Biomemories: Golden Silk Orb-Weaver--A Natural Architect

Nephila clavipes

The genus Nephila are masters of architecture; derived from the Greek words nen(thread) and philos(love), their name epitomizes their spinning skills: they have the strongest silk in the world, are skilled builders, and can basically catch anything that can fly through a forest. From the equipment to the material to the design, they've got it all.

Nephila clavipes are part of the Arthropod subphylum Chelicerata, which includes scorpions, mites, and spiders. The male is a rather uninteresting brown and about 5 times smaller than the female, which has yellow and red stripes and spots and can grow up to 4 cm in length. It is the female who does all the extensive and elaborate web-building, while the males essentially just sit around about 5 cm above her, waiting for food and sex. She, on the other hand, stays quite busy building and repairing her web and wrapping and eating her food, using venom to paralyze it as she does so. Talk about gender inequality...

A weaver in her web

The Web:
Although she consumes part of her web at night for protein and rebuilds it every day, a Nephila can make webs that can last for 2 years, stretching to over a meter in diameter. In addition to the fine mesh orb seen above, she can construct barrier webs that protect the web from weather, birds, and other larger objects and also function as an alert to invaders and prey smashing against the web.

Furthermore, she may install zig-zag lines of structural silk to increase stability or open a hole to allow wind to blow through without destroying the entire web. Her silk's slightly yellow color has been shown to act both as an enticement for flower-seekers like bees and as camouflauge in shady areas with mottled sunlight. 
 She may incorporate more or less yellow pigment in her silk depending on light conditions in the area, revealing rather a complex mind behind that pretty body. Dr. Catherine Craig, an expert in the dynamic and active nature of spider webs, has explained that "Yellow is a very generalized visual signal that both herbivorous and pollinating insects associate with flowers and new leaf material...the insects may have a much harder time evolving a mechanism to avoid it."

Spider silk spigots

The Silk:
First of all, all silk is not made equal. Both the ingredients and the process differ from species to species, and Nephila clavipes, among others, can spin up to seven different kinds of silk, each with unique properties and purposes in its web.

With a tensile strength of 4×10⁹ N/m, their dragline silk is six times stronger than steel and 10 times more effective than Kevlar at dissipating energy, it has a plethora of potential uses, from parachutes, bullet-proof vests, and ropes to sutures, tendons, and ligaments.

One excellent example is the use of Nephila clavipes dragline silk in nerve guidance. Besides being incredibly strong, it is not only nonimmunogenic but also has antibacterial and antifungal properties. Furthermore, its structure promotes cell migration and adhesion and can even obtain its own myelin sheath in the presence of human Schwann cells. You'd think it was meant to be.

Just keep spinning, spinning, spinning...

So how on earth do they do it? Think of silk making in terms of steel manufacture: in order to make it, you need not only the iron but also the Bessemer process to transform that iron into beautiful, powerful steel. Silk works in much the same way. Spiders have both the proteins to make it and the tools to transform them into long, strong silk. Using alternating parallel molecules and random-ordered regions, spider silk is able to contract or expand according to pressure, thus enabling both flexibility and strength.

Biomimic Garry Hamilton elaborates, "A single spider-silk thread can be as small as one micron (i.e., one-thousandth of a millimetre) in diameter. The protein chains that form the threads are roughly another 1,500 times smaller still. What scientists suspect is that during spinning, the spider exerts a force that causes microscopic fistfuls of these protein chains to bend back on themselves and align with one another to form tiny crystals. These crystal chiplets, containing thousands of neatly arranged amino acids, apparently prevent microscopic cracks and hold the silk together....there's not an overabundance of crystal, thereby limiting the silk's density and maximizing elasticity."

This allows Nephila dragline silk to combine incredible tensile strength with elasticity up to 40% of its original length. In contrast, capture silk trades rigidity for serious flexibility by implementing spiral structures that allow for 300% extension to snag insects flying at high speeds without breaking.

Nature's architects are the best of the best from hundreds of millions of years of development, and we have a lot to learn, if we're willing to listen.


Image Sources and Cool Links:

Images:
Keith Ramos - Golden Silk Orb-Weaver
Wikipedia Commons - Nephila Clavipes - Merritt Island
Arachnology - Spider Silk (Dennis Kunkel)
Protein Spotlight - Nephila Clavipes (Frank Starmer)

General:
Animal Diversity - Nephila Clavipes
New York Times - Crafty Signs Spun in Web Say to Prey, 'Open Sky'

Web and Silk:
YouTube - Nephila Weaving Circumferential and Radial Fibers
Protein Spotlight - The Tiptoe of an Airbus