General Information

2009/07/09 10:02:00


Spiders are small, eight-legged creatures that are best knownforspinning silk webs. Spiders spin webs so they can catch insectsfortheir food and even larger and stronger insects cannot escape.

All spiders spin silk but some don't spin webs.Bolasspiders spin a single line with a sticky end. Any insect near,getstrapped when the spider swings the sticky line near them.

All spiders have fangs and most kinds havepoisonglands. They use their fangs and poison glands to capturetheirfood. A spider's bite can kill insects and other smallanimals. Afew kinds of spiders are harmful to human beings. InNorth America,six kinds of spiders harm people, they are - theBrown Recluse,Sac, Black Widow, Brown Widow, Red-legged Widow andthe VariedWidow. Four of the Widow females are known to bitehumans. Thebites of these six spiders often cause mild reactions.Usually aperson irritates a spider several times for it to biteyou. InAustralia, the most dangerous spider is the Funnel-Web withtheRed-back, a type of Black Widow spider, also beingdangerous.

Spiders are helpful to people because theyeatharmful insects. They eat grasshoppers and locusts whichdestroycrops. Spiders also eat flies and mosquitoes which carrydiseases.Spiders feed mostly on insects but some capture and eattadpoles,small frogs, small fish and mice. Most females are largerandstronger than the males and occasionally they eat males.Spiderscan live anywhere they can find food like fields, woods,swamps,caves and deserts. One kind of spider spends most of itslifeunderwater. Another kind lives near the top of Mount Everest,theworld's highest mountain. Some live in houses, barns, andotherbuildings. Others live on the outside of buildings, onwalls,windscreens and corners of doors and windows.

The life span of arachnids in temperate areas isasingle season, therefore they rely on eggs to perpetuatethespecies from year to year. In warm regions, certain groups(somescorpions and tarantulas) appear to live more than a singleyear,in fact some tarantulas in captivity have survived for as longas20 years.

There are more than 30,000 kinds of spiders. Scientistsbelievethere may be up to 50,000 to 100,000 kinds. Some aresmaller thanthan the head of a pin but some are larger than aperson's hand. Onekind of spider, a South American tarantula ismeasured at 25centimetres but that is with its legs extended.

Most people think spiders are insects butscientistsclassify spiders as arachnids. Insects are different ina number ofways. Spiders have eight legs but ants, bees, beetlesand otherinsects have only six legs. Most insects have wings orantennaewhich are feelers. Arachnids include daddy long legs,scorpions,mites and ticks.

Scientists classify spiders as either truespidersor Tarantulas, according to certain differences in theirbodiessuch as the way their fangs point and move. In addition,spiderscan be grouped according to the way they live. Web spinningspidersspin webs to catch insects. Others lie and wait for insectstocome.

Spiders may be alldifferent shapes - shortand fat, long and thin, round, oblong, orflat and one even lookslike a stick insect. Their legs can be shortand stubby, or longand thin. Spiders are most commonly brown, grey,or black but somespecies have beautiful colours. Many of thesespiders are so smallthat their colours can be only seen  witha microscope.

A spider has no bones and its tough skin serves as aprotectiveouter skeleton. Hairs, humps, and spines (bristles ofskin) coverthe bodies of most spiders.

A spider's body has two main sections: the cephalothorax, which consists of the head joinedtothe thorax (chest); and the abdomen.Each of these sections has parts attachedto it called appendagesConnecting the the cephalothorax and theabdomen is a thin waistcalled the pedicel.

Eyes. A spider's eyes are on top and nearthefront of its head. Different species have different numbers ofeyesand the size and position also varies. Most species haveeighteyes, arranged in two rows of four each. Other kinds havesix,four, or two eyes. Some spiders have can see better thanothers.Hunting spiders have good eyesight at short distances andtheireyesight allows them to form images of their prey andmates.Web-building spiders have poor eyesight and their eyes areused fordetecting changes in light. Some species of spiders thatlive incaves or other dark places have no eyes at all.

Mouth. Below the spider's eyes isitsmouth opening. Spiders eat only liquids because they do nothavechewing mouth parts.  Around the mouth are variousappendageswhich form a short "straw" through which the spider sucksthe bodyfluid of its victim.

