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Spiders, and Their Amazing Hydraulic Legs and Genitalia

A big, hairy spider skitters across the floor of your room; its eight, long, rangy legs moving almost robotically. It stops. Horrified, you picture its fangs clicking together, and its many eyes jerkily twitching to and fro. Suddenly, the spider scuttles under your bed. Though you wait five minutes for it to reappear, it does not. Will you be able to sleep tonight, knowing that a monster lurks below?

Spiders give us the willies for a host of reasons. Their arced fangs, orb-like eyes, and segmented bodies all feed our trepidation, but the primary reason for our seemingly primordial arachnophobia likely stems from how they move. The way spiders motion their angular, jointed legs irks us like no other, psychologists say.



So why do spiders move in this unnatural, mechanical fashion? Well, it's because they fundamentally move differently than we do. Arachnid locomotion actually makes use of the same force that powers a variety of mechanical instruments: hydraulics.

Hydraulics is the process whereby power is generated, controlled, and transmitted through the use of pressurized liquids. A basic way to understand it is to picture two liquid-filled cylinders, each topped with a piston and connected by a pipe. If you apply pressure to one piston by pushing it down, the force is transferred through the liquid to the other piston, and this piston rises with the liquid. Dump trucks, skid loaders, and pumps are all man-made machines that operate on hydraulics.

While spiders have muscles to flex their spindly limbs inward, they use hydraulic pressure to extend them outward. Almost all other limbed animals have both flexor and extensor muscles, which produce smoother, less jarring, and much less unsettling movements.

To extend their legs, spiders rapidly increase pressure in their cephalothorax -- the round, bulb-like midsection to which all the legs are connected. This increase in pressure sends hemolymph (blood) flowing to the extremities, causing the legs to stretch outward. When moving, spiders innately increase and decrease body pressure in fractions of a second to quickly skitter about.

shutterstock_124988432.jpgFar from only powering basic locomotion, hydraulics endows certain spider species with prodigious jumping abilities. Jumping spiders can leap more than fifty times their own body length by swiftly boosting blood pressure in their third and fourth limbs.

Male spiders of the subgroup Entelegynae also utilize hydraulics for another function: to power their genitalia. It's theorized that hydraulics may enhance genital mobility, potentially improving the lock between male and female spiders during mating.

Though hydraulics obviously has its advantages, the fact that a spider is basically a hydraulic system gives rise to a key vulnerability. If a spider's cephalothorax is punctured, movement will be severely inhibited, as the system's pressure will drop precipitously. This also explains why the legs of dead spiders curl inward. The hydraulic system has been taken completely offline, while the flexor muscles go into rigor mortis.

Rigor mortis is a state that many of us would like to see spiders in, but remember, though these eight-legged critters may peak our ingrained fears, they aren't inherently vile or vicious. They construct beautiful works of art in the form of pristine webs and prey upon the animal that is perhaps the biggest zoological threat to mankind: the pesky mosquito.

(Image: Wolf Spider via Shutterstock)