Our Window on Nature

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Spiders – On Gossamer Wings

Filed under: Bugs — Lowell Christie -- October 15, 2006 @ 4:28 pm

Spider WebOctober is a month when the world is full of young spiders leaving home. Wisps of silken thread fill the air. Sunlight glints off gossamer parachutes following the whims of the wind. Young spiders break their ties with home by flying away on wings they build themselves.

Most of the 3,000 kinds of spiders in the United States have three things in common: eight eyes, eight legs, and six spinnerets that create the strongest natural fiber known – silk. Spiders might replace silkworms except for one problem. Silkworms eat plants and are easy to raise in captivity. But when you try to raise spiders in confined spaces, they have a strong tendency to eat each other.

As a young spider comes of age it begins looking for a piece of high ground. This may be only the tip of a blade of grass, a small rock, or perhaps a fence post, but it must be a place where air currents pass. And there the spider waits for a breeze to carry it off.

From its spinnerets the spider plays out silken lines like a flycaster testing a trout stream. Soon the spider’s parachute lengthens until it’s large enough to support the creature’s weight; then it becomes airborn. Where it lands only the spider will know.

The spider has some small degree of control by pulling in a line here, or letting out more silk there, but it is really the direction and strength of the wind that determines the landing spot – and it may be a long way away. Charles Darwin found flights of spiders 200 miles out to sea, and when a volcanic eruption creates new land in the ocean, the spider is often the first occupant.

The silk that carries the young spider on its first journey starts out as a liquid protein that hardens immediately upon contact with air. Most spiders will find a use for this silk throughout their lives. The six spinnerets produce different kinds of silk, and each kind has its own special use.

If a web-building spider had only one kind of silk, it would become entangled in its own trap. Instead it produces a non-sticky silk that forms the web frame, and the spider is very careful to walk on only this frame when it crosses the web. The spiral insect-catching part of the web is produced by a different spinneret which specializes in sticky silk. This is added after the framework is in place.

Most orb-web spiders renew their webs each night. They eat the sticky spirals, but leave the main frame in place. Then they replace the sticky silk in hope of capturing another meal. The material that they eat is recycled in their bodies, and will appear back in a web in about 24 hours. Rebuilding the web itself takes only about a half hour.

The size of a spider’s web is determined by the size of its silk gland, since it uses almost all of its available silk in its web. This means that all the webs of a single species will be approximately the same size, and in fact, they will also be of about the same shape. There is so much consistency in the way spiders build their webs that an expert can identify many spiders by web shape alone.

In one group, the Nephila, it is even possible to determine the age of the spider by looking closely at the web. Young spiders produce a complete web, but for some reason the adults build only the bottom section and leave the top of the web irregular. But what a web it is.

The Nephila is found in the southern United States and in the tropics, and the silk that it produces is the strongest ever found. Its strength surpasses that of an equal size of steel wire. South Sea islanders have discovered that the web of this spider is so strong that they can use it for fish nets.

But man isn’t the only creature that has learned to use spider webs as a net. The ogre-faced spider is a more active hunter than most, although it still captures prey with a web. It spins at night, and lies in wait for a passing insect. Then it throws its web to make the catch.

Although webs are the most easily seen, the spiders have found many other uses for silk. They build underground homes of it, they wrap their food in it, and most wrap their egg cases in a special silk which is often extremely colorful. In one species the male even drapes a light wrap of silk over his chosen mate during courtship.

Silk is truly one of the wonders of nature, and this month you’ll see it hanging everywhere. Each of those gossamer threads provided transportation for a creature that will eat around 2,000 insects a year – and who will continue to create intricate patterns in silk for our delight.

FMC1084


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