01 April 2012

Kinds of Spiders and the Secret Behind The Strength of Their Webs

Spiders weave their own silk to build webs. It is this silk that is known to be one of the strongest materials to exists. On a pound per pound basis, spider silk is stronger than steel.

Researchers have studied and analyzed the structure of the spider web to discover what makes spider webs resilient and how these properties relate to the molecular structure of silk fibers. Research such as these may open up the possibility of synthesizing spider web for practical real world applications as well as apply the same knowledge to design principles that might apply to networked systems such as the Internet or the electric grid.

A key property of spider silk that helps make webs robust is something previously considered a weakness: the way it can stretch and soften at first when pulled, and then stiffen again as the force of the pulling increases.

This stiffening response is crucial to the way spider silk resists damage. Researchers at MIT analyzed how materials with different properties, arranged in the same web pattern, respond to localized stresses. They found that materials with other responses — those that either behave as a simple linear spring as they’re pulled, or start out stretchy and then become more “plastic” — perform much less effectively.

White-tailed spiders are vagrant hunters that live beneath bark, rocks, and in leaf litter and logs, and gardens, and are often seen in houses. Tufts of specialized scopulate hairs on the ends of their legs allow them to walk easily on smooth or sloping surfaces. They make temporary silk retreats and spin disc-shaped egg sacks, each containing up to 90 eggs.
Spider webs, it turns out, can take quite a beating without failing. Damage tends to be localized, affecting just a few threads — the place where a bug got caught in the web and flailed around, for example. This localized damage can simply be repaired, rather than replaced, or even left alone if the web continues to function as before. Markus Buehler, an associate professor of civil and environmental engineering (CEE) at MIT, says, "Even if it has a lot of defects, the web actually still functions mechanically virtually the same way. It’s a very flaw-tolerant system.”

Video: The Effect of External Factors on the Construction of Spider Webs

They tested actual spider webs by poking and pulling at them. In all cases, damage was limited to the immediate area they disturbed.

The effect was somewhat surprising, Buehler says: The initial response was a deformation of the entire web, since the strands are initially relatively easy to deform. But then, because of the fibers’ nonlinear response, only the threads where the force was applied carried the load — by stretching out and then becoming stiff. As the force increased, they eventually broke.

“No matter where you pull, the web always fails exactly at that location,” Buehler says. Anyone can try this simple experiment, he adds: Simply pluck a single silk thread from a spider web, and it should break only where it’s pulled. In a web made of material with a more uniform stretching response, by contrast, local stresses cause much more widespread damage.

Daddy Long Legs: These are also known as pholcus and can easily be identified by their long legs. These spiders usually can be found hanging upside down in a loose web in the corner of the ceiling. Its strategy is to remain motionless until a prey passes, which are mostly other spiders. It has small jaws called chelicera that can deliver a very powerful poison. Its long legs are very manoeuvrable which aside from its practical uses for hunting, can also be used for hiding. The Daddy Long Legs can make itself appear invisible when needed by rapidly shaking when hanging in their web. This creates a motion-blur effect that makes them hard to see.

In a strong wind, on the other hand, it’s the initial stiffness of the silk that helps a web survive. Webs in Buehler’s simulation were able to tolerate winds up to almost hurricane strength before tearing apart.

Engineers tend to focus on materials with uniform, linear responses, Buehler says, because their properties are so much easier to calculate. But this research suggests that there could be important advantages to materials whose responses are more complex. In the unusual response of spider silk, for example — initially stiff, then stretchy, then stiff again — “each little piece of that funny behavior has a fundamental role to play” in making the whole web so robust, he says. Materials with the same ultimate strength, as measured by their breaking point, often perform very differently in real-world applications. “The actual strength is not so important, it’s how you get there,” he says.

Redbacks are very popular in Australia except in the colder regions like Tasmania. These spiders thrive in populated areas and because of this, hundreds of bites are reported each year. A little less than 30 percent of those bitten require antivenom treatment.
The basic principle of permitting localized damage so that an overall structure can survive, Buehler says, could end up guiding structural engineers. For example, earthquake-resistant buildings are generally designed to protect the whole building by dissipating energy, reducing the load on the structure. When they fail, they tend to do so in their entirety.

A new design might allow the building to flex up to a point, but then certain specific structural elements could break first, allowing the rest of the structure to survive; this might ultimately allow the building to be repaired rather than demolished. Similar principles might apply to the design of airplanes or armored vehicles that could resist localized damage and keep functioning.
As the name suggests, the Daring Jumping Spider gets its name because of its leaping ability. The spider are between 1/4 inch and 1/2 inch long while the female spiders are larger. They live in woods, fields, or gardens and are often seen on tree trunks, fallen limbs, leaves, or other ground litter. These spiders do not build webs to catch their prey, but  use silk to make a small shelter under a leaf or bark. They hunt their prey on foot. To catch prey, Daring Jumping Spiders sneak up on it, then pounce. They can leap great distances when considering their size.

Such “sacrificial elements” might be used not just for physical objects but also in the design of networked systems: For example, a computer experiencing a virus attack could be designed to shut down instantly, before its problems propagate. Someday, then, the World Wide Web might actually be strengthened thanks to lessons learned from the backyard version that inspired its name.

“It’s a real opportunity,” Buehler says. “It opens a new design variable for engineering.”

David Kaplan, a professor of engineering at Tufts University and director of its Center for Biological Engineering, calls these findings “quite exciting.” He says, “The combination of modeling and experiment makes this particularly attractive as a platform for study and inquiry into materials designs and failure modes in general, with structural hierarchy in mind.”

Funnel-web spiders are large spiders (1.5 - 4.5 cm body length) with glossy dark brown to black carapace. The abdomen is usually dark plum to black. The female funnel-web spider spends most of its life in their burrows, but do occasionally hunt on the surface at night. Adult males however leave their burrows and wander in search of females, particularly during summer and autumn.

“These principles, I believe, will have an impact in a wide range of fields such as medicine, future materials and architecture,” adds Philip LeDuc, a professor of mechanical engineering at Carnegie Mellon University.

This work was supported by the Office of Naval Research, the National Science Foundation, the Army Research Office and the MIT-Italy Program.

The most popular and famous of all spiders is Spiderman. When not fighting crime, Spiderman is Daily Bugle photographer Peter Parker. He cannot manufacture his own web but because of his expert knowledge of chemistry, he synthesizes his own web and ejects them thru web shooters built into his costume.
Hope you enjoyed the April Fools Spider Web Video if you did see it. Of course, everything else written here is accurate based on published research.


How spider webs achieve their strength
Department of Civil and Environmental Engineering
Center for Materials Science and Engineering
Center for Computational Engineering
Office of Naval Research
National Science Foundation
Army Research Office
MIT-Italy Program
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Spider Web Article written by David L. Chandler, MIT News Office