The glue on the fibres of spider webs is weaker than fibre itself - in this clever design the glue releases struggling prey before the prey breaks the fibre.
Credit: Ingi Agnarsson
BOSTON: When it comes to spider webs, stickier is not always better. Spiders cleverly optimise the stickiness so that large prey can escape without ruining the web, researchers have found.
A Journal of Zoology study reports that spiders have evolved to create webs with the optimal amount of stickiness to catch prey. Sticky thread is needed to capture insects, but too much adhesive holds bugs tightly enough for their struggles to break the web.
"If the silk is too sticky, it will stick to a large and struggling prey until the threads break, resulting in web damage and escape of the prey," said co-author Ingi Agnarsson, a biologist with the University of Puerto Rico in San Juan.
Stick to the limit
The researchers tested the webs of 17 spider species for strength, stretchiness and stickiness. To measure the first two qualities, they pulled on the silk until it broke, noting how much force was needed and how far it stretched. To measure stickiness, the researchers attached the silk to a piece of sandpaper and measured the force required to pull it away.
Initially, the researchers expected spiders would have evolved to maximise web stickiness, Agnarsson said. However, the experiments showed spiders had limited their glue use.
The researchers found a correlation between strength and stickiness; the stronger their web's fibres, the stickier spiders make them. But they also observed that the fibres would release trapped objects at between 20 to 70% of the force required to break the fibre. In other words, the glue lost its grip before the web broke.
This balance between strength and stickiness is critical, Agnarsson said. "Presumably the web functions best if stickiness is slightly less than the strength of the fibre," he said.
The evolution of stickiness
The researchers said their experiments provide insight into the evolution of spider webs. Scientists already knew that the webs of ancient spider species used dry adhesives that trapped insects by physically entangling the tiny hairs on their bodies. One group of living spiders – the deinopoid spiders – still use this method to capture prey.
However, in the early Cretaceous period, over 100 million years ago, some spiders evolved in such a way that they began to use sticky glue to trap bugs. As stickiness evolved, thread strength must have evolved with it, Agnarsson said.
Fritz Vollrath, a zoologist at the University of Oxford in the United Kingdom and the leader of the Oxford Silk Group, said he appreciated how the researchers linked their findings to the evolution of spider webs. "This is an interesting paper demonstrating the physical interaction between stickiness and mechanical strength," he said.
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