Describing earthquakes, and how they happen, without the use of illustrations is quite challenging. I must ask you to engage your imagination when reading further.

An earthquake is the result of abrupt movement along a break in the earth’s crust. Such a definition assumes that one possesses a basic understanding of what the earth’s crust is, as well as why and how it moves. People studying this phenomenon are referred to as seismologists.

The earth’s crust can be visualized as several large islands adrift on a very thick, semi-solid sphere: like pieces of a puzzle moving around on a ball of wax; or like the patchwork panels on a soccer ball. The crust of the earth is actually the outermost brittle layer of the uppermost portion of the earth’s mantle, known by geologists as the lithosphere. The individual pieces of the lithosphere are referred to as tectonic plates. Movement of these plates is powered by the circulation of semi-solid, molten material beneath the crust, heated from intense pressure and radioactive decay. Heated materials rise from beneath the earth’s crust, replaced by cooler, denser materials. An analogy by which this process may be visualized is the circulation of wax when heated in a transparent vat. Wax that has been expanded through heat will rise, while wax further from the heat source is cooler and denser, causing it to sink in relation to wax with lower density and higher temperatures – the hotter wax. In this way, heat supplies the energy necessary to move pieces of the earth’s crust. The major plates comprising the lithosphere include:

• African

• Antarctic

• Arabian

• Australian

• Caribbean

• Cocos

• Eurasian

• Indian

• Juan de Fuca

• Nazca

• North American

• Pacific

• Philippine

• Scotia

• Somali

• South American

In general, the composition of the earth’s crust can be discussed in terms of two types of rock: heavy iron-magnesium rock forming the oceanic crust and lighter aluminum-silicate rock comprising the continental crust. Referring back to the image of a soccer ball as representative of the earth’s crust, the black panels would represent the oceanic crust and the white panels the continental crust. The oceanic crust is denser and thinner than the continental crust that dominates dry land. The thickness of the oceanic crust averages about 4 miles, whereas the continental crust averages about 20 miles. Because the continental crust is only 80% as dense as the oceanic crust, continental plates override oceanic plates when collisions occur between these two distinct rock types. Thus, the oceanic crust is thrust beneath the continental crust in a process referred to as subduction. One notable exception to this general rule can be observed in the Olympic Mountains in Washington State. A hitch in the oceanic plate has produced buckling and folding of the crust during subduction so severe that a mountain range reaching an elevation in excess of 8,000 feet has formed. Places along plate boundaries where subduction occurs are called subduction zones, and can be identified by the volcanic activity that takes place within a hundred miles or so from where the plates collide.

The process of subduction contributes to the recycling of rock. Sedimentary, metamorphic, and igneous rock is thrust below the surface of the earth and melted. The magma later resurfaces as extrusive molten lava or cools beneath the surface of the earth to form intrusive solid rock. The recycling period for oceanic crust is much quicker, as it usually is forced into the subduction zone by the older, overriding continental crust. Consequently, there is no known oceanic crust older than about 200 million years, whereas continental crust exposed in places such as Death Valley is estimated to be over 2 billion years old.

Off the coast of the Pacific Northwest, the small Juan de Fuca plate is being subducted beneath the massive North American plate. Evidence of this subduction includes the eruption of Mount Saint Helens on 18 May 1980. Highly explosive volcanoes, such as the string of volcanoes lining the Pacific Rim, expel a mixture of molten rock, gas, and water vapor generated by the heat and pressure acting on the subducting oceanic plate. As the plate melts, the volatile mixture of material expands and is forced to the surface. Pressure beneath the crust builds, eventually reaching a point where energy is released in one major event: an eruption forming a composite volcano, which is formed by lava flows and accumulation of ejected debris. Volcanoes found in the Hawaiian Islands are less violent, characterized by a steady release of energy in the form of lava flows. Volcanoes of this type are the largest in the world (the island of Hawaii is one such example) and are referred to as shield volcanoes. Typically, shield volcanoes are formed when a plate passes over a mantle plume in the earth’s upper mantle. The Hawaiian Islands are evidence of one such "hotspot" in the lithosphere: not only indicating the presence of a mantle plume, but also depicting the direction in which the plate moving.

Just as some pieces of the earth’s crust collide, others are drifting apart. Areas where the earth’s crust is diverging are known as spreading centers, where sea floor spreading occurs when two plates move in opposite directions. Spreading centers are associated with rifts in the oceanic crust where molten magma boils to the surface, creating new crust that is forced away from each side of the spreading center – like two conveyer belts headed in opposite directions.

