Matt Walker

Editor, Earth News

Spiders walk a fine line when weaving their webs

That a spider's web can be too sticky? Scientists have found that if the webs of spiders were any more sticky, they would break as they trapped insects, letting the prey escape. So a spider has to balance the stickiness of its web with how strong it is. That puts a limit on how adhesive spiders' webs can become, forcing the creatures to evolve webs of an optimal stickiness, according to a study published in the Journal of Zoology. Spiders create architecturally elegant webs by first laying down radial lines of dry silk, upon which they weave regularly spaced elastic spirals of sticky capture silk. But more than 100 million years ago, in the early Cretaceous period, a major shift in the design of spiders' orb-shaped webs came about. Before then, spiders coated the spirals in their webs with puffs of dry adhesive. These dry spirals trapped insects by physically entangling around the tiny hairs known as setae on their bodies. A group of spiders living today, known as deinopoid spiders, still weave webs in this way. However, during the early Cretaceous, araneoid orb weaving spiders evolved which weaved a different type of web. These spiders replaced the dry adhesive with wet droplets of glue. These are much stickier. In theory, stickier webs should be better at catching prey and keeping it trapped. Optimally adhesive To investigate, Ingi Agnarsson of the University of Puerto Rico in San Juan and Todd Blackledge of The University of Akron, in Ohio, US examined how stickiness varies across the webs of 17 species of orb weaving spiders, representing most of the different types of modern araneoid spider. They tested the tensile strength of the silk and stickiness of the spirals, by measuring how much force it took to remove a piece of sandpaper attached to the web. They found a direct correlation between the strength of a silk fibre and its stickiness. In fact, each fibre would release the object it was trapping at between 20 to 70% of the force required to break the fibre. That means that spiders factor in a safety margin into their webs. If they made them stickier, a wriggling insect wouldn't be able to release itself from the glue, but its struggles would ultimately break the fibre, and with it the web, ensuring its escape. If the glue is only so sticky, then the insect might be able to pull free from one capture spiral on the web, but it would then likely come into contact with the same thread again, or another. By detaching instead of breaking, a capture spiral can repeatedly adhere to an insect and continue to disrupt its struggles, say the researchers.



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