E Reproduction All species of spiders have two separate sexes, and the males are usually smaller than the females. The male spider has two sperm-producing testes. A sexually mature male spider uses its large palps to transfer sperm cells into the female during mating. In this process, the male builds a small, triangular sperm web onto which he deposits a drop of sperm from his abdomen. He then dips both palps into this droplet, drawing sperm cells into the palps as if by a tiny pipette. The female reproductive system includes two egg-producing ovaries. After the male transfers sperm cells into the female's genital opening located on her abdomen, they are stored, sometimes for months, in tiny receptacles. These sperm cells fertilize the female's egg cells just before she deposits her eggs into a silky cocoon. IV LIFE CYCLE The life cycle of the spider consists of four stages: egg, larva, young spider, known as a nymph or spiderling, and adult. Like insects, spiders grow only by molting, a process that involves periodically shedding their exoskeleton. In each molting stage, young spiderlings resemble tiny adults, a process known as incomplete metamorphosis. A Courting and Mating Spiders become sexually mature after their last molt, at which time females have developed functional ovaries and males have mature testes. In most spider species the male courts the female before mating occurs. After a male spider has filled its palps with sperm cells, he begins searching for a female. A male begins by identifying himself to a female so that she does not mistake him for potential prey. In some spiders, such as American tarantulas, this identification process involves the male repeatedly touching the female. More often, a male courting a female communicates with her over larger distances using vibrations. For instance, a male wolf spider uses its legs to drum on the ground. In some web spiders, the male attaches a special signal thread to the female's web. The male then drums or plucks the thread in a rhythm that indicates the vibration is caused by another spider of the same species and not by an ensnared insect. If a female is ready to accept a courting male, she may send signals back to him. Locating the right female can be tricky for a male spider. Fortunately, female spiders produce certain chemical substances known as pheromones that aid spider courtship. A female may release these pheromones through the air (like a perfume) or she may deposit them on her silk threads. When a male spider encounters pheromones from a female of the same species, he becomes excited, even if the female is not present. Spiders with better eyesight may rely mostly on visual signals during courtship. When a male notices a female, he starts a zigzag dance in front of her in which he raises his front legs, vibrates his palps, and twitches his abdomen. Each species uses a different courtship dance with unique movements. A female will only accept a male who performs a dance with movements specific to that species. When a male finds an interested female, he inserts his sperm-containing palp into the female's genital opening. The process of mating can be very brief (a matter of seconds), or it can last several hours, depending on the species. In most species both sexes separate peacefully after mating. Contrary to popular belief, the female black widow spider does not kill her partner after mating. Depending on the species, a female may mate only once or she may mate with several males during her lifetime. B Eggs and Brood Care In most species a female determines when sperm cells from the female's receptacles will fertilize her egg cells. Fertilization may occur a few weeks after mating, a strategy that enables the female to lay her eggs when she deems external conditions are best. The female then lays her eggs. Most spider species lay several hundred eggs; species of large spiders deposit several thousand within just a few minutes. Typically the female lays her fertilized eggs in a silky case called a cocoon, which provides a protective and insulating environment for the developing spiders. Many females abandon their cocoons right after they deposit their eggs, although they may camouflage them or hang them in hidden locations. Other spiders guard and defend their cocoon until the eggs hatch. Some spiders exhibit special brood care. Female wolf spiders attach their cocoons to their spinnerets and carry them around until the eggs hatch. The newly hatched spiderlings then climb onto their mother's back, where they stay for about a week before they leave to survive on their own. Occasionally young spiderlings stay in their mother's web for weeks. During this time the mother feeds her brood, transferring regurgitated food from her mouth to their mouths. C Development and Growth Spider eggs hatch inside the cocoon. The