The spider can only eat  some of the solid tissue of itspreyby predigesting it. The spider sprays digestive juices onthetissue and the powerful juices dissolve the tissue.  Alargetarantula can reduce a mouse to a small pile of hair and bonesinabout 36 hours by predigestion and sucking.

Chelicerae are a pair ofappendagesthat the spider uses to seize and kill its prey. Thechelicerae areabove the mouth opening and just below the spider'seyes. At theend of each chelicera is a hard, hollow, pointed claw,and theseclaws are the spider's fangs. An opening in the tip of thefangconnects with the poison glands so that when a spider bitesaninsect with its chelicerae, poison flows from the fangs intothewound and paralyses or kills its prey.

In true spiders, the fangs pointcrosswise,and the poison glands extend back into the cephalothorax.The fangsof tarantulas point straight down from the head, and thepoisonglands are in the chelicerae. Spiders also crush their preywiththeir chelicerae. Some species use their chelicerae to digburrowsin the ground as nests.

Pedipalpi are a pair of appendagesthatlook like small legs. One pedipalp is attached to each side ofthespider's mouth, and they form the sides of the mouth. Eachpedipalphas six parts. In most kinds of spiders, the part closestto thebody has a sharp plate with jagged edges. The spider usesthis plateto cut and crush its food. In adult male spiders, thelast part ofeach pedipalp has a reproductive organ.

Legs. A spider has four pairs oflegs,which are attached to its cephalothorax and each leg hassevensegments. In most kinds of spiders, the tip of the lastsegment hastwo or three claws. Surrounding the claws is a pad ofhairs calledthe scopula. The scopula sticks to smooth surfaces andhelps thespider walk on ceilings and walls.  Sensitivebristles thatserve as organs of touch and perhaps organs of smellalso covereach leg. Some bristles pick up vibrations from theground or air,or the spider's leg while others detect chemicals intheenvironment.

When a spider walks, the first and third leg on one side of itsbodymove with the second and fourth leg on the other side. Musclesinthe legs make the legs bend at the joints but spiders havenomuscles to extend their legs so it is the pressure of the bloodintheir bodies that makes their legs extend. If a spider's bodydoesnot contain enough fluids, its blood pressure drops, the legsdrawup under the body, and the animal cannot walk.

Spinnerets are short, fingerlikeorganswith which the spider spins silk. They are attached to therear ofthe abdomen. Most kinds of spiders have six spinnerets, butsome mayhave four or two. The tip of a spinneret is called thespinningfield and the surface of each spinning field is covered byas manyas a hundred spinning tubes. Liquid silk flows throughthese tubesfrom silk glands in the spider's abdomen to the outsideof its bodyand the silk then hardens into a thread.

Respiratory system. Spiders as agrouphave two kinds of breathing organs - tracheae and booklungs. Tracheaeare found inalmost all kinds of true spiders and they are smalltubes whichcarry air directly to the body tissues.  In frontof thespinnerets in most kinds of true spiders is an opening calledthespiracle and air enters thetubesthrough these openings.

Book lungs are in cavities inthespider's abdomen and air enters the cavities through a tiny slitoneach side and near the front of the abdomen. Each lung consistsof15 or more thin, flat folds of tissue arranged like the pages ofabook. The sheets of tissue contain many blood vessels. Asaircirculates between the sheets, oxygen passes into theblood.Tarantulas have two pairs of book lungs. Most true spidershave onepair.

Circulatory system. The bloodofspiders contains many pale blood cells and is slightly bluishincolour. The heart, is a long, slender tube in the abdomen,andpumps the blood to all parts of the body. The blood returns totheheart through open passages instead of closed tubes, such asthoseof the human body. If the spider's skin is broken, thebloodquickly drains from its body.

Digestive system. A digestivetubeextends the length of the spider's body. In the cephalothorax,thetube is larger and forms a sucking stomach. When thestomach'spowerful muscles contract, the size of the stomachincreases. Thiscauses a strong sucking action that pulls the foodthrough thestomach into the intestine. Juices in the digestive tubebreak theliquid food into particles small enough to pass throughthe wallsof the intestine into the blood. The food is thendistributed toall parts of the body. Food is also pulled throughthe stomach intoa fingerlike cavity called the caeca.Because spiders can store food in the caeca,they can go for longperiods of time without eating.

