Wisdom of the spider
The mornings have been cool and misty this week and I have noticed webs on the fence wires on more than one occasion. Within an hour or so the dew evaporates as the air warms. The breeze hastens and this dismantles the laborious work of the night before. I like the image below. I like the way in which the dark parts of each dew droplet are highlighted in the upper half of the photo and how the light parts of each droplet are highlighted in the lower part of the photo. I like the way in which the inner-most hub looks like a densely populated solar map, complete with sun and with six planetary objects and their associated moons. The business of constructing a web such as this is a fascinating one. It involves the formation of a simple infrastructure comprised of drop lines, a perimeter, and a series of spokes which radiate from the center. Once these elements are established the spider spins an auxiliary spiral of non-sticky threads from the inner hub to the perimeter of the web. The animal then turns and deposits a sticky, capture spiral, using the auxiliary spiral as scaffolding along which to negotiate the web and as a guide. Once the web is finished the spider travels about by moving along the radial threads and traversing the auxiliary spiral. Unwary prey are caught by the sticky elements of the capture spiral. As I thought about the cognitive power needed to carry out this sort of construction I wondered about the size and complexity of the brain of a spider. I was surprised to learn that, for their size, spiders have some of the largest brains of any organism we know. Workers at the Universidad de Costa Rica and at the Smithsonian Tropical Research Institute showed that the central nervous system of the smallest spiders occupy nearly 80% of their total body space, including 25% of the volume of the legs. They suspect that very little spiders might be mostly brain. Haller’s rule predicts that as body size goes down, the proportion of the body taken up by the brain increases. Human brains represent just 2-3% of body mass. The brains of some of the tiniest ants represent nearly 15% of their biomass, and some very small spiders are much smaller than ants. [The preceding facts were taken from a 2011 issue of Smithsonian Science.] Something called an encephalization quotient measures the ratio between actual brain mass and that predicted for an animal of a given size; it is thought by some to reflect cognitive ability or intelligence. By this measure, the Jumping Spider is recognized as having the highest EQ among invertebrate animals. Consider, for a moment, that the reason we are quick to marvel at the ability of a spider to create such a structure as intricate as a web is because we can’t imagine doing it ourselves, on whatever scale, within an hour, and at least once each day. Let alone creating the silk with which to build the construction, let alone determining whether a leaf or a tiny insect, or a dangerous wasp or a preferred prey item has encountered the web by sensing the vibrations which travel the signal line to highly sensitive legs. Marvel at that for a moment and then consider where you’d like to place the spider along the entirely fictitious Great Chain of Being!