“People with an interest in North American spiders should enjoy this book.”
Common Spiders of North Americaby Richard A. Bradley, Steve Buchanan
Spiders are among the most diverse groups of terrestrial invertebrates, yet they are among the least studied and understood. This first comprehensive guide to all 68 spider families in North America beautifully illustrates 469 of the most commonly encountered species. Group keys enable identification by web type and other observable details, and species descriptions… See more details below
Spiders are among the most diverse groups of terrestrial invertebrates, yet they are among the least studied and understood. This first comprehensive guide to all 68 spider families in North America beautifully illustrates 469 of the most commonly encountered species. Group keys enable identification by web type and other observable details, and species descriptions include identification tips, typical habitat, geographic distribution, and behavioral notes. A concise illustrated introduction to spider biology and anatomy explains spider relationships. This book is a critical resource for curious naturalists who want to understand this ubiquitous and ecologically critical component of our biosphere.
“People with an interest in North American spiders should enjoy this book.”
"This is a fabulous book, worthy of a place on the bookshelves of any arachnological library."
"Bradley’s admiration of and attention to these animals is evident. . . The result is not only a remarkable compendium of information about conspicuous spiders in North America, but a gateway to their study and appreciation.”
"A critical resource for curious naturalists."
- University of California Press
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Common Spiders of North America
By Richard A. Bradley, Steve Buchanan
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2013 The Regents of the University of California
All rights reserved.
Finding, Studying, and Identifying Spiders
Naturalists know that the world is full of interesting organisms. Yet for most people, naturalists included, a large portion of our lives is lived in human-altered, domesticated environments. We don't notice the enormous variety of life that is all around us. The lawn becomes an undifferentiated green carpet; the forest edge, a green wall. Even more invisible to us is the microworld. Down among the thatch of dead grass and leaves at the soil surface is a world teeming with bacteria and fungi that break down and recycle dead tissues, creating the basis for the living soil. The microscopic organisms feed a plethora of others. By kneeling down and parting the grass with our hands or a stick, we get a glimpse into this microcosmos. Peering closely, we will likely observe one of its most active inhabitants: spiders. Spiders are ubiquitous in soil-surface communities. This abundance lives literally under our feet, yet we seldom notice.
As a biology teacher, and when presenting talks in my community, I am impressed that most people are unaware of the great variety of living organisms that share our world. They seem genuinely surprised to learn that there are so many different kinds of wild animals, even in cities and towns. When leading field trips to local parks, I can usually find dozens of species of small creatures during a relatively short walk. Children particularly embrace these experiences and relish the act of discovery. Where I can, I pass out sweep nets and set the children loose into abandoned fields or meadows. Their enthusiasm and energy yield nets full of colorful and surprising tiny beasts. We dump the contents onto white sheets on the ground and gather around to inspect the catch. Of course, many insects fly out of the nets as soon as they are opened, and some of the spiders also make a mad dash to escape. The remaining animals often feign death, remaining motionless for a while. After a few moments, they too begin to move away toward the edge of the sheet. Having spread themselves out, the organisms' true complexity and colorful diversity become evident. It is not unusual to find a few dozen spiders, including many different species.
Armed with some simple skills, you can find spiders of many kinds, even in the most mundane places. Some spiders have become regular occupants of our cities and parks. Some spiders inhabit buildings, but many more live outside. Venture outdoors and an abundance of spiders can be found in even a small residential yard. I recently compiled a list of all the types of spiders that I have found in our rural yard in central Ohio over the past 20 years. This list includes more than 160 different species of spiders representing 20 families—all this in one modest backyard acre!
NORTH AMERICAN SPIDER FAUNA
Worldwide, there are more than 42,000 spider species (Platnick 2012). The tropics host the greatest variety of spiders; nearly a quarter of the world's 110 families of spiders are found exclusively or primarily in the tropical realm. The focus of this book is the spiders of North America north of Mexico. No one knows exactly how many different types of spiders exist here, because relatively few scientists study spiders. Our knowledge of them is incomplete. At last count, a total of 3,807 species of spiders had been found in this region. Nearly the same number has been found in Europe, a substantially smaller geographic area. Judging from the continuing rate of discovery of new species in North America, it is reasonable to believe that the current list represents only a fraction of the true diversity. Why? Many of the species of spiders are small and inconspicuous. Other species are so similar that only a specialist using a microscope can distinguish them.
