Honeybee (or honey bee) is any member of the flying insect genus Apis of the tribe Apini, all of which are highly social bees and produce and store liquefied sugar ("honey") to some degree, and construct colonial nests out of wax secreted by the workers in the colony. Other types of related bees produce and store honey, but only members of the genus Apis are considered true honeybees. The term honeybee is applied especially to Apix mellifera, the European domestic bee or western honeybee. The other Apis species live in Asia.
Honeybees represent a far smaller fraction of bee diversity than most people suspect; of the approximately 20,000 known species of bees, only seven of these with a total of 44 subspecies are presently-recognized as honeybees(Engel, 1999); historically, anywhere from six to eleven species have been recognized. These bees are the only living members of the tribe Apini.
Although a honeybee's sting can be quite painful, especially because the stinger is barbed, the insect is nonetheless generally held in high regard, likely a function of the honeybees' usefulness as pollinators and producers of honey, their social nature, and their reputation as being hardworking. The harmony of their colonies has been used as a metaphor. Wilson (2004) states that a community of honeybees often has been employed historically by political theorists as a model of human society:
- 1 Origin and distribution of the genus Apis
- 2 Common Species
- 3 Beekeeping
- 4 Life cycle
- 5 Products
- 6 Defense
- 7 Communication
- 8 References
- 9 Credits
Beyond the internal harmony of a colony of bees, bees also exhibit harmony with plants. Honeybees are obligately dependent on flowering plants, from whose flowers they receive pollen and nectar, while certain plants are obligately dependent on honeybees for pollination. Thus, each species advances its own survival and/or reproduction through providing value to another species. This fits with the view of Lynn Margulis and Dorien Sagan (1986) that "Life did not take over the globe by combat, but by networking."
Origin and distribution of the genus Apis
Honeybees as a group appear to have their center of origin in Southeast Asia (including the Philippines), as all but one of the extant species are native to that region, including what are considered the most primitive living species (Apis florea and A. andreniformis). However, based on genetic studies, some consider the Western honeybee to have originated in tropical Africa and spread from there to Northern Europe and east into Asia (Whitfield et al. 2006).
The first Apis bees appear in the fossil record in deposits dating about 35 million years ago during the Oligocene period. These fossils are from Europe, but that does not indicate necessarily that Europe is where the genus originated, as the likelihood of fossils being found in Southeast Asia is very small. All of the ancestors and close relatives of modern honeybees were already social and social behavior predates the origin of the genus. Among the extant members of the genus, the species considered to be more ancient construct single, exposed combs; the species assumed to be of more recent origin nest in cavities and have multiple combs, which has greatly facilitated their domestication.
Most species have historically been cultured or at least exploited for honey and beeswax by humans indigenous to their native ranges. Only two of these species have been domesticated, and only Apis mellifera, which has been domesticated at least since the time of the building of the Egyptian pyramids, has been moved extensively beyond its native range.
Common (or European)
Apis mellifera is known as the western honeybee or European domestic bee and is the most commonly domesticated species of honeybee. Mellifera is Latin and means honey-carrying (apis, "bee," mel, melis, "honey," and fero, ferre, tuli, latum, "to carry"); hence, "Apis mellifera" is the honey-carrying bee. The name was coined in 1758 by Carolus Linnaeus, though in a subsequent 1761 publication, he referred to it as mellifica. The older name has precedence, but some people still use the incorrect subsequent spelling.
As of October 28, 2006, the Honey Bee Genome Sequencing Consortium fully sequenced and analyzed the genome of Apis mellifera.
There are many sub-species of the European honeybee that are adapted to the environment of their particular geographic and climatic area. Behavior, color, and anatomy can be quite different from one sub-species or race to another. In 1622, the first European colonists brought the sub-species Apis mellifera mellifera to the Americas. Many of the crops that depend on honeybees for pollination have also been imported since colonial times. Escaped swarms (known as wild bees, but actually feral) spread rapidly as far as the Great Plains, usually preceding the colonists. The Native Americans called the honeybee "the white man's fly." Honeybees did not naturally cross the Rocky Mountains; they were carried by ship to California in the early 1850s.
The so-called "killer bee" is a strain of this species, with ancestral stock of African origin (thus often called "Africanized"). In early 2007, abnormally high die-offs (30-70 percent of hives) of western honeybee colonies in the United States were attributed to a condition dubbed "Colony Collapse Disorder."