Nervous system.  Inthecephalothorax is the central nervous system including thebrain,which is connected to a large group of nerve cells calledtheganglion. Nerve fibres from thebrainand ganglion run throughout a spider's body. The nerve fibrescarryinformation to the brain from sense organs on the head, legs,andother parts of the body. The brain can also send signalsthroughthe nerve fibres to control the activities of thebody.

The spider's silk
Making silk:
Spider silk is made up of protein and forms in the spider'ssilkglands. As a group, spiders have seven kinds of silkglands.However, no species of spider has all seven kinds. Allspiders haveat least three kinds of silk glands, and most specieshave five.Each kind of gland produces a different type of silk.Some silkglands produce a liquid silk that becomes dry outside thebody.Other glands produce a sticky silk that stays sticky. Spidersilkcannot be dissolved in water and is the strongest naturalfibreknown.

The spinnerets, which spin the silk, work like the fingers ofahand. A spider can stretch out each spinneret, pull it back in,andeven squeeze them all together. A spider can usedifferentspinnerets to combine silk from different silk glands andproduce avery thin thread or a thick, wide band. Some spiders alsocan makea sticky thread that looks like a beaded necklace. To dothis, thespider pulls out a dry thread that is heavily coated withstickysilk. It then lets go of the thread with a snap. This actioncausesthe liquid silk to form a series of tiny beads along thethread. Aspider uses beaded threads in its web to help trap jumpingorflying insects.

Some kinds of spiders have another spinning organ calledthecribellum. It is an oval platethatlies almost flat against the abdomen, in front of thespinnerets.Hundreds of spinning tubes cover the spinning field ofthecribellum. These tubes produce extremely thin threads ofstickysilk.

Spiders with a cribellum also have a special row of curvedhairscalled a calamistrum on theirhindlegs. Spiders use the calamistrum to comb together dry silkfromthe spinnerets and sticky silk from the cribellum. Thiscombinationof threads forms a flat, ribbonlike silk structurecalled a hackledband. Spiders use hackled bands in their webs,along with the othersilk that they spin.

How spiders use silk:
Spiders, including those that do not spin webs, depend on silk insomany ways that they could not live without it. Wherever aspidergoes, it spins a silk thread behind itself. This thread iscalled adragline. The dragline issometimescalled the spider's "lifeline" because the animal oftenuses it toescape from enemies. If danger threatens a spider in itsweb, itcan drop from the web on its dragline and hide in the grass.Or thespider can simply hang in the air until the danger haspassed. Thenit climbs back up the dragline into its web. Huntingspiders usetheir draglines to swing down to the ground from highplaces.Spiders also use silk to spin tiny masses of sticky threadscalledattachment discs. They use the attachment discs to anchortheirdraglines and webs to various surfaces.

Many kinds of spiders build silk nests as their homes. Somespidersline a folded leaf with silk to make a nest. Others digburrows inthe ground and line them with silk. Still other spidersbuild nestsin the centre of their webs. Many web-spinning spidersspin stickybands or wide sheets of silk while capturing their prey.The orbweavers wrap their victims in sheets like mummies so theycannotescape. The female spider of most species encloses her eggsin anegg sac. This sac is a bag made of a special kind ofsilk.

Types of Spiders
Hunting spiders
Hunting spiders creep up on their prey or lie in wait and pounceonit. Some kinds of hunters, including jumping spiders andwolfspiders, have large eyes and can see their prey from adistance.But other hunters, such as water spiders, tarantulas, andcrabspiders, have small eyes. The powerful chelicerae ofhuntingspiders help them overpower their victims. Some huntingspidersspin simple webs that stretch out along the ground andstopinsects. These spiders are grouped as hunters because theyrunafter the insects that land in their webs. These spiders areallhunting spiders:
Jumping spiders creep up and pounce on theirprey.These spiders have short legs, but they can jump more than 40timesthe length of their bodies. Jumping spiders are the mostcolourfulof all spiders. Many thick, coloured hairs cover theirbodies. Mostmale jumping spiders have bunches of brightly colouredhairs ontheir first pair of legs.

