Spider

This article is about the spider, the animal. For other article subjects named spider see Spider (disambiguation).

Spiders are certain 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 grouped under the more general area of 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.

Anatomy

Spiders, unlike insects, have their bodies divided in only two segments: prosoma or cephalothorax (a fused head and thorax) and opisthosoma or abdomen. New World tarantulas, have a patch of urticating hairs on their abdomens, while these are lacking in Old World species. Spiders have eight legs compared to the insects' six, and their eyes (usually eight) are single lenses rather than compound eyes like those of most insects.

Eyes can be arranged differently in different species. Sometimes one pair is better developed than the rest. Some species have a pair less or are even without eyes. While several families of hunting spiders have good to excellent vision (Lycosidae and Salticidae), the members of most families of hunting spiders, most of the web weavers and the spiders that lurk on flowers and other fixed locations waiting for some insect to come their way have very poor eyesight. Instead, these spiders hunt using their extreme sensitivity to vibrations.

Unlike insects, spiders have no feelers. Instead, they have pedipalps, sometimes just called palps, which are two additional appendages next to their mouth parts that, besides having other functions, they use to help themselves manipulate the food that they eat.

Spider haemolymph (the equivalent of blood) does not circulate through vessels, but just fills the body of the spider. This is called an open circulatory system. The spider's respiratory anatomy is based on a tracheal system, with each opening to the trachea as an extension of an outer pore reaching from the spider abdomen, protected by spiracles which are pores in the spider exoskeleton, composed of chitin.

Book lungs alone, respiratory organs with openings on the ventral surface of the abdomen, enrich the blood of primitive mygalomorph spiders with oxygen, and araneomorph spiders use spiracles as well. The latter are much more efficient and allow more advanced characteristics to be displayed, such as cursorial hunting (hunting involving rapid pursuit).

Reproduction

Spiders reproduce by eggs laid in silk bundles called egg sacs, and the male (usually significantly smaller than the female down to 1% for Tidarren sisyphoides) is likely to be killed by the female after the coupling, or sometimes before intercourse has occurred. This propensity is what gave the black widow spider (Latrodectus mactans) its name, but it is the Australian red-backed spider (Latrodectus hasselti) that seems to require the sacrifice of the male.

The male Latrodectus hasselti actually dies after it inserts its second palpus in the female genital opening even if the female does not eat it. The three species of North American black widows do not seem to require the death of the male (although it may occur) and males can sometimes live in the web of a female for a while without being harmed. There has been much speculation on why the sacrifice of male mates is so common, but it may simply be that female spiders tend not to discriminate between male spiders of their species and other similar small arthropods that are their normal prey.

Spiders often use elaborate mating rituals (especially in the visually advanced jumping spiders) to allow the male to approach close enough to inseminate the female without triggering a predatory response. Assuming that the approach signals are exchanged correctly, the male spider must make a timely departure after mating to escape before the female's normal predatory instincts come back into operation.

Male spiders use modified palpi to convey seminal fluid to the genital passages of the female. For a Tidarren male, the palpi constitute 20 % of its body mass. Since this weight greatly impedes its movement, the spider detaches one of the two to gain around 50% increased mobility.

When sexually mature, a male spider will spin a web pad onto which the contents of the abdominal reproductive organs are discharged and then the seminal fluid is transferred into the cavities of the palpi; when an individual secures a mate he thrusts the palpi one at a time into her abdominal genital openings. The seminal fluid is transmitted by a hypodermic-like structure on the male palpus.

Life cycle

The spider life cycle progresses through three stages: the embryonic, the larval, and the nympho-imaginal (Foelix, 1996).

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, 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 here until its death.

Ecology

Spiders have a great range of variation and lifestyle, although all are predatory.

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.

Predatory techniques

There are many families of spiders, and the ways that they catch prey are diverse. But whether they catch insects, fish, small mammals, small birds, or some other small form of life, as soon as a spider makes contact with its prey it will attempt to bite.

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 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 food. Many spiders will store prey temporarily while this process of external digestion is going on. The prey of web weaving spiders that have made a shroud of silk to quiet their struggles while they are dying from envenomation will generally leave the prey in these shrouds and then consume them at their leisure.

Spider webs and prey capture

Main article: Spider web

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.

The spider, after spinning its web, will then wait (often, but not always, at the center of the web) for a prey animal to become trapped. They 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.

Types of spiders

Over 37,000 species of spiders have been identified but, because of their great ability for hiding, it is believed there are nearer 200,000 species. Almost all species are venomous, but only 30 species are known to be deadly dangerous.