Dwarf and Eastern
- Apis florea, the dwarf honeybee, and Apis cerana, the Asiatic honeybee or Eastern honeybee, are small honeybees of southern and southeastern Asia. The former makes very small, exposed nests in trees and shrubs, while the latter makes nests in cavities and is sometimes managed in hives in a similar fashion to Apis mellifera, though on a much smaller and regionalized scale. The stings of A. florea are often not capable of penetrating human skin, so the hive and swarms can be handled with minimal protection.
- Apis dorsata, the giant honeybee, is native to south and southeastern Asia, and usually makes its exposed combs on high tree limbs, or on cliffs, and sometimes on buildings. It is wild and can be very fierce. It is robbed of its honey periodically by human honey gatherers, a practice known as honey hunting. Its colonies are easily capable of stinging a human being to death when provoked.
Two species of honeybee, A. mellifera and A. cerana, are often maintained, fed, and transported by beekeepers to collect honey and beeswax, or for the purpose of pollinating crops, or to produce bees for sale to other beekeepers. Modern hives enable beekeepers to transport bees, moving from field to field as the crop needs pollinating and allowing the beekeeper to charge for the pollination services they provide, revising the historical role of the self-employed beekeeper, and favoring large-scale commercial operations.
Honeybees have arguably the most complex social behavior among the bees. They are highly social, living in colonies with morphologically distinct queens, worker bees, and, at certain stages in the colony cycle, drones. Like other eusocial bees, a colony generally contains one breeding female, or "queen"; seasonally up to a few thousand males, or "drones"; and a large seasonally variable population of sterile female workers. Many minor details vary among the different species of honeybees, though there are some common features.
Honeybees, like other members of Hymenoptera, have a unique genetic system, haplodiploidy, whereby mated females control the sex of their eggs, with daughters developing from fertilized eggs and sons from unfertilized eggs (Pickering 2002). Unmated females can produce only sons.
Eggs are laid singly in a cell in a wax honeycomb, produced and shaped by the workers. Larvae are initially fed with royal jelly produced by worker bees, later switching to honey and pollen. The exception is a larva fed solely on royal jelly, which will develop into a queen bee. The larva undergoes several moltings before spinning a cocoon within the cell, and pupating.
Young worker bees clean the hive and feed the larvae. After this, they begin building comb cells. They progress to other within-colony tasks as they become older, such as receiving nectar and pollen from foragers. Later still, a worker leaves the hive and typically spends the remainder of its life as a forager.
Workers cooperate to find food and use a pattern of "dancing" (known as the bee dance or waggle dance) to communicate with each other; this dance varies from species to species, but all living species of Apis exhibit some form of the behavior. The forager bees of Apis florea and Apis andreniformis, which comprise the subgenus Micrapis, do not perform a waggle dance to recruit nestmates as in the domesticated Apis mellifera. Instead they "dance" on the horizontal upper surface where the comb wraps around the supporting branch. The dance is a straight run pointing directly to the source of pollen or nectar that the forager has been visiting. In all other Apis species, the comb on which foragers dance is vertical, and the dance is not actually directed toward the food source.
Honeybees also perform Tremble dances, which recruit receiver bees to collect nectar from returning foragers.
Virgin queens go on mating flights away from their home colony, and mate with multiple drones before returning. The drones die in the act of mating.
Colonies are established not by solitary queens, as in most bees, but by groups known as "swarms," which consist of a mated queen and a large contingent of workers. This group moves en masse to a nest site that has been scouted by workers beforehand, and once they arrive they immediately construct a new comb and begin to raise a new worker brood. This type of nest founding is not seen in any other living bee genus, though there are several groups of Vespid wasps that also found new nests via swarming (sometimes including multiple queens). Also, stingless bees will start new nests with large numbers of workers, but the nest is constructed before a queen is escorted to the site, which is not a true "swarm."
Life cycle of the western honeybee
In the temperate zone, western honeybees survive winter as a colony, and the queen begins egg laying in mid to late winter, to prepare for spring. This is most likely triggered by longer day length. She is the only fertile female, and deposits all the eggs from which the other bees are produced. Except a brief mating period when she may make several flights to mate with drones, or if she leaves in later life with a swarm to establish a new colony, the queen rarely leaves the hive after the larvae have become full grown bees.