first stage hatches from the egg as an immobile, milky white larva. Larvae are covered by an embryonic membrane and receive nourishment from yolk material within their abdomen. After one to two molts over about a two-week period, the larva changes into a mobile spiderling, also referred to as a nymph. In order to grow to an adult size, spiderlings undergo a series of molts that enables them to increase in size. During molting the old cuticle slowly lifts off while a thin new cuticle forms underneath. The new cuticle is wrinkled and pliable at first, but as molting progresses and the spiderling grows, the new cuticle stretches to accommodate the larger spiderling body. It later hardens into a new rigid exoskeleton that encases the larger spiderling. The number of molts between the spiderling and adult stage varies according to the size of the species. Small species may molt about 5 times, while some large tarantulas may molt as many as 40 times. For most spiders, a spiderling's last molt marks adulthood, when functional sexual organs have developed and growth halts. Some adult female spiders, such as American tarantulas, continue to molt repeatedly. Most spiders live only 1 or 2 years. Notable exceptions are large female tarantulas, which can live up to 20 years. Male tarantulas live only 2 to 3 years. Many male spiders die soon after mating. V SILKS, THREADS, AND WEBS Nearly all spiders produce silk composed of the protein fibroin. This is the same protein produced by silkworms, the larvae stage of certain silk moths. Each spider has four to eight different kinds of silk glands in its abdomen, and each gland produces a different type of silk with different properties. For instance, spiders lay out a line of dry silk behind them as they move about. This dragline acts as a safety line like that used by a mountain climber. Other glands produce cocoon threads that blanket and protect fertilized eggs. Still other glands produce sticky capture threads that ensnare prey. Spider silk threads are very thin, about 1 micrometer (0.001 mm) in diameter. Each thread weighs very little. A spider web composed of 20 m (70 ft) of silk thread weighs less than 1/1000 of a gram. Despite its lightness, a spider silk thread is as strong as a nylon thread, but with more elasticity. The combination of strength and elasticity makes spider silk ideal for web building. Spider threads are tough enough to withstand the impact of a flying insect while being elastic enough not to tear apart with the captured prey's weight. A Types of Webs Web patterns vary considerably, depending on the species of spider. Perhaps the most recognizable web is the almost circular orb web, in which an outer framework supports a continuous spiraling thread and a series of threads radiating from the center of the web. Other web types may have a more irregular shape. Some spiders build irregular, flimsy webs. Common house spiders construct funnel webs, flat silk sheets with a raised tube in the corner that serves as a retreat for the spider. Cobweb spiders build an irregular silk meshwork with sticky threads at the bottom that trap insects. Sheet web spiders construct a horizontal silk sheet with a dome, from which the small spider hangs upside down. Many spider webs are found near the ground or in low vegetation, although orb webs often span the open spaces between bushes or trees in order to trap flying insects. The size of a web depends on the size of the spider. Whether the web has a tight or wide mesh depends on the size of the prey the spider expects to capture. B Web Building Web building is a complex process, but some spiders can complete a web in less than 30 minutes. Spiders typically build their webs at night, completely relying on their sense of touch, not eyesight, during construction. Each type of spider uses a different procedure when building a web. Spiders that weave orb webs generally begin by spinning a thread that is carried by air currents until it catches on a tree limb or other firm support. From this thread the spider lays down another thread to form a Y- shaped structure that is the basic framework of the web. The spider then climbs to the midpoint of the Y-structure, known as the hub, and begins creating radius lines, or spokes, around the web. As the spider builds radius lines, it connects these lines with a few narrow circles of thread in the center of the web that forms the auxiliary spiral. The auxiliary spiral prevents radius lines from sagging when the spider walks on them. Using the auxiliary spiral as scaffolding, the spider begins the formation of the catching spiral, fastening sticky threads to each radius line. As the spider constructs the catching spiral, it dismantles the auxiliary spiral. The catching spiral extends from the periphery of the web and stops short of the web's hub. In the finished