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Spiral orb webs in a gorge in Karijini National Park, Australia. Photo: <a href="http://en.wikipedia.org/wiki/File:Spiral_Orb_Webs.jpg">Bjorn Christian Torrissen</a>, Creative Commons, some rights reserved
Spiral orb webs in a gorge in Karijini National Park, Australia. Photo: Bjorn Christian Torrissen, Creative Commons, some rights reserved

Natural Selections: Spider Webs

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Spiders from big to tiny use their webs to snag and trap prey in fascinating ways. One spider even reels in tiny gnats that come to "roost" on the web. The silky constructions are wonders of engineering and construction. They're also highly specialized, spider to spider, as Martha Foley hears from Dr. Curt Stager in this week's edition of Natural Selections.

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Synopsis of this Natural Selections Conversation:

When you think of a spider web, you probably think of a large circular pattern, stickiness, and a spider lurking in the center, waiting for its prey. But did you know that there is a spider that can catch gnats by simply reeling in a thread? According to Dr. Curt Stager, “Most spider webs are not completely sticky, there are a lot of threads for just walking on and the spider knows where these threads are, running in on them to catch their prey.”

“Gnats know that it is safe to land on some of these threads and use them as a roosting place,” says Stager. On these threads, they are safe because other predators of the gnat will not fly into the spider web on purpose. All gnats have to do is pay attention to the vibrations in the thread. If the gnats feel the spider coming out toward them, they can jump off for a while and come back later on.

One spider in particular specializes in catching these so-called “smart gnats.” It spins a single thread through the vegetation with no glue or any other sticky residue. Naturalists speculate that the spider may use pheromones, chemicals capable of acting outside the body of the secreting individual, to draw in the gnats. Once a gnat has landed, the spider hauls it in little by little, so that the gnat does not realize it is in motion. As soon as the gnat gets close enough, the spider grabs it and ingests it.

Different spiders specialize in different kinds of web making. In the Central American tropics, if a spider wanted to feed on a colony of ants, he would have to face the guards of the column, leading to an onslaught by the entire colony. It is unlikely that the spider would make it out alive. To get around this, one spider weaves a portable net that it carries in its front legs. It climbs into the vegetation over an ant column, drops the net down by a few threads, and then pulls it back up once it has caught its prey.

Webs can be highly sophisticated. Certain kinds of mesh determine what the spider will catch when they hang the web. “When there used to be a large amount of outhouses, the black widow spider would specialize in flies that came to the septic pit. What better place to put your web than over the hole of the outhouse?” says Stager.

Some insects have a better chance of surviving if they find themselves caught in a spider web. Stager says, “Butterflies and moths have these scales on their wings that make the colors on their wings, but if they fly into a net, the scales stick to the glue and then they just fly off.”

To counter this defense system, there is a spider specialized for hunting moths. Their webs have an amazing design. It appears to be a regular old circular web, but there is a densely woven rectangular strip that runs up to the top. The spider waits at the bottom in the circular section for a moth to fly into that strip. When this happens, the moth’s scales stick to the glue, like usual, but then it continues to stick more, until enough scales fall off the wings, causing the wings themselves to get stuck.

Few insects make their way into a spider web on purpose, but there are spiders that specialize in hunting their web-making cousins. These spiders pretend that they are an insect caught in the web by going to the edge and tweaking the threads. When the unsuspecting host comes out to investigate, it is devoured by the intruder.

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