Movement of the earth’s crust, however, is imperceptibly slow. Different plates move at different speeds: none move more than a few inches annually. It is estimated that in 18 million years the land currently occupied by Los Angeles and San Francisco will be adjacent: this the result of the Pacific plate (and Los Angeles) drifting northwest, while the North American plate (and San Francisco) moves southwest. The most accurate technology for measuring movement of the earth’s surface is interferometry. This procedure involves the use of radar signals emitted from a satellite positioned about 500 miles overhead to create a landscape map. Maps of today’s landscape are compared against prior maps to determine where and how much the earth has moved.

Because plates move at different speeds, they drift apart and collide, forming a variety of geological landscapes. Moreover, plate movements are not limited to sliding past or running into one another. Plates may also rise and sink in relation to each other. One such movement is termed isostasy. Isostasy describes the movement that occurs when the crust becomes lighter or heavier as the result of material being removed from or added to the surface of the earth. Erosion, transportation, and deposition of large quantities of sediment over a long period of time by a very strong force – as in the case of the Mississippi River – displaces a tremendous load from one part of the earth’s crust to another. As the thick surface of continental crust erodes away – and becomes lighter as a result – that part of the earth’s crust ‘floats’ toward the surface. The earth’s crust becomes heavier and ‘sinks’ where material is deposited. Erosion, transportation, and deposition occurring in the Sierra Nevada Mountain Range contribute to the rise of the Sierra Nevada and the sinking of the adjacent Owens Valley.

There is another process at work in the Sierra Nevada Mountain Range that speaks directly to the topic of this article: faulting. It is along breaks or faults in the earth’s crust where earthquakes occur. To understand how earthquakes are responsible for releases of energy, it is necessary to first understand the various kinds of faults involved, as well as the movement associated with each kind of fault. There are four general types of faults. For the purpose of describing the movement of each type of fault, imagine that you are holding two blocks of wood, one in each hand.

Normal Fault: the block in one hand pulls away from the block in the other hand, with each block moving vertically in relation to the other, breaking the surface of the earth at some position along the fault line. In the Sierra Nevada Mountains, the Owens Valley Fault is the result of the pulling away and uplifting of the mountains in relation to the adjacent valley. The famous Lone Pine earthquake of 1872, which killed nearly the entire population of Lone Pine, occurred along a normal fault system.

Reverse Fault: the block in one hand moves toward the block in the other hand, with each block moving vertically over the other, breaking the surface of the earth at some position along the fault line. Although occurring on a slip-strike fault system, the Loma Prieta earthquake of 1989 demonstrated the vertical displacement characteristic of a reverse fault, coupled with horizontal motion associated with a slip-strike fault system.

Blind Thrust Fault: the block in one hand moves toward the block in the other hand, with each block moving vertically in relation to the other, without breaking the surface of the earth at some position along the fault line. It was a blind thrust fault earthquake that struck Northridge in 1994.

Slip-Strike Fault: the block in either hand slides horizontally past the block in the other hand, breaking the surface of the earth at some position along the fault line. The Hector Mine earthquake of October, 1999 was the product of energy being released on a slip-strike fault system. The San Andreas Fault system is a notorious example of this type of fault. Consequently, the notion that California will "drop" into the ocean is, thankfully, false.

It is implied by the original definition of an earthquake given earlier – an earthquake is a release of energy caused by an abrupt movement along a fault – that not all pieces of the earth’s crust move relative to each other without incident. Rather, it is the resistance of opposing plates along their boundaries that creates an enormous potential for releasing energy – stored energy that is often released during a single event: an earthquake. If one considers that faults can represent lengthy sections of plate boundaries –the San Andreas Fault, for example, is over 1,000 miles long – it is easy to understand how opposing segments of crust may grind past each other. There are many places where the plates can get hung up. One such place is marked by a bend in the San Andreas Fault near Los Angeles, called the "big bend." This one hundred mile northwesterly bend apparent along the boundaries of the North American and Pacific plates has caused buckling in the earth’s crust, resulting in the uplift of mountains east of the Los Angeles basin. The San Gabriel Mountains and the San Bernardino Mountains are among the fastest growing in the world.

