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SPIDER WEBS, SPIDER SILK

Funnel WebMaybe nothing is as special about spiders as their use of silk, and once you begin looking for spider silk, it's just everywhere.

For example, the web at the right is a very common kind of web, one that probably could be found in your backyard, too. It's a  funnel web, and as you see consists of a flat, horizontal sheet of web that "funnels" into a tunnel-like hole. That hole is where the spider stays. An insect blunders onto the web, inside the hole the spider feels vibrations of the hapless critter on his web, the spider rushes out, bites the insect, and carries it back into the funnel. As the spider grows it adds new layers to the flat web, so you can look at the web and judge how large the spider is. In the fall, female Funnel Weaver spiders deposit egg sacs in narrow places such as between rocks and loose tree bark, then dies -- sometimes still clinging to her egg sac. Funnel Weaver spiders belong to the family Agelenidae.

orb webFunnel webs are just one of many kinds of spider webs. In fact, with practice you'll learn to look at a web and know which species or general kind of spider constructed it, without seeing the spider at all. At the left you see the kind of web most people think of when they think of spider webs. It's an orb web, the classic web built by many spider species at night. Watching a spider build a web is a wonderful experience. First the spider constructs a rectangle-like frame of silk. Then from the future web's center, or hub,  inside the frame, numerous rays of sticky silk called radii (singular radius) are deposited radiating from the hub to the frame. Finally, many spirals of silk are laid down over the radii. At the left you can see the spider in its hub. If prey is caught someplace in the web, the spider knows where in the web the prey is caught by the vibrations transmitted to her via the radii. Actually, there's a lot more to building an orb web than just mentioned. You just need to watch the whole process yourself.

dome web of Sierra Dome Spider - Neriene litigiosa
Dome Web of the Sierra Dome Spider, Neriene litigiosa

Above you can see a special kind of web known as a dome web. It's created by the Sierra Dome Spider, Neriene litigiosa, of North America's Pacific Northwest region. Can you see the spider hanging upside down beneath the arching dome? The arching dome is sheetlike, so dome webs are regarded as one kind of sheet web. In sheet webs only a few of the threads are sticky, but there's such a maze of them that you can see how an insect blundering among them could get confused and eventually meet up with a sticky strand. If an insect gets snared, of course the spider gets it.

Sheet webAt the left is another sheet web, but this one is obviously built by a different species. I think the builder was the Bowl and Doily Spider, Frontinella communis. Thanks to Karen McKay of Kingston, MS for this photo.

The next picture, at the right below, is a classic cobweb, the kind found in house attics, barns, sheds and sometimes even in nature -- beneath leaves, among stones or loose bark. The photo was taken in a barn. Cobwebs such as this are produced by Cobweb Weavers, or combfooted Spiders, of the family Theridiidae. cobweb, probably of a Combfooted Spider (Theridiidae)Such webs are messy to human eyes, but they do a good job catching prey for the spider. The web's irregular outside threads are sticky. When an insects gets stuck, the spider comes and throws silk over the captive, then bites and sucks it dry. Cobweb Weavers have few or no teeth, thus don't chew their prey,Cobweb on side of barn, photo by Karen Wise of Kingtson, Mississippi which spiders of related groups usually do. The web at the left shows a neater variation on the cobweb theme.

Not all spiders produce webs. Many, most notably the large, hairy wolf spiders likely to be spotted roving across our backyards as they hunt (pictured at the top of this page), and crab spiders, found inside flowers waiting to ambush insects coming for nectar, build no webs.

spider egg sacsSpiders use silk for many purposes other than constructing webs. The females of many species construct silken egg cases in which they deposit and guard their eggs.  The cases at the left were found in the outside corner of a window. The egg cases of some spider species may contain several hundred eggs. Young spiderlings may emerge from the cases several weeks after the cases are produced, or sometimes not until the following spring. Notice how these cases are suspended among silk threads out of the reach of ants and other critters who wouldn't mind a spiderling meal.

Gnaphosid spider guarding her egg sac, with discarded exoskeleton nearbyAt the right you see a female Gnaphosid Spider guarding her white, silken egg sac. Gnaphosids are nocturnal hunters and many have eyes with a silvery layer that reflects light if you shine a flashlight on them at night. In this picture the silken sac is stuck to the bottom of a metal panel on a clothes drier in a garage in California. The crumpled-up object at the lower right is the spider's discarded exoskeleton, or external skeleton, which is explained in more detail below.

Most spiders, even if they don't construct webs, lay down silk draglines as they walk. These can help the spider find its way back home, or serve as a safety line in case the spider falls from something.

Especially in autumn, spiderlings are apt to climb to the top of almost any tall weed, bush, or fence post, and extrude strands of silk into the wind, rather like releasing a kit into the air. As the silk lines lengthen, the wind tugs harder and harder on them, until finally the spiderling releases its hold on its platform, and the wind bears silk lines and spiderling to new territory -- perhaps just to the next bush, or maybe to the next county or state. This manner of traveling is called ballooning; in the fall, when the blue sky is streaked with silvery streaks of sunlight reflecting from ballooning spiderling silk, the bright streaks are called gossamers.

curled leaf serving as resting place for spiderBesides using silk for making dragglines, for ballooning, and for constructing webs, some spiders pull together the sides of leaves, fix them in place with their silk, and form a kind of tunnel, where they rest. The picture at the right shows just such a resting place. It's a rolled-up leaf of a Roughleaf Dogwood tree. If you put your eye right up to the narrow opening between the curled-up edges you see the silk zigzagging from side to side inside the leaf, and the spider is in there, too, resting!

discarded spider exoskeletonYou just never know what a spider will do with its silk. At the right you see the discarded shell, or exoskeleton, of a spider hanging from a grassblade by a tiny thread of silk. Spiders, being arthropods like insects, must shed their rigid "skins," or exoskeletons, as they grow. So just before a spider discarded its old exoskeleton it attached its rear end, or abdomen, to the grassblade, then as it hang there its exoskeleton split and the spider emerged. What looks like a broad, brown stripe down the spider's back in the picture is actually the split in the back, or carapace, of the spider -- the split from which the growing spider emerged. A spider's carapace and head merge into a single structure known as a cephalothorax. Once a spider is outside its old exoskeleton, its new exoskeleton is soft, and the newly emerged spider in a matter of hours grows larger than it was before, like a sponge that has been under pressure suddenly being released. Once the spider reaches its new full size, its new exoskeleton hardens, and that's the end of the spider's growth until the next time the exoskeleton must be split and discarded.

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