Friday, October 15, 2010

Spooky Spiders…

October seems the appropriate month to continue on the theme of spiders and other creepy crawlies who happily decorate our homes right now in larger-than-life form. Last time I wrote about spider silk, and its amazing strength. Spiders are biological wonders from several other perspectives as well. Notably, their long and leggy legs.

First, have you ever noticed that when you find a dead spider, that its legs are all curled up (so long as it is not smashed, that is)? Their legs roll inward towards their bodies and form a sort of ball. This is because spider legs are hydraulic, like pistons. They are held rigid by the fluid inside them, and that fluid must be maintained at pressure in order for them to hold the spider up and allow it to move around. Once the spider dies, the pressure cannot be maintained, and the legs collapse.

To move, the spider has muscles within the legs that allow the seven (yes seven) sections of each leg to bend inward a coordinated manner. But, to straighten the leg, there are no opposing muscles in most spiders, because there is nothing to attach them too. Remember spiders have no bone, just their tough exoskeleton that makes up what we think of as their crunchy skin or shell. The fluid pressure in the leg is changed via changes in the spider’s blood pressure. Increases in pressure straighten the leg, decreases allow it to bend.

Jumping spiders are by far the best at this feat. They can quickly change the blood pressure in the legs which allows them to spring upwards, travelling as far as 25 times their body length. That’d be 25 to 30 feet for the average human. None of us can pull that off.

Running spiders are special again in their own way. They have given up web building entirely for the purposes of capturing food, and obtain their meals hunting them down and then overtaking them with sheer speed. Our common wolf spiders often hunt this way. Luckily, even the largest of wolf spiders is only about an inch long, because spiders running you down is enough to unnerve just about anyone. They play an important role in the control of other insects, however, so be thankful that they are there.

Fishing spiders, however, win the award for amazing legs. Fishing spiders can literally walk on water. They do this by abusing the laws of physics, and taking advantage of simple surface tension. Surface tension is the tendency of particles of water to stick together. This is why you see a drop of water form a round, bead-like drop, and not just scatter into its infinitesimally small molecules. Fishing spiders have tiny leg tips, and light bodies, and when they press these leg tips to the surface of the water, they are able to stay atop the surface, and not break the surface tension. This is due largely to a waxy coating on the legs. You see a small dimple form on the water from the pressure of the leg, but the tension does not break, unless the spider wants it to. These spiders can dash out onto the water to grab prey such as insects, or reach below the surface to grab even small fish. And, when they really need to move, they can rise up on two legs and gallop across the water, reaching speeds of about 30 feet per second, or 2 miles per hour. The average running speed for a person is 6 miles per hour. The world record 100 m dash clocked in at 28 miles per hour. But, when they are just hanging around, or casually need to move from place to place, fishing spiders can also place their legs on the water and sail with the wind.

Not a bad way to get around.

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