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Spiders and the Cobwebs
of Myth about Them
Number 31 August^ 1,^1983
LayPeople often wonder why their tax dollars should support science that seems to have no relevance to everyday problems. Why, for example, should they give money to people to study spiders or scorpions? What makes arachnology, the study of these crea- tures, relevant? The importance of arachnology became apparent to me many years ago. As I lay on a mound during an Army maneuver in Texas, I felt a shooting pain move down my left arm toward my chest. I thought I was having a heart attack. It turned out to be a scorpion sting. Many years later, I was reminded of the relevance of arachnol- ogy while living on an asparagus farm with my son in New Jersey. A small child was bitten by a black widow spider in an outhouse. Fortunately, the child sur- vived. Apart from the relevance of such ob- scure subjects as arachnology, scientists have another problem. We seem to en- joy what we do too much. The average person can’t comprehend why anyone would enjoy working with all those “things” they^ fear^ most:^ spiders, corpses, diseases, etc. Scientists need to be reminded that our lay friends have not acquired the knowledge that over- comes common myth and superstition. Like snakes and wolves, spiders have re- ceived a bum rap. Perhaps this essay will indicate why. Few animals are more universally feared than spiders, Why people fear them has never been precisely ex- plained. The late Theodore H. Savory, a British author of textbooks about
spiders, could find “no obvious justifica- tion for so dkproportionate, so wide- spread, so illogical a horror, which may welf have been an obstacle to serious arachnology. ”1 Savory believed that the fear of spiders is complex and no single explanation can cover all cases. Like many other phobias, however, it can often be traced to early chfldhood. Per- haps a chdd, warned about poisonous and ferocious animals, transferred the fear to spiders. Or maybe the child was once frightened by a spider. As Savory notes, “Few creatures are more likely than a house spider to appear unexpect- edly and give a shock to a child.” Anthropologist Marvin Harris, Uni- versit y of Florida, Gainesville, agrees that the fear is learned in childhood. Since some spiders can be dangerous, he says, it makes “good cultural sense” to teach chddren to stay away from them. The fear need not be permanent. Hu- mans can learn to love spiders and let them walk on their arms without fear.z Edward O. Wilson, Harvard Univer- sity, Cambridge, Massachusetts, be- iieves the phobia is rooted in our genes. “Wh~le there is a strong predisposition to develop phobias against spiders, snakes, closed places and cliffs-the ancient perils of humankind—there is no predis- position whatever to form phobias against knives, electric sockets and au- tomobiles, which are far more danger- ous in modem society.”z Whatever the root of human fear of spiders, these creatures have held a cer- tain fascination as well. Spiders figure in ancient myths. In 8 AD, the Roman poet
Ovid told the story of Arachne, a peas- ant girl who challenged the goddess Athena to a weaving contest. The wom- en produced cloth of equal beauty, but the jealous Athena punished Arachne, changing her into a creature that spends its liie weaving silk: a spiders Spiders belong to the phylum Arthro- pods, which they share with insects and crustaceans.q Arachnida, the subdivi- sion or class of arthropods spiders belong to, is named after Arachne. It afso in- cludes worpions, mites, and ticks. LAe most of their fellow arachnids, spiders are air-breathing invertebrates. Their bodies have two ditilons, four pairs of legs, and no antennae.s Unlike other arachnids, all spiders spin silk, though not alf build webs.b Most spiders have eight eyes, though some species have fewer. Most spiders secrete venom, although only about 12 species are harmful to humans. Their main diet is insects. They eat by first in- jecting the victim with venom and treat- ing it with digestive enzymes that dis- solve its inner organs. Then they drink the resulting nutritive broth. Spiders hatch from eggs and change little as they mature, except in size. They usually molt (shed their chttinous outer skin) seven or eight times before maturing. Females are often larger than males, and sometimes devour males after mating.b There are about 35,000 known species of spiders.b Some scholars, including Norman 1. Platnick, American Museum of Natural History, New York, befieve that at least 100,000 species inhabh the Earth.7 During the course of their 400 million-year history, spiders have adapted to almost every climate and every ecological niche, Accord~ng to Paul A. Zahl, National Geographic Society, Washington, DC, they exist in great variety among the trees, bushes, and grasses of every forest in the world. Spiders are found from the tropics to the Arctic. And some species, though they breathe air, live underwater.a Spiders clearly have efficient survival mechanisms, since they are so numerous and widespread. Newly hatched spiders are as mobile and fierce as adults. Eight
