Spider

A spider is an invertebrate animal with eight legs belonging to the order Araneae in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropods. This order is further divided into 3 sub-orders. The Mygalomorphae[?] (the primitive spiders), the Aranaeomorphae[?] (the modern spiders) and the Mesothelae[?], with one family of spiders the Liphistiidae[?].

Many spiders hunt by building webs[?] to trap insects; these webs are made of spider silk extruded from spinnerets[?] on the end of the abdomen, a thin, strong protein strand extruded by the spider. All spiders produce silk, even those which do not spin elaborate traps with them. Silk can be used to aid in climbing, forming smooth walls for burrows, coccooning prey, and many other applications.

Spiders, unlike insects, have their bodies divided in only two segments: prosoma[?] or cephalothorax (a fused head and thorax[?]) and abdomen. Some species, like Tarantula[?], have urticating hairs all over their bodies. Spiders have 8 legs compared to the insects' 6, and their eyes (usually 8) are single lenses rather than compound eyes[?] like insects have. Eyes can be arranged differently by different species. Sometimes one pair is better developed than the rest. Some species have a pair less or are even without eyes. This is very strange for a predator. How to hunt, if you're almost blind, deaf and you don't smell? The answer lies in the incredible sensitivity for vibrations. Insects have feelers but spiders don't.

Digestion is carried out internally and externally. The spider injects a venom into its prey[?] from a series of ducts perforating its jaws, dissolveing the prey's internals. Then, the spider feeds by sucking them out. Spiders consume only liquid food. Spiders will typically store prey temporarily while this is done, wrapping them in a coccoon of silk. Most spiders are too small to bite a human, though some are capable of inflicting painful or even dangerous injuries if threatened and prevented from escaping.

Spider blood does not circulate through vessels, it just fills the body of the spider. This is called an open circulatory system. Book lungs, respiratory organs with openings on the ventral surface of the abdomen, enrich the blood with oxygen.

They reproduce by eggs laid in silk bundles called egg sacs, and the male (usually significantly smaller than the female) is often exposed to being killed by the female before or after the coupling. This propensity is what gave the "black widow" spider (Latrodectus tredecimguttatus) its name, and there has been much speculation on why it 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 to allow the male to approach close enough to inseminate the female without triggering a predatory response.

Male spiders use modified palpi to convey seminal fluid to the genital passages of the female. When sexually mature, a male spider will spin a web into which the contents of the reproductive organs are discharged and then transferred into the cavities of the palpi; when an individual secures a mate he thrusts the palpi one at a time into her genital openings.

Spiders have a great range of variation and lifestyle, though all are predatory. Some 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 are the most important link in the regulation of the number of insects. Every day they devour over 100 kg on 1 hectare of a meadow.

Spiders do not usually stick 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.

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

• Brown recluse spider
• Tarantula[?]
• Black widow spider
• Kimura-gumo
• Thomisidae -- Crab spiders

Other species of spider:

• St Andrew's Cross spider/argiope

Sources

1. Spider info by Ed Nieuwenhuys (http://www.xs4all.nl/~ednieuw/Spiders/Info/spiderinfo.htm)

The spider is sometimes used as symbol for scariness, for example in the Dutch movie rating system. Spider lovers are not happy with this.

Web crawlers, automated software agents that gather pages from the World Wide Web are also known as spiders.

A spider is a frothy drink made from soft drink (soda), flavouring and ice cream. Spiders can also be made by adding ice cream to beer (beer spiders).