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ANATOMY

THE SPIDER'S SENSE OF TOUCH:

If you were a spider, you would have a very good sense of touch, but you wouldn't be able to see much more than light, dark, and basic shapes. Spiders learn more about the world around them by feeling vibrations, than they learn from using their eyes. They can tell what is happening around them by feeling the differences between different vibrations. Eg: wind & trapped insects can both make the spider's web move, but the spider can tell the difference between the jerky, uneven vibrations caused by the insect & the gentle , smooth vibrations caused by the wind. When trying to mate with a female,many male spiders will cause the female's web to vibrate in a special way that tells her that he is another spider, & not an insect to be eaten.

THE SPIDER'S HAIR & ITS FUNCTIONS:

All of the hairs on a spider 's body do not do the same thing, and each hair is sensitive in many ways. Some hairs help the spider hold on to different kinds of surfaces. Each of these "holding haires" divide into thousands of smaller hairs which are like many tiny hands, giving the spider a better grip. Other types of hairs are used by spiders as little combs to keep their silk from getting tangled. Still other hairs sense movement around the spider.
Also see 'Urticating Hairs'.

SENSORY ORGANS:

Spiders have as many as eight simple eyes, arranged in two groups, but, though some spiders can see images, none have eyes as well developed as those of the insects. Instead, the world of spiders is one of vibrations that are sensed through the surface on which the spider lives. Such a world is almost unknown to humans. For example, imagine tightrope artists walking on a network of fine threads and communicating by plucking and vibrating the surrounding threads. This is the world of web-weaving spiders. All of their activities-including feeding, mating, and egg laying-take place while they are suspended from silk threads.

HOW SPIDER LEGS WORK:

The seven segments of a spider's legs make them much more flexible than human legs, which only have two segments. Like humans, spiders have muscles that bend the legs closer to the body. However, spiders do not have muscles that move the legs away from the body. Each time a spider needs to stretch a leg back out, it must pump fluid into that leg. Then, to bend the leg back, pressure is relaxed and the fluid flows out of the leg as the muscles do their work. This is similar to the way a garden hose gets stiff and moves around when filled with water, and then gets limp again when the water drains out.

SPIDER SILK:

Spider silk is made inside the spider's body as a liquid leaves the body through little tubes called spinnerets. These spinnerets have "faucets" or valves on them that the spider can turn on or off when it wants. When the spider opens this "faucet", pressure inside it's body helps to push the liquid silk out through the spinnerets. The silk becomes solid as it leaves the body. The spider uses it's legs to help pull the silk out of the spinnerets.

CIRCULATION AND LOCOMOTION:

Spiders have what is called an open circulatory system. The heart pumps blood through a series of vessels and arteries, but spiders lack the complex system of capillaries that in vertebrates exchange oxygen, nutrients, and wastes between the blood and body tissues. Instead, blood seeps between the spider's tissues, collects in little pockets on the underside of the body, and flows back to the heart. Not all blood passes through the spiders respiratory organs. An efficient, high-pressure ciculatory system is crucila for a spider's locomotion. Spiders have seven leg segments, and their movements are controlled by muscles and by pressure changes in the body's cirulatory fluid. Spiders use muscles to retract their legs, but they lack extensor muscles. Instead, spiders extend their legs by means of changes in body-fluid pressure. When spiders do not receive enough water to replenish their body fluids, their legs fold up and they are unable to extend them.

RESPIRATION:

Spiders have different types of respiratory systems. Some have book lungs, some have tubular tracheae, and others have both tracheae and book lungs. Book lungs are located by two hairless patches on the underside of the spider's abdomen. Each lung has an open slit for air intact and a stack of leafletlike, blood filled structures called lamellae. As air passes into the spider's body, blood passes through the lamellae is oxygenated.

EXTERNAL ANATOMY:

Like insects, spiders have a hard cuticle or body shell, called an exoskeleton. The cuticle covers the cephalothorax and legs and prevents the spider from losing moisture and drying out. In addition, the cuticle provides the spider with structural support. Spiders also have an internal skeleton that is actually an extension of the external cuticle. This internal skeleton serves as a surface for muscle attachment. Unlike insects, spiders have no antennae. They do, however, have two appendages near their mouths that are often confused with insect antennaes. These structures, called pedipalps, are used by spiders to manipulate their prey while feeding. The palps of immature males are expanded and look like boxing gloves. As the male matures the palps are transformed into highly complex organs that are used to inseminate females. The females palps are slender.

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