The queen deposits each egg in a cell prepared by the worker bees. The egg hatches into a small larva that is fed by nurse bees (worker bees who maintain the interior of the colony). After about a week, the larva is sealed up in its cell by the nurse bees and begins the pupal stage. After another week, it will emerge as an adult bee.
For the first ten days of their lives, the female worker bees clean the hive and feed the larvae. After this, they begin building comb cells. On days 16 through 20, a worker receives nectar and pollen from older workers and stores it. After the twentieth day, a worker leaves the hive and spends the remainder of its life as a forager. The population of a healthy hive in mid-summer can average between 40,000 and 80,000 bees.
The larvae and pupae in a frame of honeycomb are referred to as frames of brood and are often sold (with adhering bees) by beekeepers to other beekeepers to start new beehives.
Both workers and queens are fed "royal jelly" during the first three days of the larval stage. Then workers are switched to a diet of pollen and nectar or diluted honey, while those intended for queens will continue to receive royal jelly. This causes the larva to develop to the pupa stage more quickly, while being also larger and fully developed sexually. Queen breeders consider good nutrition during the larval stage to be of critical importance to the quality of the queens raised, good genetics and sufficient number of matings also being factors. During the larval and pupal stages, various parasites can attack the pupa/larva and destroy or damage it.
Queens are not raised in the typical horizontal brood cells of the honeycomb. The typical queen cell is specially constructed to be much larger, and has a vertical orientation. However, should the workers sense that the old queen is weakening, they will produce emergency cells known as supersedure cells. These cells are made from a cell with an egg or very young larva. These cells protrude from the comb. As the queen finishes her larval feeding, and pupates, she moves into a head downward position, from which she will later chew her way out of the cell. At pupation, the workers cap or seal the cell. Just prior to emerging from their cells, young queens can often be heard "piping." The purpose of this sound is not yet fully understood.
Worker bees are infertile females; however, in some circumstances they may lay infertile eggs, and in one subspecies these eggs may be fertile. Worker bees secrete the wax used to build the hive, clean and maintain the hive, raise the young, guard the hive and forage for nectar and pollen.
In honeybees, the worker bees have a modified ovipositor called a stinger with which they can sting to defend the hive. Unlike other bees of any other genus (and even unlike the queens of their own species), the stinger is barbed. Contrary to popular belief, the bee will not always die soon after stinging: this is a misconception based on the fact that a bee will usually die after stinging a human or other mammal. The sting and associated venom sac are modified so as to pull free of the body once lodged (autotomy), and the sting apparatus has its own musculature and ganglion that allow it to keep delivering venom once detached. It is presumed that this complex apparatus, including the barbs on the sting, evolved specifically in response to predation by vertebrates, as the barbs do not function (and the sting apparatus does not detach) unless the sting is embedded in elastic material. Even then, the barbs do not always "catch," so a bee may occasionally pull the sting free and either fly off unharmed, or sting again.
Drone bees are the male bees of the colony. Since they do not have ovipositors, they also do not have stingers. Drone honeybees do not forage for nectar or pollen. In some species of honeybees, drones are suspected of playing a contributing role in the temperature regulation of the hive. The primary purpose of a drone bee is to fertilize a new queen. Multiple drones will mate with any given queen in flight, and each drone will die immediately after mating; the process of insemination requires a lethally convulsive effort.
The average lifespan of the queen is three to four years. Because queens successively run out of sperm, towards the end of their life, they start laying more and more unfertilized eggs. Beekeepers therefore frequently change queens every year or every other year.
The lifespan of the workers vary drastically over the year in places with an extended winter. Workers born in the spring will work hard and live only a few weeks, whereas those born in the autumn will stay inside for several months as the colony hibernates.
Honeybee queens release pheromones to regulate hive activities, and worker bees also produce pheromones for various communications. Honeybees use special pheromones, or chemical communication, for almost all behaviors of life. Such uses include (but are not limited to): mating, alarm, defense, orientation, kin and colony recognition, food production, and integration of colony activities. Pheromones are thus essential to honeybees for their survival.