When an earthquake occurs, a sudden release of energy is emitted in all directions in the form of waves from the point of initial seismic activity, called the focus. The focus is generally situated below the surface of the earth. The epicenter of an earthquake is the location on the surface of the earth directly above the focus. Earthquakes typically emit three kinds of seismic waves: P-waves, or primary waves; S-waves, the secondary waves; and surface waves. P-waves cause particles to vibrate back and forth along the direction that the wave is traveling. S-waves cause particles to move perpendicular to wave direction. Surface waves, as the name suggests, travel along the surface of the earth generating horizontal and vertical vibrations, like the movement of a rope that is whipped. Generally, it is the surface waves – like ocean waves – that are responsible for the most of the damage resulting from an earthquake, since surface waves produce more ground movement and, as a consequence, are slow to pass. Surface waves can be extremely disastrous in areas where the soil in not consolidated, such as an evaporated lake basin or a river floodplain. Areas of loose, unconsolidated soils may actually take on the form of a passing surface wave, causing extensive damage to any structure on the surface. This condition is referred to as liquefaction.

Any location where there exists the possibility for displacement of the earth’s crust is a potential earthquake area. Higher risk areas are along known tectonic plate boundaries. However, there are stress points located in mid-continent locales that are capable of producing earthquakes as damaging as those associated with plate boundary activity. The New Madrid Fault zone near Memphis, Tennessee has produced some of the most powerful earthquakes described in North America. Three earthquakes generated along this zone in 1811 and 1812 were estimated to have been more powerful than the great earthquake of 1906 in San Francisco, California. In fact, church bells rang in Philadelphia as a result of these quakes. In the region of the New Madrid Fault zone, seismologists believe that tensional forces have released energy from a failed rift – an area where the earth began to break apart, but ceased to spread – deep in the earth’s crust. Adjustments within the crust can produce very powerful and deadly earthquakes.

Predicting earthquakes is one of several important goals being pursued by seismologists. Earthquake prediction is usually defined as the specification of the time, location, and magnitude of a future earthquake within stated limits. At this point, seismologists are searching for any signal that could suggest a pattern in the development of earthquakes. Conventional thinking about earthquakes is that a region remains quiet for years following a significant quake, while stress slowly accumulates underground. Over time, the number of small and moderate tremors increases, building up to the next big earthquake. This theory seems to apply in northern California. However, the quakes of southern California do not follow this pattern. Along the southern region of the San Andreas Fault, the magnitude of earthquakes remains somewhat constant.

Seismologists are also reviewing tidal data to make predictions about earthquakes. From timing more than 13,000 small to moderate quakes on California's San Andreas Fault system, seismologists from the University of California at Los Angeles have shown a correlation between tectonic activity and the lunar and solar cycles. The gravitational pull of the moon and sun, evident in ocean tides, influences tectonic stresses. As tides wax and wane, they alternately increase and relieve stress along faults. According to this scheme of earthquake prediction, seismic events should be more common when the tidal pull is strongest. To date, data has not supported this theory.

Another environmental signal that seismologists are investigating is the potential link between earthquakes and weather. It was observed that seismic activity during a five year period established a pattern in California, whereby predominantly tiny quakes occurred most frequently in September and least often in April. After examining many potential causes of the cycle, seismologists hypothesized that atmospheric pressure may be the controlling factor. When atmospheric pressure on the earth’s surface remains low, as it does during the warm summer months (the average atmospheric pressure is greater during cooler months), it lessens the weight of the atmosphere pressing on the ground. This situation reduces the friction on rocks, allowing earthquakes to occur more easily.

A natural phenomenon that has recently been evaluated for potential in earthquake prediction is upper atmospheric disturbance. The ionoshpere is that part of the atmosphere beginning approximately 40 miles from the surface of the earth, consisting of high concentrations of ions and free electrons. Through utilization of a network of satellites – Global Positioning System (GPS) – data can be obtained depicting the electron density of the atmosphere at a given altitude. This is accomplished by sending radio signals to ground-based receivers. Displacement of the ground as a result of an earthquake generates waves that cause fluctuations in ionosphere electron density. This research is aimed at discovering patterns in atmospheric activity that precede large-scale ground displacement.

The notion of preventing earthquakes seems absurd at a first glance. What is implied by this idea is not preventing the occurrence of an earthquake, but reducing the magnitude – and the resulting destruction – of a seismic event. Like many great scientific discoveries, controlling the effects of earthquakes was discovered by accident: water forced underground during an oil recovery procedure caused several small tremors. From this event, geophysicists hypothesized that the pressure of water forced underground may gradually relieve the strain accumulating in the rocks below. Furthermore, water pumped beneath the surface could potentially act as a lubricant, allowing rock surfaces to slide by each other. In essence, hydro-pressure techniques could be deployed to trigger smaller, more frequent tremors along high-risk fault zones, thus reducing the likelihood of a large earthquake.

