? Spiders | ||||||||
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| [image] A long-jawed orb weaver spider | ||||||||
| Scientific classification | ||||||||
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| Suborders | ||||||||
| Araneomorphae Mesothelae Mygalomorphae See the taxonomy section for families |
Spiders are Invertebrate animals that produce silk, have eight legs and no wings. More precisely, a spider is any member of the Arachnid order Araneae, an order divided into three sub-orders in newer systems: the Mygalomorphae (the primitive spiders), the Araneomorphae (the modern spiders) and the Mesothelae, which contains the Family Liphistiidae, rarely seen burrowing spiders from Asia. The study of spiders is known as Arachnology, although it is often included in the more general term Entomology.
Many spiders hunt by building webs to trap insects. These webs are made of Spider silk, a thin, strong Protein strand extruded by the spider from spinnerets on the end of the abdomen. All spiders produce silk, although not all use it to spin elaborate traps. Silk can be used to aid in climbing, forming smooth walls for burrows, cocooning prey, and for many other applications.
[image]Spiders have developed several different respiratory anatomies, based either on book lungs, a tracheal system, or both. Primitive mygalomorph spiders generally have only a pair of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allows air to enter and diffuse Oxygen. Modern araneomorph spiders often have a single book lung in addition to spiracles which deliver air into the tracheae, where oxygen is then diffused into the haemolymph. In the tracheal system oxygen interchange is much more efficient, enabling Cursorial hunting (hunting involving rapid pursuit) and other advanced characteristics.
Between the time an egg is fertilized and the spider begins to take the shape of a spider is referred to as the embryonic stage (Foelix, 1996). As the spider begins to look more like a spider it enters the larval stage (Foelix, 1996). It enters the larval stage as a prelarva and, through subsequent molts, it reaches its larval form, a spider-looking, non self-sufficient animal feeding off its yolk supply (Foelix, 1996). After a few more molts, also called instars, body structures become differentiated; all organ systems are complete and the animal begins to hunt on its own; it has reached the nympho-imaginal stage (Foelix, 1996). This stage is differentiated by two sub-stages: the nymph, or juvenile stage and the imago, or adult stage (Foelix, 1996). A spider does not transition from the nymph to the imago until it has become sexually mature (Foelix, 1996). Once a spider has reached the imago stage, it will remain there until its death. Many spiders may live only about a year, but a number will live two years or more, overwintering in sheltered areas (the annual influx of 'outdoor' spiders into houses in the fall is due to this search for a nice warm place to spend the winter).
Unusually, sperm transmission is an indirect process. When a male is ready to mate, he will spin a web pad onto which the contents of the abdominal reproductive organs are discharged. He then dips his palps (also known as 'palpi'), the small, leg-like appendages on the front of his Cephalothorax, into the sperm, absorbing it. Mature male spiders characteristically have swollen bulbs on the end of their palps for this purpose, and this is a useful way to identify the sex of a spider in the field. With his palps thus 'charged' he then goes off in search of a female. The act of copulation occurs when the male inserts one or both palps into the female's genital opening, known as the Epigyne. He transfers his sperm into the female by contracting his palps.
Very unusual behaviour is seen in spiders of the Genus Tidarren, as the male amputates one of his palps before maturation and enters his adult life with one palp only. The palpi constitute 20% of its body mass, and since this weight greatly impedes its movement, the spider detaches one of the two to gain mobility. In the Yemeni species Tidarren argo, the remaining palp is then torn off by the female. The separated palp remains attached to the female's epigynum for about four hours and apparently continues to function independently. In the meantime the female feeds on the palpless male. (Journal of Zoology (2001), 254:449-459 Cambridge University Press DOI: 10.1017/S0952836901000954 )
However, despite these examples, and many other similar reports, the story of the 'sacrificial male' has become greater than the truth. Mating of spiders is not invariably followed by cannibalism. Rainer F. Foelix, (1982), says "The supposed aggressiveness of the female spider towards the male is largely a myth... only in some exceptional cases does the male fall victim to the female.". Michael Roberts (1995) says "It is rare for a fit male to be eaten by the female..."
However, there has been speculation on why this sacrifice of male mates might occur at all. One theory is that once the male has mated, if he is unlikely to mate again then further extension of his life serves no evolutionary purpose, while the sacrifice of the male may help increase egg production through increased Nutrition for the female. Having more offspring would give the male an advantage of having his genes passed on over other males that were not eaten. This would be consistent with the hypothesis (Roberts, 1995) that old or unfit males get eaten, whilst younger and fitter ones may survive to mate again.