Within the region covered in this book, North America north of Mexico, the areas with the greatest diversity of spiders are along the southern boundary. Florida has many tropical and subtropical species. California and Texas are both large states with considerable topographic and climatic variety, thus it is not surprising that they host many spider species. Although many spiders have adapted to existence in the arid regions of the Southwest, humid habitats and forests typically harbor the highest numbers of spiders. This guide provides information on most of the larger, more conspicuous, and common species of spiders found in North America north of Mexico. For some genera of spiders, many species look the same. In these cases, one species from the group is illustrated. Representatives of all 68 families of spiders found in the region studied here are included. Nearly half of the species in North America north of Mexico never grow to more than 3 mm (about an eighth of an inch) long. Many of these tiny spiders live near the soil surface, often in the loose layers of fallen leaves or under rocks, and are rarely observed. Others inhabit caves or crevices that are rarely visited. This guide is biased to include those larger spiders that a curious naturalist is likely to encounter. In total, 469 species representing 311 genera are included.
THE ROLE OF SPIDERS IN NATURE
As small or intermediate-sized predators in the food web, spiders are pivotal to the normal function of both natural and human-modified habitats. One study concluded that spiders accounted for more than half of the predator biomass in the forest floor community. Most spiders are considered generalist predators because they will capture and consume a variety of different kinds of prey. Some spiders are specialists that feed on only one type of prey. Because of their small size, the remarkable abundance of spiders is the key to their importance. Density estimates of spiders range from hundreds to millions of spiders per acre. Of course, these are mostly tiny spiders. Despite their bad reputation with the general public, spiders are mostly beneficial to humans. Scientific studies have established that spiders play an important role by eating insects in natural environments as well as in agricultural croplands. Although spiders cannot completely control explosive pest outbreaks, their presence probably dampens fluctuations in pest insect populations in farm fields and orchards.
Each natural environment has its own community of small animals, including many kinds of spiders. Consider the tiny spaces under the leaf litter of a deciduous forest: here you will find an abundance of tiny sheet-weaving and hunting spiders that prey on the resident mites and springtails. These spiders themselves will become food for larger spiders, ants, beetles, and perhaps wasps that also forage among the fallen leaves. On first glance, deserts seem to be inhospitable places. A closer look reveals that life abounds. In the dry brush of a desert wash in Arizona, sweeping with a net captures large numbers of small spiders searching for their insect prey. Up close, these are sometimes marvelously colored and beautiful animals, such as jumping spiders, but we overlook them in favor of the larger lizards and birds that also forage in this habitat. At night this same desert comes alive with both spider and scorpion predators.
Under the canopy of a soybean field in the Midwest live both sedentary cobweb weavers and active hunting lynx and jumping spiders. Near the ground are the tiny webs of numerous sheet-weaving species. Running on the soil surface are a variety of wolf spiders—small, medium, and large. Each of these is both predator and prey. They feed on small insects but may also eat spiders smaller than they are. In turn, they fall prey to large predatory spiders and insects, such as wasps and praying mantises.
Among the reedy growth at the edge of a freshwater pond can be found numerous horizontal orb webs of longjawed orbweavers waiting for emerging mosquitoes. Perhaps a fishing spider clings to a twig of emergent vegetation, patiently waiting for an unwary minnow, tadpole, or aquatic insect nymph to stray within reach. Along the muddy shore, small pirate wolf spiders stalk their insect prey. For each of these situations, the most numerous and important small predators in the ecosystem may be the spiders. They are inconspicuous but crucial components in every terrestrial environment. Tiny species, often living near the soil surface, are usually the most diverse residents. Pick any spot and the first small predator that you encounter will probably be a spider.