web, only remnants of the auxiliary spiral remain and there is an empty space in the web's center known as the free zone. The spider may sit upside down with its legs placed in the center of its web to detect vibrations in the web when prey gets caught in the sticky catching spiral. Others may hide nearby under a curled leaf and use the vibrations from a signal thread attached to the hub to stay informed when prey has struck the web. The orb web is built anew every day. Some species eat their old web before starting a new web, while others roll up the web and discard it as a tiny silk ball. VI EVOLUTION Spiders evolved around 400 million years ago. The ancient spiders that lived millions of years ago were relatively large, and their bodies were segmented. In contrast, almost all spiders living today have an unsegmented abdomen. Only members of the suborder Mesothelae still exhibit a segmented abdomen, and these spiders are generally considered the most primitive types of spiders. The fossil record of spiders is rather limited. The oldest spider fossil was found in New York State in rocks dating back to the Devonian Period (about 410 million to 360 million years ago). This fossil was remarkably well preserved. Using a microscope, scientists were able to recognize the spider's spinnerets and chelicerae. Some fossil spiders with segmented abdomens have been identified in rocks dating from the Carboniferous Period (360 million to 290 million years ago). Very few fossils have been found from the Mesozoic Era (240 million to 65 million years ago). Spider webs rarely preserve as fossils, but scientists theorize that ancient spiders initially built irregular webs located near the ground and that webs located higher up in trees evolved only later. Orb webs, for instance, may be an adaptation for snaring insects flying higher up in the vegetation. Some scientists suggest that spiders may have influenced the evolution of insects--and vice versa. In an effort to escape ground-living spiders, insects may have evolved ways to fly. At a later time, spiders may have evolved ways to develop aerial webs in order to catch flying insects. VII TYPES OF SPIDERS Spiders belong to the order Araneae. Scientists typically divide the 40,000 species of spiders into three suborders: the Mesothelae, the Mygalomorphae, and the Araneomorphae. The suborder Mesothelae is a small, primitive group of spiders that have a segmented abdomen resembling that of early fossil spiders. Spiders of this suborder are sometimes called living fossils since they still exhibit primitive traits that no other living spiders share. Only 40 species from this suborder have been identified, and they live in Indonesia and Japan. The suborder Mygalomorphae contains about 2,000 species, including tarantulas, trap-door spiders, and purse-web spiders. Although these spiders are not segmented, this group still retains some primitive characteristics, including four book lungs and chelicerae in which the biting parts lie parallel to each other and strike forward and down. About 32,000 species, or almost 90 percent of all spiders, belong to the suborder Araneomorphae. These spiders are characterized by the presence of two book lungs and a tracheal tube breathing system. They have chelicerae with opposing fangs that work together like a pair of pliers. Many araneomorphs are ground hunters. Although these spiders do not use a web to capture prey, they may use silk for other purposes such as for silk cocoons to protect eggs. The remainder of this section discusses some interesting spiders and their natural history. The spiders highlighted are organized according to their hunting ability. Ground spiders hunt prey from the ground, while web spiders use webs to ensnare prey. A Ground Spiders A1 Tarantula True tarantulas are a type of wolf spider that belong to the family Lycosidae in the suborder Araneomorphae. But most people use the term tarantula to refer to about 700 species of spiders belonging to the family Theraphosidae in the suborder Mygalomorphae. Sometimes known as American tarantulas, they are found in tropical regions throughout the world, with many species in the southwestern United States. These giants of the spider world can attain a body length of 10 cm (4 in) with a leg span up to 20 cm (8 in), making it possible for some species to overpower small vertebrates (animals with backbones), such as frogs or lizards, for their meal. Their large body and long legs are covered with hairs, and they have powerful chelicerae, giving them a fearsome appearance. Most tarantulas stay on the ground (often in burrows). As ground hunters, tarantulas are typical sit-and-wait predators--they wait for insects or small vertebrates to come near before they pounce and kill the prey with their strong chelicerae. Perhaps as a result of their frightening size and