legs allow them to step quickly over rug- ged temain. Chitinous exoskeletons make them unappetizing to many preda- tors. And venoms, in tiny amounts, par- alyze their insect prey. A spider caught by the leg can leave it behind and grow another at next molt .s Thomas Eisner and Scott Camazine, Cornell University, Ithaca, New York, recently made an amazing dkcovery. When a spider is stung in the leg by a venomous insect, such as a wasp or honeybee, it lets go of the leg within seconds, before the venom spreads through its body,g In addition to these adaptations, the spider’s use of sifk is one of its tools for survival. According to F. Lucas, Shirley Institute, a fiber-research organization in England, spider silk is stronger than nylon. It is also finer, lighter, and more tenacious than silk from the common silkworm, Bornbyx men”.lo All silks are made of a protein called fibroin. Spiders produce it in up to six sets of glands in their abdomens. They secrete it through valve-like openings, or spinnerets, which help regulate the flow. 1I Spiders’ silk is the basis for one of their most prominent survival tactics: the web. About half of all spider species build them, Peter N. Witt, an arachnolo- gist-pharrnacologist retired from the North Carolina Department of Human Resources, points out in a recent news story that webs are great energy savers for spiders. Web-builders do not have to roam in search of prey. They simply wait for insects to get caught in the web. Buildlng a web costs a spider the energy equivalent of two or three insects; on an average day, a spider may catch 30. And many spiders recycle their webs, They eat all or part of the silk at the end of the day and use its amino acids in their next web.lz According to Witt and colleagues, spiders build their webs according to in- structions encoded in their genes. They can build the day they hatch, though most produce their fwst web after two weeks, 13 W hlle each species spins a characteristic web, individuals vary it in minor ways, making each web almost as distinctive as a fingerprint. And even
shape of a flaring tube with a rim extend- ing into a sheet web. At the apex of the tube is the spider’s retreat, which maybe buned underground or below vegeta- tion. lb One type of funnel weaver, the European water (^) spider Argyoneta aquatica, constructs an airtight silk dome beneath the surface of slow-mov- ing waters. It carries bubbles of air to its web nest, where it lives, mates, lays its eggs, and raises its young. la One group of spiders builds the tr- iangle web which, unlike sheet and fun- nel webs, Lougee notes, is characterized by the careful positioning of each strand. The result is a strong, triangle-shaped silk net anchored with a long thread to a nearby plant, wall, or other structure. One species, Hyptiotes ca vatus, anchors the main thread to tree trunks and sits at the junction disguised as a bud. It holds the anchoring thread taut, and when prey strikes the web, lets slack out of the line, further entangling the victim. lb (See Figure 2.) About 2,500 species build the most beautiful and complex of webs, the orb
FIsure 2: The triangle web of ffyptio~er cavatus.
web. 19 The word orb comes from the Latin or-his, for ring, circle, or dkk.zo Savory and many naturalists before him have found that in orb webs, dry silk fila- ments radiate from a central point like spokes of a wheel, while sticky threads cross over them in a spiral that ends near the center.zl (See Figure 3.) The orb web of an adult can have up to 60 radii, 80 spiral turns, and more than 1,500 in- tersections, 19 Orb webs are so finely made that they are almost invisible, (^) except when drenched with rain or dew. But, Witt notes, they are strong, efficient prey- catching devices. They stop fast-flying insects and foif their attempts to escape. They support the weight of the spider, its mates, and sometimes several accumu- lated prey.zz Witt and his colleagues have also found that orb-weavers take about half an hour each day to build the web. Some work in the hours before dawn, others in the early evening. One species, Zygiella .r-nokzta, keeps a 60 degree slice of the web free of sticky spirals, where it leaves
(From: The Wodd ./ Spid.r$. Bridowe, WS, William Collins Sons & Co. Ltd.)
FfgrJre 3: Left: the orb web of Zygiel/a x-notata has a free sector with a signaf thread Ieadkg to the spider’s retreat, Right: the orb web of the cross spider A mneu$ diadematu$.