Initially, the risks associated with testing methods for managing seismic activity would be substantial. The impact of triggering an inaugural tremor along the San Andreas Fault – or any fault associated with the San Andreas Fault system, for that matter – must be seriously considered. It is likely that only a series of controlled experiments along the entire length of a particular fault zone would lead to predictable results with respect to that area. The challenge for scientists working to control seismic activity is to achieve a level of predictability without a disastrous learning curve. The complexity of variables influencing seismic activity, and the enormity of potential energy under consideration, render the consequences of miscalculating the results of inducing earthquakes a responsibility not likely to be assumed by any public or private agency in the near future.

1) Tangshan, China; 1976; 7.4 magnitude; 655,000 deaths

2) Gansu and Shaanxi, China; 1920; 8.3 magnitude; 200,000 deaths

3) Qinhai, China; 1927; 7.7 magnitude; 200,000 deaths

4) Tokyo, Japan; 1923; 7.9 magnitude; 142,810 deaths

5) Messina, Italy; 1908; 7.0 magnitude; 110,000 deaths

6) Northern Peru; 1970; 7.9 magnitude; 67,000 deaths

7) Western Iran; 1990; 7.5 magnitude; 50,000 deaths

8) Erzincan, Turkey; 1939; 7.6 magnitude; 32,700 deaths

9) Quetta, Pakistan; 1935; 8.1 magnitude; 30,000 deaths

10) Armenia; 1988; 6.8 magnitude; 25,000 deaths

Format: Author; Title; Publisher; Date Published; Edition; ISBN; LOC; Category; Index Number

Allaby, Michael & Ailsa, Editor; Dictionary of Earth Sciences; Oxford University Press; 1991; 1st; 0-19-286125-5; ; Reference; Geology; Climatology; B; 20

Alt, D./Hyndman, D. W.; Roadside Geology of Northern California; Mountain Press Publishing; 1975; 1st; 0-87842-055-X; ; Geology; ; ; B; 60

Ballard, Robert, D.; Exploring Our Living Planet; National Geographic Book Service, Washington, D.C.; 1983; 1^st; ; ;Geology

Brown, Kenneth A.; Cycles of Rock and Water; HarperCollins, New York; 1994; 1st; 0-06-258533-9; QE71.B76 1994; Geology; ; ; B; 484

Busch, Richard M., Editor; Laboratory Manual In Physical Geology; Macmillan Publishing Company; 1986; 3rd; 0-02-301071-1; ; Geology; ; ; B; 56

California Geology; ; Department of Conservation, DMG; ; Periodical; Subscription; ; Geology; ; ; B; 48

Chronic, Halka ; Roadside Geology of Arizona; Mountain Press Publishing; 1983; 1st; 0-87842-147-5; ; Geology; ; ; B; 59

Chronic, Halka ; Pages of Stone: II; The Mountaineers; 1986; 1st; 0-89886-114-4; ; Geology; ; ; B; 62

Conner, Cathy/O'Haire, Daniel ; Roadside Geology of Alaska; Mountain Press Publishing; 1988; 1st; 0-87842-213-7; ; Geology; ; ; B; 61

Fleisher, M./Mandarino, J. A. ; Glossary of Mineral Species; The Mineralogical Record Inc.; 1995; 7th; ; ; Geology; ; ; B; 70

Gore, Rick; Living With California’s Faults; National Geographic, Vol. 187, #4; April, 1995; ; ; Geology

Hill, Mary ; Geology of the Sierra Nevada; University of California Press; 1975; 1st; 0-520-02698-5; ; Geology; ; ; B; 66

Hill, Russell B. ; California Mountain Ranges: I; Falcon Press; 1986; 1st; 0-934318-77-8; ; Geology; ; ; B; 63

Kennedy, M. P./Peterson, G. L. ; Geology of the San Diego Metro Area; CA Division of Mines & Geology; 1975; Bulletin 200; ; ; Geology; ; ; B; 105

Key/Pesavento/Trujillo; Earth Science 100; W.C. Brown, Dubuque, Iowa; 1995; 1st; 0-697-28333-X; ; Geography; Geology; Climatology; B; 512

Lacock, Janice, Editor; Rocks & Minerals; Dorling Kindersley; 1988; 1st; 0-394-89621-1; ; Geology; Mineralogy; ; B; 72

Laudan, Rachel; From Mineralogy To Geology; University of Chicago, Chicago; 1987; 1st; 0-226-46947-6; ; Geology; Mineralogy; ; B; 409

