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There are round 34 000 species worldwide of those invertebrates, classified in the order of Araneae with 105 families.
They are mostly terrestrial but a few can live in water and are relatively small, being less than one cm but the biggest have a body length of 9cm.

Spiders have a two-part body divided into an anterior part called cephalothorax and a posterior abdomen called opisthosoma separated by a narrow stalk or  pedicel.
The cephalothorax carries 4 pairs of simple eyes, fangs with poison glands, a pair of pedipalps, which are feelers and where in the case of the male the sexual organ is located and 4 pairs of walking legs.
In the posterior abdomen are located the spinnerets which is used in secreting silk. There are also respiratory openings or tubes for respiration.

There are basically two kinds of spiders, the web spinners which build webs to trap insects and the hunting spiders, which lay in wait or actively pursue and catch their prey without a web.
Although not building webs the hunters are still able to produce silk. Spiders lay their eggs in a silk wrapped cocoon guarded by the female or carried on her body. From those, small spiders hatch without undergoing metamorphosis.

Spider's prey
Once a spider caught its prey either trapped in the web or hunted, it injects it with venom from its poison glands and wraps it up using silk. Its digestive system can only take up liquid food, therefore it sucks the juices out of its trapped victim and leaves then an empty shell.

All spiders are venomous and mostly carnivorous feeding on live prey. They all possess poison glands which are connected with the fangs and thus are able to inject their victims with venom. To humans the bite of most spiders is not harmful as it cannot pierce the human skin. Some bigger spiders, however can do so with the Black widow spiders being the most hurtful to humans. They only bite in  self defense and  in healthy humans the bites are never mortal.

Spiders are generally useful to humans by devouring insects. The silk, they produce is very strong and are used for cross hairs of optical instruments.
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