ECOLOGY:

Some spiders' dragline silk can absorb a great deal of energy before breaking. It can be tougher than steel, as well as the most advanced synthetic polymers ever developed.

The search for spider silk of optimum mechanical properties, and consequently the quest to further improve upon synthetic materials, has perhaps made less progress to date than what could have been achieved. Scientists typically either sample silk at random, or attempt to sample silk from a very large number of spider species.

The former doesn't easily yield useful trends. While the latter may appeal to scientists' methodical nature and desire to fully characterize their subjects, it's a never-ending task in this case.

Most spiders produce dragline silk. There are over 40,000 known spider species in the world.

Consequently, a systematic study of spider silk is unrealistic, and unlikely to yield useful results in the short-term. Ingi Agnarsson (University of Puerto Rico, Slovenian Academy of Sciences and Arts, and University of Akron) and coworkers have taken a different approach.

By investigating the dragline silk of a spider species which creates very large webs, they have found dragline silk which is twice as tough and stretchy as any other known spider silk. This research is complimentary to that which questioned why orb-weaving spiders decorate their webs.

Observing spiders.

The scientists studied Darwin's bark spider, Caerostris darwini (named after Darwin but not discovered by him), from Madagascar. Unlike other spiders, they build ther webs over rivers and lakes, catching prey mid-flight over the water.

These orb-weaving spiders create the longest-spanning webs ever seen (up to at least 25 meters long), and one of the largest-area webs currently known. It's reasonable to suspect that these spiders' dragline silk is exceptionally tough, a hypothesis put to the test by Agnarsson and coworkers.

The scientists' study took place over the course of one month in Spring 2008, within Andasibe-Mantadia National Park. After characterizing the webs, they took female spiders back to a greenhouse for analysis.

The spiders started building webs somewhere between one day and two months after transport to the greenhouse. Spider silk was sampled within 24 hours of its deposition, and was sometimes forcibly extracted from the spiders.

After digital photography (superior to electron microscopy in that photography is non-destructive and allows one to measure variations in diameter within a single strand of silk), the strands were pulled until they broke. The force at strand failure was recorded to a precision of roughly 1 microNewton.

The scientists recorded four mechanical properties of their silk. Specifically, toughness (energy absorbed prior to rupture), initial stiffness, extensibility, and ultimate strength (force required for rupture relative to the instantaneous cross sectional area of the fiber, i.e. area calculated by viewing it from the same angle) were all measured.

Web and silk properties.

Curiously, the scientists found that Caerostris darwini constructs webs which are less dense than most other Caerostris species in the reserve. Their webs ranged in area from 0.9 to 2.8 square meters, while their bridgelines were up to 25 meters long.

Spiders captured and grown in the laboratory typically produced much smaller webs, averaging 0.08 square meters. Their webs were still sparse.

The spiders likely maintain their webs for several days. This is indicated by holes and repair work seen in the webs.

Testing against multiple spiders, spider silk toughness was 27±69 Joules per cubic centimeter (twice that of other spiders). Initial stiffness was 11.5±5.1 gigaPascals (average relative to other spider silks).

Spider silk extensibility was 0.33±0.08 mm/mm (twice that of other spiders). Ultimate strength was 1850±340 megaPascals (higher than many other spider silks).

Silk toughness and ultimate strength in captured spiders was less than that from spiders in the field, but extensibility was greater. Values obtained from the field are likely more representative of the spider population than values obtained via forcibly-extracted silk.

The silk toughness is well above steel and any synthetic polymer developed to date. Also notable is its enhanced elasticity; in conjunction with its high strength, this is what imparts the extreme toughness.

Final comments.

What explains the spider silk properties, from an evolutionary standpoint? The draglines are very long, and the scientists suggest that extra toughness may be needed to prevent the webs from collapsing into the water.

What explains the spider silk properties, from a molecular standpoint? This is the million-dollar question.

An investigation of the molecular basis of enhanced toughness and other properties of Caerostris darwini dragline silk should provide insight into how to synthesize super-tough artificial fibers. Further research should also question how such long webs are put in place in the first place.

NOTE: The scientists' research was funded by the Slovenian Research Agency, the National Geographic Society, the National Science Foundation, the European Community 6th Framework Programme, and the Sallee Charitable Trust.

for more information:
Agnarsson, I., Kuntner, M., & Blackledge, T. A. (2010). Bioprospecting Finds the Toughest Biological Material: Extraordinary Silk from a Giant Riverine Orb Spider PLoS ONE, 5 (9) DOI: 10.1371/journal.pone.0011234