While spiders are generalist predators, in actuality their different methods of prey capture often limits the type of prey taken. Thus web-building spiders rarely capture caterpillars and crab spiders that ambush prey in flowers capture more bees, butterflies and some flies than other insects. Groups of families that tend to take certain types of prey because of their prey capture methods are often called guilds. A few spiders are more specialized in their prey capture. Dysdera captures and eats sowbugs, pillbugs and beetles, while pirate spiders eat only other spiders. Bolas spiders in the family Araneidae use sex pheromone analogs to capture only the males of certain moth species. Despite their generally broad prey ranges spiders are one of the most important links in the regulation of the populations of insects. Every day on a meadow they devour over 10 g/m² of insects and other arthropods.
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Spiders bite their prey, and occasionally animals that cause them pain or threaten them, to do two things. First, they inflict mechanical damage, which, in the case of a spider that is as large as or larger than its prey, can be severe. Second, they can choose to inject venom through their hollow fangs. Many genera, such as the widow spiders, inject neurotoxins that can spread through the prey's entire body and interfere with vital body functions. Other genera inject venom that operates to produce tissue damage at the site of the bite. Genera such as that of the Brown recluse spider produce a necrotoxin. The necrotoxin is injected into prey where it causes the degradation of cell membranes. In the larger victims that do not die from these attacks, painful lesions over a fairly wide area of the body can remain active for fairly long periods of time.
Digestion is carried out internally and externally. The spiders secrete digestive fluids into their Prey from a series of ducts perforating their jaws. These digestive fluids dissolve the prey's internal tissues. Then, the spider feeds by sucking the partially digested fluids out. Spiders consume only liquid foods. Many spiders will store prey temporarily while this process of external digestion is going on. Web weaving spiders that have made a shroud of silk to quiet their envenomed prey's death struggles will generally leave them in these shrouds and then consume them at their leisure.
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Some spiders spin Funnel-shaped webs, others make irregular webs, and still others make the Spiral "orb" webs which are most commonly associated with the order. These webs may be made with sticky capture silk, or with "fluffy" capture silk, depending on the type of spider. Webs may be in a vertical plane (most orb webs), a horizontal plane, or at any angle in between. Some webs will have loose, irregular tangles of silk above them. These serve to disorient and knock down flying insects, making them more vulnerable to being trapped in the web below. They may also help to protect the spider from aerial predators such as birds and wasps.
The spider, after spinning its web, will then wait on, or near, the web for a prey animal to become trapped. The spider can sense the impact and struggle of a prey animal by vibrations transmitted along the web lines.
Other species of spiders do not use webs for capturing prey directly, instead pouncing from concealment (e.g. Trapdoor spiders) or running them down in open chase (e.g. Wolf spiders). Spiders do not usually adhere to their own webs. However, they are not immune to their own glue. Some of the strands of the web are sticky, and others are not. The spiders have to be careful to only climb on the non-sticky strands.
The ability to spin webs allows a spider to catch prey without having to expend energy by running it down. Thus it is a very efficient method of gathering food. However, constructing the web is in itself an energy costly process, due to the large amount of protein required, in the form of silk. It is not uncommon for spiders to eat their own web after a period of time to recoup some of the energy used in spinning. The silk proteins are thus 'recycled'. The spider, in the middle of the web, also makes for a highly visible prey for birds and other predators. Many day-hunting orb-web spinners reduce this risk; for example, by hiding at the edge of the web with one foot on a signal line from the hub, or by appearing to be inedible or unappetizing.
The Net-casting spider balances the two methods of running and web-spinning in its feeding habits. This spider weaves a small net which it attaches to its front legs. It then lurks in wait for potential prey and, when such prey arrives, lunges forward to wrap its victim in the net, bite and paralyse it. Hence, this spider expends less energy catching prey than a primitive hunter such as the Wolf Spider. It also avoids the energy loss of weaving a large orb-web.
Some spiders manage to use the 'signalling snare' technique of a web without spinning a web at all. Several types of water-dwelling spiders will rest their feet on the water's surface in much the same manner as an orb-web user. When an insect falls onto the water and is ensnared by surface tension, the spider can detect the vibrations and run out to capture the prey.
Key to bite severity:
A couple of things should be noted when considering the amount of danger posed by spider bites. First, it is often the case that a spider bite is "dry"--the skin may be pierced, but little or no venom injected into the victim; thus reducing or eliminating any harmful effects. Second, there have been reports of spider bites (by spiders considered otherwise harmless) causing allergic reactions in some individuals, up to and including anaphylactic shock, a life-threatening condition (much the same as a sting from an ant, Bee, or wasp). Third, many spiders listed as dangerous are seldom encountered, or have dispositions that make them unlikely to bite despite the high toxicity of their venom. Finally, little is known about the toxicity of many spiders, due to infrequent encounters with Man; the list of venomous spiders is limited to those who are linked to medical events in humans or who otherwise have been extensively studied.
