Secrets of the silk of cave-dwelling spiders

Ontogenetic shift toward stronger, tougher silk of a web-building, cave-dwelling spider

Piorkowski, D., Blamires, S. J., Doran, N. E., Liao, C.-P., Wu, C.-L. and Tso, I.-M., 2018, Journal of Zoology, vol 304, pp. 81-89

With my left foot positioned on a wobbly, semi-submerged stone, my right foot slowly losing traction on the gravel edge of the cave’s stream bank, and my right shoulder balancing the rest of my body on a large stalactite, I reached out to delicately sandwich a section of a spider’s web between two cardboard frames. I had to perform a sort of cave yoga; maintaining complete control of my body as I was covered in limestone dust and mud, with water dripping from the ceiling onto the brim of my helmet. Even a breath too heavy could have resulted in breaking or overstretching the silk fibers I was trying to collect, ruining my sample. These silk fibers measure less than six micrometers in diameter, making them difficult to see without the use of an eighty-lumen head torch contrasted with the cave’s dark background. Collecting sections from twenty webs built by spiders of various sizes required two full days of work in caves nestled in the wet Eucalyptus forests of southern Tasmania, Australia.

Figure 1
Collecting a section of web built by H. troglodytes. Photo by Niall Doran

The aim of our project was to determine whether the body size of the Tassie cave spider, Hickmania troglodytes, influences the stretchiness, strength and toughness of their silk fibers. We planned to tackle an inquiry that had been seldom addressed and not previously linked to significant shifts in spider silk mechanical performance.

Spider silk is a proteinaceous fiber renowned for its impressive combination of high breaking stress (~1.5 GPa) and strain (~30%), making it the toughest biological material known in nature. Several attempts have been made to recreate these outstanding mechanical properties in a synthetic fiber, but none have been successful. One challenge in this endeavor is the high variability observed in these properties between and within species, and even within a single individual. Understanding the sources of this variability is the key to unlocking the hidden knowledge of these silken super-materials.

Figure 2


Figure 4
Images of H. troglodytes hanging from their webs in their cave habitat. Copyright: SIXTEEN LEGS/Bookend Trust, photo by Joe Shemesh

Once we had collected web samples from the caves, and sufficiently cleaned off, I returned to Tunghai University, more than 7800 km away in Taiwan, to perform mechanical analyses of individual fibers. Our tests revealed a new source of spider silk variability, as we found that the strength and toughness of major ampullate silk fibers, threads used to build the frame and scaffolding of the web, significantly increased with spider ontogeny, or growth. From a biological perspective, this means that as these spiders grow up their webs may become more efficient at catching insect prey or more tolerant to damage. Given that silk is metabolically quite expensive to produce, better performing silk might also be a way for a spider to increase silk economy. Additionally, stronger silk would allow large spiders to better support their own body weight as they walk around their web or hang upside down from a single thread.

Whatever the case, the endemic Tasmanian cave spider, one of the oldest web building spiders in the world, has kept the secret about its silk for over 200 million years since its origin on the Gondwanan supercontinent. This enigmatic and ancient spider may have other secrets hiding within the caves of Tasmania and as curiosity grows, more light may be shined into the darkness.

Dakota Piorkowski

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