Spiders’ ability at building spiral webs is fascinating. They spin the radial and spiral threads that give shape to orb webs, which weigh close to nothing, catch prey, and resist winds and insect impacts even if some threads are broken.
Spiders are able to accomplish such feats because spider silk, the protein fibre that they produce, has phenomenal properties: Its tensile strength — the maximum stress it can withstand without breaking or deforming permanently — is superior to that of high-grade steel; its ductility — or ability to lengthen without breaking — is such that a typical thread of spider silk can be stretched about half its length before it breaks; and its weight is minimal. Indeed, a filament of spider silk long enough to circle the Earth would weigh less than 500 grams.
But so much greatness does not come for free. Spiders must spend energy to produce the protein fibers that make up their silk. It is therefore not surprising that throughout the at least 140 million years they have evolved, spiders have learned a few tricks to spend less energy in building the web while increasing its efficiency at prey capture. For instance, they can use the wind to bridge two tree branches with a strand of silk. Also, they can produce sticky and non-sticky threads, to both catch insects and walk around the net safely — since spiders are not immune to their own glue. They can also recycle its own web by eating the protein fibre from damaged or deteriorated threads, and then refortify the strands that have lost stickiness.
Recently, a more subtle trick involving the geometry of the spiral orb web has been found by two Japanese researchers from Ochanomizu University in Tokyo. Writing in Physical Review Letters, the researchers describe a simple model for the mechanics of the orb web that demonstrates that the web’s highly tolerance to damage is enhanced when the radial threads are much stronger than the spiral threads. And, indeed, previous research has shown that the radial threads in orb webs have a much higher resistance to extension than the spiral threads. To understand why having a two-tier thread quality matters, it is helpful to look at the distribution of forces in the web. The diagram below, taken from the research article, uses a color code to show the magnitude of the force in the orb web when the radial-to-spiral ratio for the strength of the threads is 10. Clearly, forces are accumulated towards the periphery of the radial threads, and the stronger the spiral threads, the larger the forces accumulated in them. The same principle holds for a simple horizontal rope holding a weight. The higher the weight of the object, the larger the force pulling the two extrema of the rope.
Consequently, because of the accumulation of forces, the softer the spiral threads are, the less they contribute to the tension on the radial threads. Now, why is this important? Well, if a spiral thread breaks (radial threads are more resistant to damage), the web has to make up for the sudden loss of tension. This leads to a concentration of stress, as indicated in the figure by the arrows, which point to the radial and spiral threads with the highest tension. These threads are the most susceptible to breakage. Therefore, improving the resistance to damage involves reducing the maximum force appearing in the web. It becomes clear now why spiders that build orb webs do not spin all threads equally. By spinning spiral threads that are much softer, spiders can put more of them to make a denser net — and catch smaller insects — without reducing the damage tolerance of the web.
If Spider-Man would have known, the villains would be hung by a thread.