Water spiders are the only spidersthatlive most of their life underwater. The water spiderbreathesunderwater from air bubbles that it holds close to itsbody. Itsunderwater nest is a silk web shaped like a small bell.The spiderfills the web with air bubbles, which gradually push allthe waterout of the bell. The animal can live on this air forseveralmonths. Water spiders are found only in Europe and partsofAsia.

Tarantulas aretheworld's largest spiders. The biggest ones live in theSouthAmerican jungles. Great numbers of tarantulas also are foundin thesouthwestern United States. Many kinds of tarantulas digburrows asnests. The trap-door spider covers the entrance to itsburrow witha lid. A California tarantula builds a turret (smalltower) ofgrass and twigs at the entrance to its burrow. This spiderthensits on the tower and watches for insects moving in thenearbygrass. A few kinds of tarantulas live in trees.

Fisher spiders live near water andhuntwater insects, small fish, and tadpoles. These spiders havelargebodies and long, thin legs. But because of their lightweight, theycan walk on the water without sinking. These spidersalso can diveunderwater for short periods of time. Some fisherspiders are callednursery-web weavers because the female builds aspecial web for heryoung.

Crab spiders have short, wide bodies andlooklike small crabs. They can walk backwards and sideways aseasily ascrabs do. Some brightly coloured crab spiders hide inflowers andcapture bees and butterflies. A few kinds of crabspiders candisguise themselves by changing the colour of theirbodies to matchthe colour of the flower blossom.

Wolf spiders are very common andareexcellent hunters. Many kinds have large, hairy bodies, andrunswiftly in search of food. Others look and act like other typesofspiders. For example, some wolf spiders make their homes nearwaterand resemble fisher spiders in appearance and habits. Otherslivein burrows, or spin funnel-shaped webs.

Web-spinning spiders, like hunting spiders, live in caves, ingrassor shrubs, or in trees. They cannot catch food by huntingbecauseof their poor vision. Instead, they spin webs to trapinsects. Aweb-spinning spider does not become caught in its ownweb. Whenwalking across the web, it grasps the silk lines with aspecialhooked claw on each foot. These spiders are allweb-spinningspiders:
Tangled-web weavers spin a webthatconsists of a jumble of threads attached to a support, such asthecorner of a ceiling.

The cellar spiders spin tangled webs in dark,emptyparts of buildings. One cellar spider that looks like adaddylonglegs has thin legs more than 5 centimetres long.

The comb-footed spiders spin atangledweb with a tightly woven sheet of silk in the middle. Thesheetserves as an insect trap and as the spider's hideout. Thesespidersget their name from the comb of hairs on their fourth pairof legs.They use the comb to throw liquid silk over an insect andtrap it.The black widow and the Australian redback spider arebothcomb-footed spiders.

Some spiders spin a tangled web containing a hackled band of dryandsticky silk. The ogre-facedstickspider spins a web that is made up largely of hackledbands.The web is only about as large as a postage stamp. Thisspiderspins a structure of dry silk to hold the sticky web inplace. Thespider hangs upside down from the dry silk. It holds thesticky webwith its four front legs. When an insect crawls or fliesnear, thespider stretches the sticky web to several times itsnormal sizeand sweeps it over the insect.

Funnel-web spiders live in largewebsthat they spin in tall grass or under rocks or logs. The bottomofthe web is shaped like a funnel and serves as the spider'shidingplace. The top part of the web forms a large sheet of silkspreadout over grass or soil. When an insect lands on the sheet,thespider runs out of the funnel and pounces on the victim.

Sheet-web weavers weave flat sheetsofsilk between blades of grass or branches of shrubs or trees.Thesespiders also spin a net of crisscrossed threads above thesheetweb. When a flying insect hits the net, it bounces into thesheetweb. Often, an insect will fly directly into the sheet web.Thespider, which hangs beneath the web, quickly runs to the insectandpulls it through the webbing. Sheet webs last a long timebecausethe spider repairs any damaged parts. Dwarfspiders, which are less than 1.3 millimetreslong, spinsmall, square sheet webs near rivers and lakes.

Whip or tailed spiders liveinSoutheast Asia and Australia. They have long, thin,tubelikebodies, 2 to 4 centimetres long. At night the whip spiderlets outa few long, thin strands of silk. It then waits foritsprey--mainly smaller spiders--to use these silklines as"guideropes." When the unsuspecting small spider climbs up such aropethe whip spider snares it by wrapping it in a broad bandofsilk.