appearance, tarantulas have gained a deadly reputation among humans. For the most part, however, tarantulas do not attack unless provoked, and their venom is not harmful to humans, although their strong chelicerae can cause painful wounds. Tarantulas are popular pets, and some become so tame that they can be picked up and handled safely. Although the chance of receiving a bite is small, there is another danger: Many tarantulas brush off their abdominal hairs when they feel threatened. These barbed hairs fly through the air and can penetrate skin and the mucous membranes of the nose, causing a strong burning sensation. A2 Wolf Spider About 2,000 species of wolf spiders belong to the family Lycosidae in the suborder Araneomorphae. Found throughout the world, these spiders have dull brown or black coloration, stout bodies, and long, thick legs. Some species have hairy bodies. Wolf spiders are ground hunters, but their name inaccurately suggests that they actively hunt their prey just like their wolf namesake. However, like tarantulas, most wolf spiders usually sit in a hidden spot. When prey happens by, they ambush the prey by jumping on it. Wolf spiders are sensitive to vibrations, such as the buzzing wings of insects, as well as to visual signals. For instance, during courtship males drum their legs on the ground or wave their legs and palps in a rhythmic dance in order to catch a female's attention. Female wolf spiders are renowned for their brood care. After laying eggs, a female spider carries the cocoon attached to her spinnerets. When the spiderlings hatch, she allows them to ride on her back for about a week. Some larger wolf spiders dig burrows in the soil, which they may line with silk and provide with a door. At night they leave their burrows to hunt for insects. A3 Jumping Spider The jumping spiders are active hunters. Depending on the species, they can leap up to 25 times their body length when stalking prey. They belong to the family Salticidae, the largest spider family with more than 4,000 species, in the suborder Araneomorphae. Jumping spiders are found mostly in tropical regions throughout the world, although some species live in high elevation regions in the Himalayas. Jumping spiders are small spiders, seldom growing larger than 2 cm (0.8 in) in length. Most jumping spiders have somber brown or gray coloration, but a few male species are quite colorful, with iridescent scales and spines and tufts of bright hair. The most striking feature of jumping spiders is their eyes. They have two primary eyes on the front of their cephalothorax that provide exceptionally acute vision. For example, at 20 cm (8 in) they not only see sharp images, but also recognize members of their own species. Their six secondary eyes detect motion. Their excellent eyesight makes these spiders reliant on visual cues for courtship and hunting, and as a result they are active mostly during the day. At night they hide in crevices or under bark, often in small silken cells that they weave for themselves. During courtship, the male jumping spider identifies himself by dancing in front of a female, waving his legs in a pattern specific to that species. Hunting is also guided by visual cues. The spider's secondary eyes are able to detect a moving insect, which prompts the spider to turn toward the insect and scrutinize it with its primary eyes. This overlapping visual field produced by the primary and secondary eyes enables the spider to accurately calculate the distance to the prey. Jumping spiders approach their prey like a cat stalks a mouse. When the spider comes within a few centimeters of its prey, it suddenly leaps by pushing off the ground with its hind legs and then grabs the prey with its front legs. Muscle power and the hydraulic action of body fluids fuel the explosive force of the jumping spider's legs. B Web Spiders B1 House Spider The common house spider belongs to the funnelweb spiders in the family Agelenidae in the suborder Araneomorphae. House spiders are found throughout Europe and North America. This spider is so named because its horizontal sheet web is often seen in wall corners of houses, but it can also be found in any cool, dark place such as dense vegetation or crevices of logs or rocks. The spider's web forms a tube, and the narrowed end serves as a retreat where the spider can hide. When an insect walks over the sheet web, the spider immediately rushes out from the funnel, grabs its victim, and delivers a poisonous bite. The spider then carries its prey back to its retreat, where it begins to feed. Sometimes house spiders leave their webs and can be seen wandering around or trapped in a bathtub. These are usually males who have given up building webs, but are always in search of a female. Male house spiders lack adhesive hairs on their feet and therefore cannot climb on