McKnight, Tom L.; Physical Geography: a landscape appreciation; Prentice Hall, New Jersey; 1996; 5th; 0-13-440215-4; GB54.5.M39 1996; Geography; Geology; Climatology; B; 513

Plummer/McGeary; Physical Geology; William C. Brown Publishers; 1996; 7th; 0-697-26676-1; ; Geology; ; ; B; 55

Remeika, Paul; Geology of Anza-Borrego: Edge of Creation; Kendall-Hunt, Dubuque, Iowa; 1992; 1st; 0-932653-17-0; ; Geology; ; ; B; 410

Rhodes, Frank H. T. ; Geology; Golden Press; 1972; 1991; 0-307-24349-4; ; Geology; ; ; B; 68

Schumann, Walter ; Rocks, Minerals & Gemstones; Houghton Mifflin Company; 1993; 1st; 0-395-51137-2; ; Geology; Mineralogy; ; B; 71

Spear, Clifford, Bergen; Geology of San Diego County; Sunbelt Publications, Inc., San Diego; 1997; 1st; 0-932653-21-9; ; Geology; ; ; B; 421

Spear, S./Pesavento, J. P. ; Physical Geology Study Guide; Paladin House; 1977; 2nd; 0-88252-173-X; ; Geology; ; ; B; 65

Storer, T. I./Usinger, R. L.; Sierra Nevada Natural History; University of California Press; 1963; 1st; 0-520-01227-5; ; Ecology; Geology; ; B; 85

Pough, Frederick H. ; Rocks & Minerals; Houghton Mifflin Company; 1953; 4th; 0-395-24049-2; ; Geology; Mineralogy; ; B; 115

Whitten, D. G. A. ; Dictionary of Geology; Penguin Books; 1972; 1st; 0-14-051049-4; ; Geology; ; ; B; 67

THE BLACK-WIDOW SPIDER

I remember being about 4 years old when I had my first encounter with a Black-widow spider. Living in the Central Valley of California, most of the older homes were situated on raised foundations. A person could access beneath the house from outside through a "crawl space."

Our house had at least two crawl spaces. The crawl space at the back of the house was located near a hose bib. The entry for access below the house was deeper on the outside perimeter of the foundation than beneath the house by several inches. Surrounding the entry area was a short stemwall in the shape of a rectangle, maybe 30 inches wide by 36 inches long, filled with sand. A screen prevented access beneath the house by animals – and small children. This little area was very inviting to me as a small child. I thought of it as a sandbox.

One summer afternoon, I filled the "sandbox" with water. I sat in the large puddle of water and began splashing and playing when I noticed a large, black spider emerge from the corner of the entry toward the house. It was floating on the surface of the water within inches of my right elbow. I had flooded the spider from its habitat.

I remember being terrified. I scrambled out of the box and sought out my mother and father. I was told that the spider was probably a Black-widow spider, based on my description. I was further instructed not to touch or play with this type of spider, as they were capable of killing a young child. My respect for the Black-widow spider was born at that moment.

My respect for the Black-widow spider continued to grow, as there were ample opportunities for encounters with this conspicuous creature in the dry valley heat. I remember being only slightly older when I nearly put my hand on a large Black-widow while reaching for the garage light switch one summer morning. I walked away from the episode with a tremendous adrenaline rush, and knowing the feel of the spider’s unusually strong web.

Throughout my childhood I feared two animals: bears and Black-widow spiders. Bears were only a part-time fear, as our family only spent part of the summers in the mountains. Moreover, bears were easy to spot and usually frequented the cabin at night when I was in bed on the second floor of our cabin. Black-widow spiders, on the other hand, were stealthy, reclusive, and not particularly easy to spot for a hyperactive, mischievous child. It was not until early adulthood that my fear of the Black-widow spider began to arouse my curiosity. I started to look closer at this notorious creature at each encounter: the conspicuous markings; the long front and back legs; the taunt web; and the telltale excrement. Another interesting thing that I noticed was the difference in size, termed sexual dimorphism, between the carnage of a male Black-widow spider, found on the fringe of the female’s web, and the female of the species. I had many questions about this unnerving animal.

Eventually, I decided to capture and observe a female that appeared ready to produce an egg sac. With great caution I captured this little creature that had been an object of fear for my entire life. The first glass jar that I used to contain the spider did not offer good viewing. It was an old mayonnaise jar. So I relocated the spider into a small fishbowl, which offered a view from every angle except the top. I covered the top with aluminum wrap and had punctured the foil with several small breathing holes.