Orb weavers build the mostbeautifuland complicated of all webs. They weave their round websin openareas, often between tree branches or flower stems. Threadsof drysilk extend from an orb web's centre like the spokes of awheel.Coiling lines of sticky silk connect the spokes, and serve asaninsect trap.

Some orb weavers lie in wait for their prey in the centre oftheweb. Others attach a signal thread to the centre of the web.Thespider hides in its nest near the web, and holds on to thesignalthread. When an insect lands in the web, the thread vibrates.Thespider darts out and captures the insect. Many orb weavers spinanew web every night. It takes them about an hour. Suchspidersoften eat their old webs to conserve silk. Other orb weaversrepairor replace any damaged parts of their webs.

Bolus or angling spiders are classed as orbweavers,but they catch flying insects by swinging a silk line witha stickyglobule at one end. Some bolus spiders are thought to giveoff ascent that attracts male moths. Anglingspiders have large, cream-coloured bodieswith pink, yellow,and brown markings. The magnificentspider of Australia is a type of bolusspider.

The life of a spider
Each species of spider lives a different life. Many kinds ofspiderslive for only about a year. Large wolf spiders live severalyearsand some female tarantulas have lived for up to 20 yearsincaptivity. Spiders become adults at different times of theyear.Some mature in the autumn, and then mate and die during thewinter.Others live through the winter, mate in the spring, andthendie.

Courtship and mating.
As soon as a male spider matures, it seeks a mate. The femalespidermay mistake the male for prey and eat him, but most malespidersperform courtship activities that identify themselves andattractthe females. The male of some species vibrates the threadsof thefemale's web. Some male hunting spiders wave their legs andbodiesin an unusual courtship dance. Male jumping spiders use thecolouredhairs on their legs to signal females. Male nursery webspiderspresent the female with a captured fly before mating.

Before mating, the male spider spins a silk platform called aspermweb. He deposits a drop of sperm from his abdomen ontotheplatform. Then he fills each of his pedipalpi with sperm. Heusesthe pedipalpi to transfer the sperm to females during mating.Aftermating, the female stores the sperm in her body. When she layshereggs, several weeks or even months later, the eggs arefertilizedby the sperm. Usually, the female does not eat the maleaftermating as is commonly believed. Females can continue to layeggsfor many months after mating because of the stored sperm.

Thenumberof eggs that a spider lays at one time varies with the sizeof theanimal. An average sized female lays about 100 eggs but someof thelargest spiders lay more than 2,000 eggs. In most species,themother spider encloses the eggs in a silken egg sac. The sacofeach species differs in size and shape. In many species, themotherdies soon after making the egg sac. In other species, shestayswith the eggs until they hatch. Some spiders hang the sac in aweb.Others attach the sac to leaves or plants. Still others carryitwith them. The female wolf spider attaches the sac toherspinnerets, and drags it behind her and then carriesthespiderlings on her back after they have hatched.

Spiderlings hatch inside the egg sac and remain there untilwarmweather arrives. If the eggs are laid in autumn, thespiderlingsstay quietly inside their egg sac until spring. Afterleaving theegg sac, the spiderlings immediately beginspinningdraglines.

Many spiderlings travel to other areas. To do this, aspiderlingclimbs to the top of a fence post or some other tallobject andtilts its spinnerets up into the air. The moving airpulls silkthreads out of the spinnerets. Then the wind catches thethreadsand carries the spiderling into the air. This unusual wayoftravelling is called ballooning. A spider may travel agreatdistance by ballooning. Sailors more than 300 kilometres fromlandhave seen ballooning spiders.

Spiderlings moult (shed their outer skin) several times whiletheyare growing. A new, larger skin replaces the skin that hasgrowntoo tight. Most kinds of spiders moult from five to ninetimesbefore they reach adulthood. Tarantulas moult more than20times.

Spiders have many enemies including snakes, frogs, toads,lizards,birds, fish, and other animals that also eat insects. Evensomeinsects eat spiders like the wasp which is one of thespider'sworst enemies.  One group of spiders called piratespiderseats nothing but other spiders.

Researched by Stacey (reference source "WorldBookEncyclopaedia")

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