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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 related to scorpions, mites, ticks and daddy longlegs,
with which they make up the Arachnids. They are
believed to be one of the first land animals
becoming terrestrial some 400 million years ago. Although mostly
terrestrial, a few can live in water
by trapping air bubbles underwater. Spiders are relatively small,
with most having a body length
of less than one centimeter but
the biggest ones can reach some nine cm
like in the case of tarantulas. Their long legs however gives them
a much bigger appearance. |
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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. |
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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. |
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The silk is secreted as a liquid
from the back spinnerets. Those fibrous proteins solidify at contact with the
air to form strong and elastic polymers. One species of
spider can prodce various kinds of silk, which is used for many purposes, like forming a cocoon for laid
eggs, constructing a parachute, wrapping up caught victims (like
in the picture to the right) and of course for building spider webs. |
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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 harmful 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 is used for
crosshairs of optical instruments. |
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Spider Webs
range from simple ones, put on the ground to very elaborate
ones, enveloping whole shrubs (like picture above) or trees
spanning several meters. In all cases
they are used to
trap insects, which unwillingly get entangled in them
and are then devoured by the spiders. |
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Hunting Spiders on the other
hand do not use webs and often
use
camouflage to get close to
catch their prey, which are
often other invertebrates.
Day-hunting species hunt by vision and the nocturnal ones by
touch.
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 an empty shell. |
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