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Spiders:
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| Jumping spiders, such as this one, belong to the family Salticidae. Courtesy of Ed Nieuwenhuys. Learn more about spiders at these sites: "Arachnology Home Page" and "Spiders of North-West Europe". |
Spiders have a peculiar system by which they move their legs. We are used to the "normal" way of moving limbs with muscles and sinews. Spiders, on the other hand, use the same technique as in robots or car brakes - hydraulic pressure.
It has been known for long that spiders cannot extend their legs with the aid of muscles. But the method by which they actually extend them was still to be revealed.
If muscles are not used - then what? Two theories were quickly proposed: the use of elastic forces (it was known from other animals that elastic forces could be strong enough) or the use of hydraulic pressure. However, it was revealed that no leg extension by elastic forces occurred. Then it had to be hydraulic pressure. But how? What kind of fluid is used? Have spiders got a special fluid, like the brake-fluid in cars? How are the pressure developed?
Actually it was discovered that spiders use their blood as hydraulic fluid. The blood pressures in spiders are, in fact, substantially higher than those in other related animals. This is a necessity for the use of their hydraulic system.
What causes the pressure increase needed to extend the legs? This question has not yet been settled, but they probably use muscles to compress the forepart of their body, thereby increasing the blood pressure inside it.
The hydraulic system seems to be a slow, laborious way to move, but some spiders are really fast, for example a group called jumping spiders. They are small, from less than one centimeter to a couple of centimeters. Some jumping spiders catch their prey with a, for their size, huge jump of about twenty centimeters. The jump is produced entirely by the hydraulic forces created in their last (fourth) pair of legs. In fact, the jump is so fast that the exact movement can only be followed by video recording. There may be even more extreme examples. Possibly, some South American species may to jump over one meter.
References
K. Schmidt-Nielsen: Animal Physiology (5th edition, Cambridge University
Press, 1997).
J.W. Schultz: Evolution of locomotion in Arachnida: the hydraulic pressure
pump of the giant whipscorpion, Mastigoproctus giganteus (Uropygi)
(Journal of Morphology 210:13-31, 1991).