It should also be noted that for healthy adults, a bite by even the most toxic spiders on the list will require hours before death ensues; if timely appropriate emergency medicine is administered, victims should be expected to recover. The scenario given in movies such as Arachnophobia, where bite victims die within minutes, does not occur. One exception to this is with very small children; there is at least one recorded case of a small child dying within 15 minutes of a bite from a Sydney funnel-web spider; that occurred before the development of an antivenin.
Widows (Latrodectus spp.) - a large, cosmopolitan group; all with relatively dangerous bites. These are relatively large (about the size of a nickel), 'burly-looking' house spiders; generally dark, often with a red mark on the glossy, smooth abdomen, either above or below. examples:
Steatoda--a large genus which includes the false black widows; these are sometimes mistaken for widows, but have more flattened abdomens, and abdominal markings are generally white stripes or dots rather than red dots. None of these is truly dangerous but some of them are medically significant:
others - the common "yuck!" spiders - large, globular abdomen, thin, spindly legs. Often rather non-descript patterns in gray or brown and white. examples:
This is a "catch-all" category, comprising members of several different groups that spin non-sticky webs in a variety of structural styles. Some (the Linyphiidae) make various forms of bowl- or dome-shaped web with or without a flat sheet or a tangled web above or below. Some make a flat platform extending from a funnel-shaped retreat, with generally a tangle of silk above the web. The common northern hemisphere 'funnel-web', 'house' or 'grass' spiders are only distantly related to the notorious Sydney funnel-web spider, and are generally considered to be quite harmless (with one notable exception - the Hobo spider, below). Some of the more primitive group Atypidae may make tubular webs up the base of trees, from inside which they bite insects that land on the webbing. These spiders look quite ferocious, but are not generally considered to be particularly dangerous to humans.
Dangerous spiders in the United States include widow spiders, brown recluse spiders, hobo spiders, and yellow sac spiders.
None of these spiders will intentionally "come after you," but they should be removed from one's house to avoid accidental injury. Many authorities warn against spraying poisons indiscriminately to kill all spiders, because doing so may actually remove one of the biological controls against incursions of the more dangerous species by ridding them of their competition.
If dangerous spiders are present in your area, then be mindful when you move cardboard boxes and other such objects that may have become the shelter of a poisonous spider. There is no need to be fearful; just do not grab a spider.
Brown recluse spiders, (or "Violin Spiders" from the dark violin-shaped marking on the cephalothorax) Loxosceles reclusa, are slow-moving, retiring spiders which wander about in dim areas and under things, and so are more easily trapped against one's skin by clothing, bed sheets, etc. Even so, most encounters with this spider occur from moving boxes or rooting about in closets or under beds. The range of this spider in the US is approximately the lower 2/3 of the country by the eastern 3/4 of the country. A number of related Recluse spiders (some non-native introductions) are found in southern California and nearby areas, as well. The Hobo spider, Tegenaria agrestis, may wander away from its web, especially in the fall, and thus come into contact with people and bite. This spider is found in the northwestern United States and throughout much of Europe. Oddly enough, in Europe it is considered a harmless outdoor relative of the common House Spider (Tegenaria domestica). The yellow sac spiders, Chiracanthum sp., take shelter in silk tubes during the daytime and generally come out to hunt at night. These pale yellow or whitish spiders are often found in houses at the top of walls, or wandering across ceilings. They are also commonly found outdoors on foliage. The drag-lines they leave while hunting are one of the most common "cob-webs" that are removed with broom and vacuum cleaner. People may unintentionally make contact with them in the dark and so be bitten. However, most people will live their entire lives in close proximity to them and never suffer a bite.
Brown recluse spider bites can produce very severe local symptoms, death of tissue around the wound, and, sometimes, severe systemic symptoms, including organ damage. The bites of hobo spiders and yellow sac spiders can cause both pain and necrosis (tissue death). Typically, all these bites are characterized by open, sore-like wounds that heal very slowly and may leave scarring. It has been suggested that steroid treatments may speed healing and reduce scarring.
It is believed that many spider bites which are attributed (often by physicians and other medical personnel) to the brown recluse are in fact caused by the hobo spider (if caused by a spider at all). Many brown recluse bites are reported in the U.S. west coast states (Washington, Oregon, and northern California) where populations of brown recluse spiders have not been found.
There are other species of spider in the genus Loxosceles which are found in southern California and other southwestern states; most of these species live in remote areas and are rarely encountered by humans. Bites of Loxosceles spiders found in South America are more serious in their consequences than their North American relatives.
There is one spider in California and Japan, probably a huntsman (tentatively identified as a member of the Sparassidae family, Heteropoda venatoria), that might run over and bite your finger if you touch the wall that it is clambering over. That behavior may well occur because its eyesight is good enough to see movement and general shape, but not sufficient to avoid mistaking something else for its natural prey.
Suborder Mesothelae
Suborder Mygalomorphae [image]
The Italian dance and music Tarantella is related to tarantulas, either as a folk remedy for bites or from its vigorous movements.