Initially, the spider did not move often. It seemed to be in a state of torpor. This made viewing the anatomy of the spider much easier than trying to study it on the move. The abdomen, or opisthosoma, was striking: disproportionately large with the distinctive red hour glass design on the ventral side of the abdomen left no doubt what spider I was looking over. I had never before noticed that all of the spider’s appendages were attached to the front portion of the spider, called the prosoma. In fact, there are six pairs of appendages attached to the prosoma, which resembles a sandwich. The four pairs of legs, one pair of pedipalps, and one set of chelicera are attached to the soft middle section of the prosoma, called the pleurae. The bottom part of the prosoma is called the sternum, which consists of a hardened cuticle layer. Capping the prosoma, also formed from cuticle material, is the carapace. Truly fascinating were the four pairs of eyes, mounted on the front of the carapace like a set of dune buggy lights.

Have you ever wondered how spiders eat? It is quite disgusting by most people’s standards. The first set of appendages, the chelicera, function primarily as tools for acquiring food. The chelicerae consist of an arm and a fang, constructed much like a hypodermic needle, situated at the end of the arm. Running through the fang is a tube originating from a poison gland, through which a toxin is delivered to the victim of the spider’s bite. Depending on the species of spider, the prey may be subdued with a poisonous bite or speared with the fang and subsequently masticated.

Attached to the chelicerae are the maxillae, which have tiny serrations for grinding, shredding, and filtering food. Spiders ingest food as a liquid. Upon seizing control of its prey, the spider begins to feed by vomiting – yes, vomiting – digestive enzymes that liquefy the soft body parts of the food source. Thus, the digestive process for spiders originates outside of the body.

The liquid meal is then sucked through a flattened esophagus into the intestinal tract by the suction force generated by the stomach. The stomach functions in a similar manner to an accordion: when muscles attached to the side of the spider’s pleurae contract, muscles attached to the carapace and sternum expand. Alternating contraction and expansion of perpendicular musculature creates a vacuum in the stomach, drawing the liquid meal into and through the intestinal tract. By now you should have a clear picture of what happens to the insects snared by the spider’s web.

8

After a day or so of capturing the spider, I obtained a few crickets from the reptile store for the purpose of feeding my captive. I dropped one particularly large cricket into the container expecting an instant battle between the giant insect and the smaller arachnid. My anticipation was met with disappointment. The Black-widow spider deliberately moved away from the cricket, climbing to the top of the jar. The cricket seemed content to hang out on the bottom of the jar. I sat waiting patiently for an hour or so, eventually arriving at the conclusion that either my spider was not hungry or that the cricket was just too big and intimidating for the spider. I was mistaken.

Very slowly, the Black-widow spider began to stalk the cricket. After several minutes, the spider turned away from the cricket and began backing toward it, very slowly. The spider then made its move: using its back pair of legs, it began ‘throwing’ silk strands onto the cricket’s legs. After a few short flurries of ‘silk throwing,’ the cricket’s ability to hop had been sufficiently hindered so that the spider could turn and quickly inflict a bite on the back leg of the cricket. Within seconds, the cricket began to convulse and the spider retreated. During the next few minutes, the convulsions became less violent, until the cricket appeared to finally succumb to the spider’s toxin. Once all movement stopped, the spider walked over to the cricket and began wrapping it in a cocoon of silk so thick that the species of the insect was barely distinguishable. For the finishing touch, the spider hung the cricket from the top of the jar, reminding me of the Ivanhoe Meat Locker my father took me into as a child: the way in which all of the cattle and hogs were stored dangling from meat hooks.

It wasn’t until several minutes later that the spider began to feed on its victim. The cricket, hanging upside down, was approached by the spider from the front, or the bottom in this case, which began vomiting – yes, vomiting – on the area behind the head of the cricket. A foamy liquid soon appeared. The spider’s body gently rocked as it was feeding, almost as if enjoying some strange form of sex. Within the next 36 hours the cricket had been sucked dry, with only the chitinous body remaining as evidence for its brief life – unless, of course, one considers the engorged abdomen of the Black-widow as additional evidence.

Are you wondering how spiders digest and defecate their liquid diets? This won’t take long. You will be surprised to find out that spiders retain energy from their meals for long periods of time. In an experiment done with Black-widow spiders, it was observed that they were able to live to 200 days without feeding (Foelix, 1996; p. 47). As mentioned earlier, spider digestion is initiated outside of the body. Liquid material ingested by spiders is sucked into the digestive tract by the vacuum action of the stomach. The accordion effect generated by the suspended musculature of the stomach chamber draws the food from outside the body, into the esophagus, through the stomach, and into the midgut. During the digestive process, unacceptable materials are trapped by fine hairs lining the digestive tract. Filtered materials dry out and are then ejected through the mouth.

