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More About Spiders ...
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.

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).

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.

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/m of insects and other arthropods.

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.
Phylum: Arthropoda
Class: arachnida
Order: araneae
Family: Mesothelae
Source: Wikipedia Read more about Spiders
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