Tangleweb spiders

• Black widow spider (dangerous)
• Red-back spider (dangerous)
• American house spider (not dangerous)

Orb weavers

• St Andrew's Cross spider (also known as an argiope) (not dangerous)
• Long-jawed orb weaver (see above) (not dangerous)
• Cyclosa conica (not dangerous)
• Golden silk spider (see above) (not dangerous)

Other forms of webs

• Sydney funnel-web spider (dangerous)
• Bowl and doily spiders (not dangerous)
• Hobo spider (dangerous)
• Grass spiders (not dangerous)
• Filmy dome spider (not dangerous)
• Hackled orbweavers (no venom)

Spiders that live on flowers

• Crab spiders (Thomisidae) (not dangerous)

Hunting spiders

• Brazilian Wandering Spider (dangerous)
• Brown recluse spider (dangerous)
• Huntsman spiders (not dangerous)
• Jumping spiders (not dangerous)
• Lynx spiders (not dangerous)
• Nursery web spiders (not dangerous)
• Spitting spiders (not dangerous)
• Tarantulas (not dangerous)
• Wolf spiders (Lycosidae) (not dangerous)
• Yellow sac spider (dangerous)

Others

• Kimura-gumo (Liphistiomorphae) (not dangerous)

Spider bites

Most spiders are unlikely to bite humans because they do not identify humans as prey. Spiders, even small ones, may however bite humans when pinched. For instance, a common jumping spider (Family: Salticidae), around 3/8 inch (1 cm.) long, when pinched between the folds of a human's palm may inflict a bite that is about as painful as a bee sting.

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 actually removes one of the biological controls against incursions of the more dangerous species.

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.

Black widows

Normally, black widow spider bites are fatal only to children, due to the fact that children have much smaller body weights than adults and so the poison is more concentrated in their bodies when a bite does occur.

There are several widow spiders, i.e., spiders of the genus Latrodectus, and they are generally regarded as all being about equally venomous. Widow spiders are practically blind, and move with difficulty when they are not on their web. Unlike the other problematical spiders, the females of these most toxic of U.S. spiders stay on their web, and the males (which wander around seeking mates) are too small to deliver a dangerous amount of venom.

The website of the Carlsen Ranch asserts that almost half the black widow bites reported in the medical literature from 1901-1941 "were inflicted on the male genitalia by spiders on the underside of outdoor toilet seats." From the standpoint of a spider that likes to make its web under overturned wooden boxes resting on the ground, and in other such sheltered places near the ground, and which eats flies, an outdoor toilet is an almost ideal location. If a black widow happens to be resting near the front edge of the hole when a male human visitor sits down, she may find herself pinned against the wood by his penis, and bite it.

Brown recluse spiders

Brown recluse spiders frequently wander about and so are more easily trapped against one's skin by clothing, bed sheets, etc. The so-called "aggressive house spider" or hobo spider (Tegenaria agrestis) is also a wanderer that may come into contact with people and bite. The yellow sac spiders take shelter in silk tubes during the daytime and come out to hunt at night. People may unintentionally make contact with them in the dark and so be bitten.

Brown recluse spider bites can produce very severe local symptoms, death of tissue around the wound, and, sometimes, severe systemic symptoms. The bites of hobo spiders can cause both pain and necrosis (tissue death), and yellow sac spider bites can be extremely painful.

Huntsman spiders

The huntsman spiders have a worldwide reputation for scaring people. They are large, defend their nests, and may move toward people and make threat displays. They frequently enter houses and hunt over the walls and ceilings where they may run rapidly for long distances without pausing. When they actually do bite people, the bites are very unpleasant, but these spiders are not regarded as dangerous.

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.

Redback jumping spiders

Some people have reported being bitten by redback jumping spiders (Phidippus johnsoni). Most reports seem to be from California. These spiders have bright red abdomens (the females have a black stripe), and should be clearly visible even if they are not particularly large. It is unclear how the bites occur. Accidental contact seems rather unlikely since jumping spiders have excellent vision and could easily avoid being brushed by a human hand. It is also unlikely that they would mistake a human finger for their natural prey. Once source suggests that since they are quite attractive children may try to pick them up and in that way elicit a defensive bite. Fortunately, the worst consequences reported have been three to four days of discomfort, with no permanent damage. Since they do not frequent human habitations it should ordinarily be easy to avoid unpleasant contact with them.

Brazilian wandering spiders and Sydney funnel-web spiders

The Brazilian wandering spider and the Sydney funnel-web spider frequently bite people and are regarded as among the most dangerous in the world. The Sydney funnel-web spider is restricted to a relatively small area around Sydney, Australia. The Brazilian wandering spider reportedly may hitch a ride in clusters of bananas. As a result, any large spider appearing in a bunch of bananas should be treated with due care.

Further reading

• The Life of the Spider by John Crompton. Mentor, 1950.
• How to Know the Spiders by B. J. Kaston. Dubuque, 1953.
• Biology of Spiders, by Rainer F. Foelix, second edition, 1996

External links

• Arachnology Home Pages: Araneae
• Spider info by Ed Nieuwenhuys
• List of spiders found in Great Britain
• Platnick, N.I. 2004. World Spider Catalog
• Webs made by spiders fed on drug-dosed flies