Nutrients are absorbed in the midgut. Excess nutrients are stored in the hypodermis, the cellular layer that secretes the exoskeleton, in the form of crystals. Waste products pass to the stercoral pocket, where they are periodically discharge through the anus. The excrement from the spider I was observing was a white, pasty material similar to bird feces. Moreover, I could smell the gas produced by the highly concentrated excrement. Features of the Black-widow spider excrement have become useful clues for me in locating the whereabouts of the spider from a safe distance. I can easily smell a spider that has been feeding in a confined area, such as the corner of a room. More often, I recognize the white uric acid droppings underneath furniture as the current or past location below a Black-widow spider.

I have always been curious about the toxicity of the Black-widow’s venom. The literature explains that the venom is a neuromuscular toxin, composed primarily of proteins: actually, seven proteins and three non-protein components. Considering equal quantities of venom, the Black-widow spider is 15 times more toxic to humans than the venom of a rattlesnake. However, the rattlesnake, because of its much greater size, can deliver up to 900 times the amount of venom than the Black-widow spider: 35mg compared to 300mg (Cornett, 1994; p. 9).

It was interesting to note that the toxicity of Black-widow venom varies with the time of year. According to James Cornett, in his excellent handbook on Black-widow spiders (q.v.), the venom can be up to 10 times more toxic in the fall than at other times during the year.

Another question for which I wanted an answer was, "How many fatalities result from Black-widow spider bites?" Cornett gives some idea of what this number might be (p. 9):

"In the United States there were approximately 1,300 Black-widow spider bites reported between 1926 and 1943. Fifty-five of the people bitten died, a mortality rate of four percent. From 1960 to 1969 only four people died from the bite of the Black-widow. Both of these statistics may be misleading, however, since many Black-widow bites go unreported, resulting in a lower mortality rate statistic. With modern medicines and medical technology, the percentage in 1994 is probably lower still."

Cornett further discloses that many of the bites early in the century were received on human genitalia while perched on outhouse seats.

When death does occur as the result of a bite from the Black-widow, it is from suffocation, the consequence of paralysis of respiratory muscles. The bite itself may not be felt. However, within 90 minutes or so, the painful symptoms begin to appear and progress as described below (Hare, 1995: p. 10):

Muscle pain and respiratory dysfunction;

Headache, dizziness, and restlessness;

Abdominal pain and muscular rigidity;

Thoracic breathing;

Delirium and cold sweats;

Convulsions, nausea, and other flu-like symptoms.

Symptoms generally become more severe during the initial 24 hours from the bite, then diminish over the next 72 hours. In extreme cases, aside from death, some symptoms may persist for several weeks. Multiple bites from the same spider can greatly enhance complications. An antivenin does exist for Black-widow venom. If administered early on, the antivenin may cause the symptoms to subside in less than 6 hours from the bite. Some people, however, are extremely allergic to the antivenin. In the absence of antivenin, pain can be relieved with injections of calcium gluconate.

The Black-widow spider is not particularly active once it has established a hunting territory. It hangs, usually upside down, in its web, which is the tool it uses to capture its prey. It is generally accepted among spider biologists that the spider’s web is indicative of its evolutionary origin. If this assumption is accurate, the most highly evolved spiders are those that spin "orb" webs, such as the common garden spider. The web of a common garden spider has a definite structure: a frame and the radial lines, which are constructed first; the catching spiral; and the free zone and the hub, situated in the center of the web. Orb-weavers can construct a web in less than 30 minutes, using about 60 feet of silk. A typical web will have between 25 to 30 radial lines, radiating from the hub at an angle of 12 to 15 degrees (Foelix; pp. 128-138).

The Black-widow spider belongs to the group of spiders referred to as "cobweb weavers." Cobwebs lack the geometric uniformity and repetition that characterizes the web of the "orb weavers." Cobweb-spinning spiders are considered to be of a much earlier origin than the orb-weaving spiders. The cobwebs characteristic of the Theridiidae spider family, of which the Black-widow is a member, can be a relatively large, 3-dimensional expanse of unusually strong silk, often located near the ground. The feel of a Black-widow cobweb is not easily confused with that of any other spider.