(From: Biology of Spiders. Foelix RF, Harward University Press.)
a single dry filament leadhg to its re- treat. 13(See Figure 3. ) Some use special combs on their feet to knit dry spirals in- to loops of woolly lace. 12 Some species weave white, conspicu- ous slashes, crosses, or ovals across their orb webs. Until recently, thk puzzled arachnologists. Webs, after all, are sup- posed to be invisible, to trick insects into flying into them. The mystery was solved in January 1983 by Eisner and Stephen Nowicki, Cornell. They reported that these markers, called stabilimenta, serve as detour signs for birds. Otherwise, birds would fly through a web and de- molish it. They found that webs with stabilimenta have a 60 percent chance of surviving untif noon. Webs without these markers have only an eight percent chance.~ The orb web is more than a trap. It compensates for the poor vision some spiders have. Diemut K~arner and Fned- rich G. Barth, Zoology Institute, Johann Wolfgang Goethe University, Frankfurt, Federal Republic of Germany, have found that when prey strikes an orb web, the spider mns to the hub and feels each
radial thread, orienting itself toward the quadrant sending the strongest nbra- tion. If the spider feels vibrations that are too strong, it may deem the captive too big or lively to handle. In this case, the spider cuts the threads surrounding the captive and lets it fall to the ground. It does the same if a twig or other in- animate object fails into the web. It can determine the size and position of a twig by plucking the threads and “reading” the reverberations. Of course, spiders also use these methods to find edible prey, which they bite and wrap in silk to eat or store.% Webs, besides helping individual spiders catch food, serve in some species as communal homes for up to several hundred thousand spiders. About 30 species build communal webs in tropical and subtropical climates, where prey is superabundant. J. Wesley Burgess, Uni- versity of California, Davis, found a web of the species Mallos gregalis near Guadalajara in Mexico that covered the upper three-quarters of a 60-foot mimo- sa tree. Most communal webs, however, are smaller. The spiders collaborate in
they sit in to wait for prey. The camou- flage prevents the spiders from being seen by btrds or other predators. When prey comes near, the spiders snap their legs shut on it and inject an extra-potent venom. These smalf spiders attack wasps and bumblebees that are large by com- parison.B Probably the most famous spider of all is the tarantula. In North America, what is commonly called the tarantula be- longs to the family Theraphosidae. It has a leg span of up to five inches. It lives longer than most spiders—sometimes up to 30 years—and takes nearly a decade to grow to full size. Its bhe is rarely worse than a wasp or bee sting, and it shies away from humans, usually biting only if provoked. Many species live in the dry, sun-baked regions of the Ameri- can Southwest and Mexico. South American tarantulas live in the temperate and tropical zones of that continent. Their bodies alone can be up to six inches long, and they can catch and eat smalf birds.J The European tarantula, about an inch long, belongs to the family Lycosi- dae and lives in the regions surrounding the Mediterranean. It is also known as the wolf spider. It has been studied by Zvonimir MaretiE and Drago Lebez, University of Ljubljana, Yugoslavia. They have found that the European tarantula’s bite is usually no worse than a bee sting. In sensitive persons, however, it can cause redness, swelling, and the eventual death of large patches of tissue surrounding the bite.% Mareti6 and Lebez found that in the fifteenth to seventeenth centuries, this species was believed responsible for epidemics of spider bites in towns throughout southern Europe. Bitten people ran through towns, alternately laughhg and crying, and performing unusual, sometimes vulgar, acts. The “tarantati,” as they came to be called, suffered pain, sweating, convulsions, paralysis, delirium, and death.% The cure was dancing. Victims danced to lively tunes, sometimes for days, until they fell to the ground, ex-
hausted. It was important to play the right tune for the victim. Otherwise, it was believed, the symptoms could wors- en. Alf that remains today of the taran- tati is the lively dance, the tarantella, and an old German saying, “bitten by tarantula,” meaning crazy.~ Since the time of the tarantati, the cul- prit has been found to be not the taran- tula, but the black widow spider, Latro- dectus.~ Black widows get their name from the habit of females of consuming males after mating. The ten or so species of black widow inhabit warm to moder- ate climates throughout the world. The venom of a mature female, drop for drop, is 14 times as potent as rattlesnake venom. Venom from males is weak by comparison, and poses no serious danger to humans.~,J Herbert E. Longenecker and col- leagues, Rockefeller University, New York, found that female black widow venom makes the body’s nerve endings release their supply of neurotransmit- ters, the chemicals that carry messages across the spaces between nerves.