Attack of the killer spiders!
Giant killer spiders are the stuff of Hollywood legend. In fact not just spiders as snakes, gorillas, and ants have all had their proportions dramatically increased all in the name of a Hollywood blockbuster. So would it be possible for a giant spider to exist? That is the question we pose today.
So what is the world's largest known spider? That would be the Goliath Bird-eater Tarantula. They are known to have up to a 12 inch leg span and weigh over 6 ounces. It is native to the rain-forests of South America and is commonly found in marshy or swampy areas. Despite its name the Goliath Bird-eater normally eats insects and other invertebrates. Like most tarantulas it is fairly harmless to humans.
The Tarantula family produce some of the largest spiders in the animal kingdom.
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However even though a foot wide spider is likely to scare any number of people it certainly would not be classified as giant by Hollywood standards. Take for example the 2002 horror/comedy film "Eight Legged Freaks" in which a small town is terrorized by four foot tall spiders. Could these types of spiders possibly exist?
If you look at the animal kingdom there is a great deal of variety in very small creatures as opposed to larger animals. Spiders, bees, centipedes, worms and ants all have remarkably different body structures. On the other hand larger creatures such as cows, tigers, dogs and elephants all have four legs, a tail and the same rough proportions. Why is this? The reason is simple: volume versus area.
Take your own body for example. Imagine if it was twice as wide, twice as tall and twice as deep. The proportions of your body would still be exactly the same except that you would be eight times (2x2x2 = 8) heavier. However the surface area of your body would only be four times (2x2 = 4) larger. So what does this mean? It means that when you double your size your volume increases twice as much as your surface area. So even though the surface area of the soles of your feet have increased four times, they have to support eight times the weight. Your lungs have increased four times but they now have to supply oxygen to a body eight times its original size. As a result you would need to breath twice as fast to get the same amount of oxygen into your body. There are many other aspects of your body that also fall into this category of volume versus area. The larger you get the worse it becomes. If you increased your size four times your volume would increase 64 times but your surface area would only increase 16 times. As a result small creatures can often have remarkably different shapes as the volume to surface ratio is quite low.
As another example imagine a drop of water on a leaf. It is not possible to scale this up as the weight of the water is perfectly balanced by the surface tension of the water drop. The moment you try to scale it up the volume to surface ratio problem results in a puddle rather than a big ball of water.
It is this volume to surface problem that limits how large a spider can be. Larger animals have already evolved into the most efficient shape for their size which is typically four legged with a sturdy body.
While Hollywood continues to fire our imaginations with tales of giant spiders wreaking havoc just remember that in reality there is no way that this could happen. Of course this doesn't mean that a foot long Goliath Bird-eater Tarantula couldn't crawl over your face in the middle of the night...
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