SPIDERS IN BUILDINGS
A few spiders are closely associated with human-created habitats. These spiders can tolerate dry conditions indoors and may become locally abundant. Most of the spiders in buildings reside in relatively undisturbed or dark areas under furniture, in corners, closets, basements, cellars, garages, and warehouses. Outbuildings (such as barns and sheds) sometimes support large numbers of spiders. Spiders in buildings provide a service by capturing and devouring insect pests. Most will live out their lives without ever encountering their human landlords.
SPIDER LIFE HISTORIES
The lives of most spiders are relatively short. The majority of species live less than a year. The seasonal timing of reproduction in spiders usually conforms to one of a few basic patterns. One pattern, characteristic of large orbweavers, begins with emergence from the egg case in the spring. The young spiders grow throughout the summer and mature in the autumn. Egg cases are laid at this time, before the first hard frosts in the North, and the adults usually die before winter. The population continues through the winter as eggs or tiny juveniles in egg cases destined to emerge in spring. In a second common pattern, maturity is achieved in the spring, eggs are produced in late spring or early summer, and growth extends through the autumn. Individuals spend the winter as adults or subadults. Subadult spiders are one molt away from maturity. In the first appropriate weather of early spring, spiders become active, the subadults molt into adults, and courtship and mating occurs. Other spiders actually mature and reproduce during the winter months. This pattern is common among dwarf sheet weavers that live close to the ground. Even in the North, where winter is cold, courtship may happen under the insulating cover of snow near the soil surface. Some multigenerational populations of spiders, particularly in warmer climates, can mature at any season and adults are found year-round.
Courtship and Mating
Some of the most fascinating aspects of spider behavior are their courtship and mating. Soon after a male spider molts into his mature form, he prepares for mating in a behavior called "charging the palps." The palps (short for pedipalps) are small leglike appendages at the front of the spider's head (see Fig. 7). In adult male spiders the last segment (the tarsus) of the palps is modified into a receptacle for the transfer of his sperm. Charging the palps entails spinning a small web, sometimes a single silk line, and depositing the seminal fluid containing sperm as a droplet onto this sperm web. The male then repositions himself and draws the droplet up into one of his palps and then the other.
Most male spiders live a relatively brief period as adults. It has been estimated that the average lifespan of an adult male orbweaver is only about one week, and they spend this time searching for females. In some species adult males may not eat during this period. For many spiders the males use special odors called pheromones to locate the females. Chemical cues may be present in the silk web, or draglines, of females. For example, some wolf spider males find the females by following her silk draglines. When a male encounters a female, he usually begins to court her immediately. This courting may involve a combination of plucking the web, vibrating or waving his palps and legs to provide a visual signal, or producing soft sounds by stridulation (rubbing two body parts together). Some spiders will tap the surface of the substrate (leaf, ground) with their legs or their body to produce the courtship sounds. This vibratory signal is usually transmitted to the female through the substrate. Some species of diurnal (day-active) spiders—in particular, the members of the jumping spider family—use brightly colored body parts in visual courtship displays. It is important to remember that male spiders run the risk of being confused with prey, attacked, and eaten by the (usually larger) female. Elaborate courtship behavior may make this less likely.
If the female accepts the male, the two will mate. In some species the male has specific structures that permit him to grasp the female, or her jaws, during mating. In some crab spiders the male spins a few thin silk lines over the female as a mating veil. The male then reaches out to the female and inserts a syringe-like structure called the embolus (part of the palp) into her reproductive opening and pumps in the seminal fluid. In some spiders thin-walled sacs in the palps, called the hematodochae, inflate and assist in insertion and insemination (Fig. 1). Female spiders have bilaterally symmetrical reproductive structures, so after the first mating, the male may change sides and inseminate the other side. In some spiders the male will pause to recharge his palps between matings. The entire process may last a few moments, or sometimes it may extend for hours. The female stores the seminal fluid, containing the sperm, in special internal structures called spermathecae. When the female lays eggs, she adds sperm from the spermathecae to fertilize them.
Excerpted from Common Spiders of North America by Richard A. Bradley, Steve Buchanan. Copyright © 2013 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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