~ Resulting symptoms are those exhibited by the tarantati. Mareti& and Lebez say that intense pain is the most prominent feature. In- terestingly enough, the pain is somewhat alleviated by physical exertion. A 23-year-old farmer, bitten in a field in Yugoslavia, bicycled nearly 12 and a half miles over bad roads in severe heat to the nearest hospital. He reported that the pain lessened when he cycled, but whenever he stopped to rest, it flared up quickly. Three hours after he was bitten, he was diagnosed as in good condhion. In another case, a victim who was inac- tive could not even stand up three hours after the bite.~ Bite victims usually have a flushed face; swollen, tearful eyes; and a look of apprehension and pain.ss Though wri- OUS, bites are rarely fatal, Zahl reports. Hospitals use antivenoms to treat vic- tims. Only four or five of the 1, Americans bhten each year dle. MaretiE and Lebez list four species of black widow as common to the Ameri-
cas. The most venomous and wide- spread is L. mactuns, which has a black body with a red hourglass shape on the underside of its abdomen. It hangs up- side down in silk nests in low bushes and grass, and can be found in trash heaps and outhouses. Its relative, L. variolus, looks similar, but builds its webs in trees. It lives mostly in the western areas of North America. Black with a gray or brown abdomen, L. geometn”cus lives in tropical cities. And a native to the scrub pine of Florida, L. bishopi, has an orange-red body and legs.s’f The brown recluse spider, Loxosce[es reclusa, is another spider native to North America whose bite is harmful to hu- mans. In a letter to Science in 1957, J.A. Atkins, C.W. Wingo, and W.A. Sode- man, University of Missouri, Columbia, first described the symptoms of the Lox- osceles bite: the skin surrounding the bite becomes painful, raised, and scabrous; turns black and dry; and even- tually sloughs off.sT A scar remains that takes several weeks to heaLsG South American species cause larger and more painful lesions than other vaneties.ss Ironically, the spider survival tactic that people fear most may provide a way to enhance human life. Black widow venom is being used in laboratory ani- mals to study the effects of botulism and progressive muscular dystrophy. One characteristic of both afflictions is that nerve endings are prevented from releas- ing neurotransmitters, resulting in paral- ysis. This is exactly the opposite effect of black widow venom, which forces nerves to release neurotransmitters. P. Stem and K. Valjevac, University of Sarajevo, Yugoslavia, and S. Gomez, National Hospital, Institute of Neurol- ogy, London, England, have found that rats with botulism live longer when treated with extracts from black widow spider glands. If they survive, they recover more quickly with the treat- ment.38, Rats with progressive muscufar dys- trophy, Stem and Valjevac found, retain muscle control longer and recover much more quickly when treated with gland
extracts.~ While the side effects of the venom must be treated along with the original ailments, these encouraging re- sults may soon be surpassed as methods of collecting and purifying black widow venom improve .38-@ Although some spiders are harmful, they are greatly outnumbered by the thousands of species that, directfy or in- directly, benefit mankind. Spiders are increasingly recognized as a valuable natural means of controlling caterpil- lars, aphids, mites, and grasshoppers in apple and peach orchards; in soybean, cotton, and alfalfa fields; and in avocado and citrus groves.qt M. Nyffeler and G. Benz, Swiss Federal Institute of Technology, Zurich, report that spiders have been introduced into rice fields in the Peopfe’s Republic of China and other Asian countries to help control the more than 800 species of rice pests,qz And around the world, spiders quietly and consistently keep insect populations in balance—annually destroying 100 times their number in insects. 17 There are about 800 arachnologists in the world, according to Jonathan Reis- kind, Department of Zoology, Univer- sity of Florida, Gainesville, Florida 32601, president of the American Arachnological Association. His address is also the present address of the associa- tion, which publishes the Journal of Arachno[ogy.qs Reiskind’s figure includes amateur arachnologists, who contribute a great deal to the field, but excludes acarolo- gists, who study mites and ticks.qo The number of professional arachnologists worldwide “is probably 300 to 400 at most, ” says Herbert W. Levi, Harvard University Museum of Comparative Zo- ology.~ Furthermore, many arachnolo- gists combine their study with other fields, such as biochemistry, ecology, or physiology,’ls Arachnologists are sometimes ento- mologists as well, or are closely associat- ed with them. Many belong to entomo- logical societies such as the Entomologi- cal Society of America, 4603 Calvert Road, College Park, Maryland 20740,
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