Have you thought about how spiders reproduce? It is not a story of undying intimacy. Both spiders possess a ventral opening in the abdomen, referred to as the epigastric furrow. Sensing the presence of a female, male spiders typically exude sperm through the epigastric furrow onto a sperm web. The sperm is dabbed up by the donor spider using a pedipalp. The end of the male pedipalp is equipped with an organ, called a bulb, that draws up and stores the sperm, not unlike like the turkey baster that my wife uses at Thanksgiving. When the male spider senses that a female is receptive to his presence, usually by means of a species-specific courtship ritual, the much smaller male spider carefully approaches the female prepared to deliver his genetic cargo. Once in contact with the female, the male spider locates and penetrates the female’s epigastric furrow with his pedipalp, at which time the sperm is released into the reproductive tract of the female. One such mating episode may provide ample sperm to fertilize all future eggs produced by the female. A male spider that is destined to experience another day quickly retreats from the females hunting territory after mating.

For all of this effort to propagate and sustain life, the female Black-widow spider lives on average for about 1 year. If the female spider is fortunate enough to survive a winter, its lifespan may approach two years. The smaller males, which are brown with white markings and look similar to the immature Black-widows of both genders, are less fortunate. They live only a few weeks – just long enough to perform their phylogenic function. If the female Black-widow can avoid predation by wasps, lizards, and parasitic flies (which enter the egg sac while it is under construction and lay their eggs to feed on the spider eggs before they hatch), she will molt up to 7 times during her life. The female Black-widow will also produce up to 9 egg sacs in one year, each containing about 750 eggs on average. The spiderlings will molt once before emerging from the egg sac in about 14 to 30 days. They will reach maturity in about 3-4 months, depending on the ambient temperature. Because spiders are poikilothermic animals, warmer temperatures will accelerate metabolic functions and, consequently, the spider’s ontogenic development (Cornett; pp. 15-17).

8

My inquiry into the biology of spiders, initiated by my reverence of the female Black-widow, has transformed the remnants of my long-standing fear of these creatures into a sense of appreciation and understanding. I have always been amused by casual acquaintances who assumed that, because of my interest in nature, I would protect and never harm another living creature. Unfortunately, this has not been the case. Any animal that I perceived as a threat, whether real or imagined, has either been killed or acquiesced the right-of-way. I am not particularly proud of the fate of the animals that were not acquiesced the right-of-way, but fear can bring about this type of human behavior.

Any sign of a spider now easily distracts me: with great interest, I now observe spiders earning their living, rather than compulsively annihilating them. I am learning from my arachnid companions how understanding and appreciation can transcend the powerful emotion of fear.

Allaby, Michael, Editor; Dictionary of Zoology; University of Oxford Press; 1991; 1st; 0-19-286093-3; ; Reference; Zoology; ; B; 23

Cornett, James W.; The Black-Widow Spider; Palm Springs Desert Museum, Palm Springs, CA; 1994; 1^st; 0-937794-15-5; ; Arthropods; Arachnids; Zoology; B;

Emerton, James H.; Common Spiders of the U.S.; Dover Publications, Inc., New York; 1961; 1st; 0-486-20223-2; ; Arthropods; Arachnids; Zoology; B; 4

Foelix, Rainer F.; Biology of Spiders; Oxford University Press & Georg Thieme Verlag, New York; 1996; 2^nd; 0-19-509594-4; QL458.4.F6313 1996; Arthropods; Arachnids; Zoology; B;

Hare, Trevor ; Poisonous Dwellers of the Desert; Southwest Parks & Monuments; 1995; 1st; 1-877856-53-3; ; Arthropods; Zoology; ; B; 152

Kaston, B.J.; how to know the spiders; W.C. Brown Co., Dubuque, Iowa; 1978; 3rd; 0-697-04898-5; ; Arthropods; Arachnids; Zoology; B; 411

Levi, Herbert W.; Spiders & Their Kin; Golden Press, New York; 1987; 1990; 0-307-24021-5; ; Arthropods; Arachnids; Zoology; B; 1

Literature Cited

Cornett, James W.; The Black-Widow Spider; Palm Springs Desert Museum, Palm Springs, CA; 1994; 1^st; 0-937794-15-5; ; Arthropods; Arachnids; Zoology; B;

Foelix, Rainer F.; Biology of Spiders; Oxford University Press & Georg Thieme Verlag, New York; 1996; 2^nd; 0-19-509594-4; QL458.4.F6313 1996; Arthropods; Arachnids; Zoology; B;

Hare, Trevor ; Poisonous Dwellers of the Desert; Southwest Parks & Monuments; 1995; 1st; 1-877856-53-3; ; Arthropods; Zoology; ; B; 152