Bees produce honey by collecting nectar, which is a clear liquid consisting of nearly 80% water with complex sugars. The collecting bees store the nectar in a second stomach and return to the hive where worker bees remove the nectar. The worker bees digest the raw nectar for about 30 minutes using enzymes to break up the complex sugars into simpler ones. Raw honey is then spread out in empty honeycomb cells to dry, which reduces the water content to less than 20%. When nectar is being processed, honeybees create a draft through the hive by fanning with their wings. Once dried, the cells of the honeycomb are sealed (capped) with wax to preserve the honey.
When a hive detects smoke, many bees become remarkably non-aggressive. It is speculated that this is a defense mechanism; wild colonies generally live in hollow trees, and when bees detect smoke it is presumed that they prepare to evacuate from a forest fire, carrying as much food reserve as they can. In this state, defense from predation is relatively unimportant; saving as much as possible is the most important activity.
Thermal regulation of the western honeybee
The western honeybee needs an internal body temperature of 35°C to fly, which is also the temperature within the cluster. The brood nest needs the same temperature over a long period to develop the brood, and it is the optimal temperature for the creation of wax.
The temperature on the periphery of the cluster varies with the outside air temperature. In the winter cluster, the inside temperature is as low as 20 - 22°C.
Honeybees are able to forage over a 30°C range of air temperature largely because they have behavioral and physiological mechanisms for regulating the temperature of their flight muscles. From very low to very high air temperatures, the successive mechanisms are shivering before flight and stopping flight for additional shivering, passive body temperature in a comfort range that is a function of work effort, and finally active heat dissipation by evaporative cooling from regurgitated honey sac contents. The body temperatures maintained differ depending on expected foraging rewards and on caste (Heinrich 1996).
The optimal air temperature for foraging is 22 - 25°C. During flight, the rather large flight muscles create heat, which must dissipate. The honeybee uses a form of evaporative cooling to release heat through its mouth. Under hot conditions, heat from the thorax is dissipated through the head. The bee regurgitates a droplet of hot internal fluid—a "honeycrop droplet"—which immediately cools the head temperature by 10°C (Heinrich 1996).
Below 7-10°C, bees become immobile due to the cold and above 38°C bee activity slows due to heat. Honeybees can tolerate temperatures up to 50°C for short periods.
Western honeybee queens
Periodically, the colony determines that a new queen is needed. There are three general triggers.
- The colony becomes space-constrained because the hive is filled with honey, leaving little room for new eggs. This will trigger a swarm where the old queen will take about half the worker bees to found a new colony, leaving the new queen with the other half of worker bees to continue the old colony.
- The old queen begins to fail. This is thought to be recognized by a decrease in queen pheromones throughout the hive. This situation is called supersedure. At the end of the supersedure, the old queen is generally killed.
- The old queen dies suddenly. This is an emergency supersedure. The worker bees will find several eggs or larvae in the right age-range and attempt to develop them into queens. Emergency supersedure can generally be recognized because the queen cell is built out from a regular cell of the comb rather than hanging from the bottom of a frame.
Regardless of the trigger, the workers develop the larvae into queens by continuing to feed them royal jelly. This triggers an extended development as a pupa.
When the virgin queen emerges, it has been commonly thought that she seeks out other queen cells and stings the infant queens within, and that should two queens emerge simultaneously, they will fight to the death. Recent studies, however, have indicated that colonies may maintain two queens in as many as 10 percent of hives. The mechanism by which this occurs is not yet known. Regardless, the queen asserts her control over the worker bees through the release of a complex suite of pheromones called queen scent.
After several days of orientation within and around the hive, the young queen flies to a drone congregation point—a site near a clearing and generally about 30 feet above the ground, where the drones from different hives tend to congregate in a swirling aerial mass. Drones detect the presence of a queen in their congregation area by her smell, and then find her by sight and mate with her in midair (drones can be induced to mate with "dummy" queens if they have the queen pheromone applied). A queen will mate multiple times and may leave to mate several days in a row, weather permitting, until her spermatheca is full.
The queen lays all the eggs in a healthy colony. The number and pace of egg-laying is controlled by weather and availability of resources and by the characteristics of the specific race of honeybee. Honeybee queens generally begin to slow egg-laying in the early-fall and may even stop during the winter. Egg-laying will generally resume in late winter as soon as the days begin to get longer. Egg-laying generally peaks in the spring. At the height of the season, she may lay over 2,500 eggs per day—more than her own body mass.
