Saturday, January 7, 2012

Spiders Have Huge Brains

Do you see spiders lurking in their webs at the corner of your house? Disgusting? Scary?
Don't think of it that way. These arachnids are incredible creatures.

We all know from the movie Spiderman that spiders are lethal and have amazing physical abilities. Jumping spiders, for example, can jump several times the length of their body. The silk they produce is stronger than steel and tougher than Kevlar. A thread with a radius of 3-cm can stop a Boeing 747 in full flight. And tiny spiders have brains so huge the neurons spill into their legs.

Horizontal sections of several spider species showing extension of neurons (black arrows) into leg joints in tiny individuals. 
Image: R. Quesada et al. , Arthropod Structure & Development, 40 (2011) 521-529
Scientists were baffled as to why young spiders are able to build complicated webs like adult spiders. Well, their enormous brains may hold the answer. Our brain makes up only 2 to 3 percent of our body weight. A spider brain takes up almost 80 percent of the animal's body cavity, and it spills into its leg, making up about a quarter the mass of its legs. Insects like ants and minute beetles have brains that weigh roughly 15 percent of their weight.
Dorsal view showing the cephalothorax (C), the supra-oesophageal (white arrow) and sub-oesophageal (black arrow) ganglia of adult female orb-weaver Anapisona simoni (A) and Faiditus elevatus (B). 
 Image: R. Quesada et al. , Arthropod Structure & Development, 40 (2011) 521-529
The tiny lump of fat in our head consumes 20% of the body's energy. Similarly, much of the energy netted by the tiniest of spiders goes straight to their head. In fact, the percentage of space devoted to cognition in spider brains dwarfs that of humans.
Given the huge brain size, why are they not the dominant species on Earth?

For biology-majors, you have probably heard of Haller's rule, which holds that the brains of smaller animals are larger relative to body size than large-bodied forms. Well this is a classic case of animals adhering to the rule. Human beings do not have the heaviest brains in overall weight, or even weight in proportion to their bodies, but our brains appreciate humor, beauty, death, and many more, whereas animal behaviors are driven largely by instinct.
The power of contemplation only exists in the minds of human and some highly intelligent animals--well at least that's what scientists found so far.

Size doesn't matter; it's the way the brains work that does.
But still, whenever you choose to smack a tiny spider, remember that it is way brainier than you can ever be.

R. Quesada et al. , Arthropod Structure & Development, 40 (2011) 521-52

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