Some 225 million years ago, the first spider webs appeared on the scene. The cribellar threads that primitive spiders use to spin their webs are composed of looped fibrils tangled into a mass `a bit like 3D Velcro' explains Brent Opell, an evolutionary biologist at Virginia Polytechnic Institute and State University. Modern orb-weaving spiders improved on the design of fuzzy cribellar threads; orb-weavers make viscous capture threads by coating their threads with a chemical `glue', which coalesces into regularly spaced droplets along the thread. Relative to the amount of material invested, viscous threads are 13 times stickier than cribellar threads. Intrigued by the properties that make viscous threads superior to cribellar threads, Opell decided to investigate viscous thread stickiness().
Opell explains that when a load (like a juicy insect) hangs from a cribellar thread, most of the force applied to the thread is perpendicular to the load. This means that little of the force is directed inwards along the thread in a way that would allow the middle part of the thread touching the insect to contribute to adhesion. Perhaps a potential advantage of viscous thread, Opell reasoned, is that it enhances the stickiness of the entire length of thread in contact with a load by directing the force inwards along the thread, enabling droplets nearer the centre of the strand to contribute their stickiness. `We imagined each droplet acting like a cable on a suspension bridge, distributing the force,' Opell says. If this suspension bridge theory is correct, a thread's stickiness should increase as the number of droplets contacting a load increases.
To test this, Opell and Mary Lee Hendricks measured the stickiness of viscous threads touching contact plates ranging from 963 to 2133 μm in width. Having collected some spiders' webs, they pressed the threads against a contact plate that transferred force to a load cell, then pulled the thread back. To measure the thread's stickiness, they recorded the maximum force registered by the load cell before the thread pulled free of the contact plate. Using four different widths of contact plate, Opell and Hendricks showed that a viscous thread's stickiness increases with increasing plate width, since more droplets contacted the wider plates. This was just as they expected to see if a suspension bridge mechanism was operating. But something odd was happening: the droplets were not acting like a perfect suspension bridge. When Opell and Hendricks plotted the increase in the number of droplets contacting a plate against the change in stickiness per droplet, they saw a decrease in the average adhesion per droplet. In a perfect suspension bridge, each droplet should be equally sticky. The fact that average adhesion per droplet decreases suggests that the droplets at either end of a thread contacting a load contribute the most adhesion, while droplets towards the middle of the thread contribute progressively less.
To investigate the limitations of the suspension bridge mechanism, Opell and Hendricks developed a model of the mechanism. They showed that only about six droplets at either end of a contacting thread contribute to thread stickiness, suggesting that there is an upper limit beyond which a longer contact surface does not increase a thread's stickiness. `This may have consequences for web architecture,' Opell says, as spiders may have to place their threads closer together to compensate for this limitation.