The queen fertilizes each egg as it is being laid using stored sperm from the spermatheca. The queen will occasionally not fertilize an egg. These eggs, having only half as many genes as the queen or the workers, develop into drones.
The western honeybee is the third insect, after the fruit fly and the mosquito, to have its genome mapped. According to the scientists who analyzed its genetic code, the honeybee originated in Africa and spread to Europe in two ancient migrations (Whitfield et al. 2006). They have also discovered that the number of genes in the honeybees related to smell outnumber those for taste, and they have fewer genes for immunity than the fruit fly and the mosquito (HBGSC 2006). The genome sequence revealed several groups of genes, particularly the genes related to circadian rhythms, were closer to vertebrates than other insects. Genes related to enzymes that control other genes were also vertebrate-like (Wang et al. 2006).
Species of Apis are generalist floral visitors and will pollinate a large variety of plants, but by no means all plants. Of all the honeybee species, only Apis mellifera has been used extensively for commercial pollination of crops and other plants. The value of these pollination services is commonly measured in the billions of dollars.
Honey is the complex substance made when the nectar and sweet deposits from plants and trees are gathered, modified and stored in the honeycomb by honeybees. All living species of Apis have had their honey gathered by indigenous peoples for consumption, though for commercial purposes only A. mellifera and A. cerana have been exploited to any degree. Honey is sometimes also gathered by humans from the nests of various stingless bees.
Worker bees of a certain age will secrete beeswax from a series of glands on their abdomens. They use the wax to form the walls and caps of the comb. As with honey, most indigenous peoples will gather beeswax for various purposes.
Bees collect pollen in the pollen basket and carry it back to the hive. In the hive, pollen is used as a protein source necessary during brood-rearing. In certain environments, excess pollen can be collected from the hives of A. mellifera and A. cerana. It is often eaten as a health supplement.
Propolis (or bee glue) is created from resins, balsams, and tree saps. Those species of honeybees that nest in tree cavities use propolis to seal cracks in the hive. Propolis is also used in some cosmetics.
All honeybees live in colonies where the workers will sting intruders as a form of defense, and alarmed bees will release a pheromone that stimulates the attack response in other bees. The different species of honeybees are distinguished from all other bee species by the possession of small barbs on the sting, but these barbs are found only in the workers. The sting and associated venom sac are also modified so as to pull free of the body once lodged (autotomy), and the sting apparatus has its own musculature and ganglion, which allow it to keep delivering venom once detached.
It is presumed that this complex apparatus, including the barbs on the sting, evolved specifically in response to predation by vertebrates, as the barbs do not usually function (and the sting apparatus does not detach) unless the sting is embedded in fleshy tissue. While the sting can also penetrate the flexible exoskeletal joints in appendages of other insects (and is used in fights between queens), in the case of Apis cerana defense against other insects, such as predatory wasps, is usually performed by surrounding the intruder with a mass of defending workers, who vibrate their muscles so vigorously that it raises the temperature of the intruder to a lethal level. This is also used to kill a queen perceived as intruding or defective, an action known to beekeepers as balling the queen, named for the ball of bees formed.
Honeybees are known to communicate through many different chemicals and odors, as is common in insects, but also use specific behaviors that convey information about the quality and type of resources in the environment, and where these resources are located. The details of the signaling being used vary from species to species. For example, the two smallest species, Apis andreniformis and Apis florea, dance on the upper surface of the comb, which is horizontal (not vertical, as in other species), and workers orient the dance in the actual compass direction of the resource to which they are recruiting.
Honeybees are an excellent animal to study with regard to behavior because they are abundant and familiar to most people. An animal that is disregarded every day may have very specific behaviors that go unnoticed by the causal observer. Karl von Frisch studied the behavior of honeybees with regards to communication and was awarded the Nobel Prize for physiology and medicine in 1973.
Von Frisch noticed that honeybees communicate with the language of dance. Honeybees are able to direct other bees to food sources through the round dance and the waggle dance. The round dance tells the other foragers that food is within 50 meters of the hive, but it does not provide much information regarding direction. The waggle dance, which may be vertical or horizontal, provides more detail about both the distance and the direction of the located food source. It is also hypothesized that the bees rely on their olfactory sense to help locate the food source once the foragers are given directions from the dances.
