"New" Spider Species Weaves Uncommonly Regular Webs
National Geographic Channel
|June 24, 2004|
As Spider-Man 2 swings into movie theaters, a new species of spider has been discovered, and it has a knack for creating well-ordered webs.
Until now there were just four known instances of spiders evolving the ability to measure and create symmetrical webs: The fifth was discovered in Peru last month, prompting questions as to how and why some spiders develop the skill.
"It's interesting because it doesn't make any sense. There doesn't seem to be any advantage to having a symmetrical web, yet it evolved independently among spiders more than once," said Jonathan Coddington, a senior scientist at the Smithsonian Institution in Washington, D.C. Coddington has studied spiders for over twenty years.
"It's not possible that this is a just random drift in evolution and these spiders are stumbling into the ability to measure things. It must have evolved for a reason, but we don't know what that reason is yet."
The Webs They Weave
Of the more than 37,000 species of spiders, all of them can make silk, but only about half use the silk to spin webs. The rest use silk to wrap prey or eggs; weave small, temporary shelters; or create a safety line if they are jumping, Spider-Man style.
The silk emerges from short, muscular projections called spinnerets, usually at the posterior of the abdomen. "The silk is in liquid form in their abdomen which emerges as a solid thread: Researchers are still working out how that happens," explained Linda Rayor, an assistant professor in the department of entomology at Cornell University in Ithaca, New York.
"The silk is stronger than steel for its width and of course far more flexible. It can stretch to 200 times its length."
Each species of spider has a style of weaving that is both innate and easily recognizable to an expert. "Show me any web on Earth and I can tell you what species of spider built it," Coddington said. "Just like an art expert can recognize a Michelangelo or a van Gogh as soon as they see one."
However, just as each painting is unique, each web is customized by a spider to fit the specific space where they're building. "Spiders will alter the design of the web based on wind conditions or the surrounding vegetation," explained Robert B. Suter, professor of biology at Vassar College in Poughkeepsie, New York.
Among the best known symmetrical webs are those of orb spiders. "There are around 5,000 species that spin orb webs," Coddington said.
Orb webs are circular with radial supports jutting out from the middle and a spiral woven in to ensnare prey.
"We don't have any proven reason as to why these or other spiders create symmetrical webs. They don't catch more prey then irregularly formed webs, so why bother with such a well measured design?" Coddington said.
Most regularity in nature evolved for one of two reasons. It's either a function of growth rate, which accounts for the spiral of a nautilus shell or a snail shell, "or it's a packing issue," Coddington said. "If there are a set number of identically shaped objects, the most efficient way to pack them is in a regular array, like a sunflower's seeds."
Neither of these reasons explain why there is symmetry in a spiderweb. However, Rayor suspects that for webs with radial symmetry, the answer is a matter of biodynamics. For a web to be effective, it needs to be built so that an insect doesn't snap the web or bounce out of it.
"As the insect crashes into the web, that impact has to be absorbed by the radial threads. So perhaps the advantage in symmetry is that the force is spread more evenly throughout the web and reduces the thrust, making the web less likely to tear," Rayor said
New Spider Discovered in Peru
The species Coddington recently discovered lives in Peru, though he surmises it probably exists throughout the Amazon Basin.
The species has no common name yet, so it's referred to by its Latin name, Ochyroceratidae. Adults are an iridescent blue or purple, less than half a centimeter (one-fifth of an inch) in length.
Coddington was in the leaf litter on the forest floor with a group of Peruvian students when he first spotted an Ochyroceratidae web.
"Leaf litter, from a bug's point of view, is a catacomb of caverns, tunnels, and amphitheaters. There is an entire fauna that exists there," Coddington said. "It is the bottom of the food chain, where insects and the like are eating leaves, causing them to decompose and turn into soil."
For the tiny animals that live among leaf litter, something as simple as the curl of a dead leaf is like the opening of a cavern. Ochyroceratidae spiders weave webs to cover those openings and catch bugs as they are coming and going.
"When I looked closely at their webs, I realized they were made of regular arrays of fibers. But the strange thing is that the arrays are irregularly assembled," Coddington said.
He compared it to making a pile of bricks, each created identically, which are then thrown together in a random assemblage.
"The pieces of the web are regular, but the overall web is not. So these spiders have evolved this very rare trait of being able to measure something out in regular intervals," Coddington said, "but for reasons I can't explain, they don't exploit that ability like other spiders do by creating symmetrical webs."
The Search for New Spiders
Coddington's next goal is to examine whether other spiders in the same family also measure out their webs in regular intervals. "They probably do, but no one's ever looked."
The more than 37,000 identified species of spiders probably represent only one-third to one-fifth of what is actually out there. "There are far more undescribed, unknown species of spiders on Earth than what is known. We are not even over the hump," Coddington said. "That's what makes my job so interesting."
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