smooth vertical surfaces, such as those found in sinks and bathtubs. Most house spiders are harmless and may actually prove beneficial to humans since they feed on insects that stray indoors. B2 Garden Spider Garden spiders belong to the family Araneidae, a group of 2,500 different species of spiders that weave orb, or circular, webs. Marked with varying shades of brown, garden spiders have a distinctive white cross on their abdomens, and some people refer to them as cross spiders. They are found throughout the continental United States, Canada, and Mexico. Some species are found in Europe and Hawaii. Like most orb weavers, a garden spider typically sits upside down in the middle of the orb web, placing its eight feet on the threads radiating out from the center of the web. The feet act as sensors, picking up vibrations when prey enters the web. When an insect becomes stuck in the web's sticky spiral threads, the spider rushes out to wrap the victim with silk and give it a paralyzing bite. The spider then carries the prey back to the hub of the web to feed on it. VIII HUMANS AND SPIDERS Spiders suffer an unsavory reputation, perhaps because of their appearance, their tendency to lurk in dark places or dangle ominously from a thread, and a gross exaggeration of their ability to poison humans. Some people are absolutely terrified at the mere sight of a spider, a condition known as arachnophobia. Although all spiders have poison glands that they use for defense and to kill or paralyze prey, only about 30 of the 40,000 spider species produce venom that can cause serious illness in humans. Humans are more likely to be harmed by bee or wasp stings than by the relatively few spider species that can inflict a harmful bite. Some of the spiders most dangerous to humans are the black widow spider, the brown recluse spider, the Australian funnelweb spider, and the South American hunting spider. Bites from these spiders can be fatal to humans without proper treatment. For instance, the bite of a black widow spider causes severe pain that may last for days. If not treated properly with an injection of calcium and a specific antivenin, a person may take weeks to recover, and in rare cases the bite may result in death. Most spiders that are poisonous to humans prefer to avoid human contact and only bite when they feel threatened. The South American hunting spider is an exception--it is quite aggressive. Its bite is very painful, but it is rarely deadly for humans, most likely because the spider uses only a small amount of venom when it bites. Although spiders pose minimal danger to humans, human activity seriously threatens some spider species. Such activities include habitat destruction, in which forestlands are destroyed for agriculture or for building homes and business developments. The widespread use of pesticides in agriculture targets specific insect pests but also kills harmless spiders and their insect prey. To protect spider populations, 16 spider species are included on the 2000 Red List of Threatened Species compiled by the World Conservation Union (IUCN), a nongovernmental organization that compiles global information on endangered species. Certain tarantulas, threatened by the pet trade, are now protected by the Convention on International Trade in Endangered Species (CITES), which prevents or limits the international trade in threatened plants and animals. Protecting spiders is necessary because they play a critical role in the environment as aggressive predators of insects. Studies estimate that each year spiders consume as much as 100 kg (220 lb) of insects per 1 hectare (2.5 acres) of land. Such efficient predators would normally make good candidates for use in the biological control of insect pests in agriculture. However, most spiders are not particular about the type of insects that they eat--they consume insects that are harmful to agricultural crops as well as those that are beneficial. As a result of their inability to target a specific harmful insect species, spiders are of limited use in pest control. Spiders may one day be used as an alternate source of silk. Over the past 200 years scientists have studied the properties of spider silk in order to develop ways to mass-produce these fibers for use in fabrics and other textiles. These efforts have been stymied by the cumbersome and costly process of drawing out individual silk threads from spiders. More recently, scientists have attempted to identify and isolate a silk gene in spiders in order to duplicate a silk thread with the wonderful strength and elasticity of spider silk, but their success has been limited. Based on information provided by R. F. Foelix, curator of the Natural History Museum in Aarau, Switzerland, and author of Biology of Spiders (2nd edition, 1996). Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.