Another signal for communication is the shaking signal, also known as the jerking dance, vibration dance, or vibration signal. It is a modulatory communication signal because it appears to manipulate the overall arousal or activity of behaviors. The shaking signal is most common in worker communication, but it is also evident in reproductive swarming. A worker bee vibrates its body dorsoventrally while holding another honeybee with its front legs. Jacobus Biesmeijer examined the incidence of shaking signals in a forager’s life and the conditions that led to its performance to investigate why the shaking signal is used in communication for food sources. Biesmeijer found that the experienced foragers executed 92.1 percent of the observed shaking signals. He also observed that 64 percent of the shaking signals were executed by experienced foragers after they had discovered a food source. About 71 percent of the shaking signal sessions occurred after the first five foraging successes within one day. Then other communication signals, such as the waggle dance, were performed more often after the first five successes.
Biesmeijer proved that most shakers are foragers and that the shaking signal is most often executed by foraging bees over pre-foraging bees. Beismeijer concluded that the shaking signal presents the overall message of transfer work for various activities or activity levels. Sometimes the signal serves to increase activity, when bees shake inactive bees. At other times, the signal serves as an inhibitory mechanism such as the shaking signal at the end of the day. However, the shaking signal is preferentially directed towards inactive bees. All three types of communication between honeybees are effective in their jobs with regards to foraging and task managing.
ReferencesISBN links support NWE through referral fees
- Biesmeijer, J. 2003. "The occurrence and context of the shaking signal in honeybees (Apis mellifera) exploiting natural food sources." Ethology 109: 1009-1020.
- Collet, T., K. M. Ferreira, M. C. Arias, A. E. E. Soares, and M. A. Del Lama. 2006. "Genetic structure of Africanized honeybee populations (Apis mellifera L.) from Brazil and Uruguay viewed through mitochondrial DNA COI–COII patterns." Heredity 97: 329–335.
- Engel, M. S. 1999. "The taxonomy of recent and fossil honey bees (Hymenoptera: Apidae: Apis)." Journal of Hymenoptera Research 8: 165-196.
- Engel, M. S., and D. Grimaldi. 2005. Evolution of the Insects. New York: Cambridge University Press. ISBN 0521821495.
- Gross, C. L., and D. Mackay. 1998. "Honeybees reduce fitness in the pioneer shrub Melastoma affine (Melastomataceae)." Biological Conservation 86(2): 169-178.
- Heinrich, B. 1996. How honeybees regulate body temperature. Bee World 77: 130-137.
- Heinrich, B. 1981. Insect Thermoregulation. New York: Wiley. ISBN 0471051446.
- Honeybee Genome Sequencing Consortium (HBGSC). 2006. "Insights into social insects from the genome of the honeybee Apis mellifera." Nature 443(7114): 931-949.
- Kak, S. C. 2001. "The honey bee dance language controversy." The Mankind Quarterly 31: 357-365.
- Lindauer, M. 1961. Communication among social bees. Cambridge, MA: Harvard University Press.
- Margulis L., and D. Sagan. 1986. Microcosmos. New York: Summit Books. ISBN 0671441698.
- Myerscough, M. R. 2003. "Dancing for a decision: A matrix model for nest-site choice by honeybees." Proc. Royal Soc. London B 270: 577-582.
- Pickering, J. 2002. Bombus. Discover Life. Retrieved May 31, 2007.
- Schneider, S. S., P. K. Visscher, and S. Camazine. 1998. "Vibration signal behavior of waggle-dancers in swarms of the honey bee, Apis mellifera (Hymenoptera: Apidae)." Ethology 104: 963-972.
- Wang, Y., M. Jorda, P. L. Jones, R. Maleszka, X. Ling, H. M. Robertson, C. A. Mizzen, M. A. Peinado, and G. E. Robinson. 2006. "Functional CpG methylation system in a social insect." Science 314(5799): 645-647.
- Whitfield, C. W., S. K. Behura, S. H. Berlocher, A. G. Clark, J. S. Johnston, W. S. Sheppard, D. R. Smith, A. V. Suarez, D. Weaver, and N. D. Tsutsui. 2006. "Thrice out of Africa: Ancient and recent expansions of the honey bee, Apis mellifera." Science 314(5799): 642-645.
- Wilson, B. 2004. The Hive: The Story Of The Honeybee. London, Great Britain: John Murray. ISBN 0719565987.
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.