Window on Nature
By Lowell & Kaye Christie, F47246
If you’re leery of spiders, you’re in good company. It’s such a common complaint, it has its own name: arachnophobia. For decades Kaye has used a “know more, fear less” technique to reduce her dread of spiders. And it worked “” eventually. She finally got to the point that she could hold a tarantula in her bare hand as an interpreter at the Arizona-Sonora Desert Museum in Tucson, Arizona. Once! She hasn’t chosen to repeat the experience.
But spiderwebs are a different story. We both enjoy their beauty as they shimmer in a soft spring rain, or when their complex structure becomes more obvious as the first frost of the year makes them glisten in the sunlight. On some mornings the outdoors can become a fairyland after these night-weavers have prepared for a new day. And to understand spiderwebs, it helps to know a little about their creators.
Spiders live in almost all habitats and at almost all elevations. And very few of them are dangerous to humans. Of the approximately 36,000 spider species, most of them spin webs. But even those that don’t rely on webs to capture their prey may still produce silk for other purposes, such as a cocoon to protect eggs.
Did you ever wonder why a spider doesn’t get snared in its own web? It’s because these web-weavers make several types of silk, and not all are sticky. A spiderweb begins with a non-sticky frame on which a spider can safely walk. The spiral portion has a sticky surface designed to capture the insects that make up a spider’s diet.
The silk itself is produced in a liquid form in special glands inside the spider’s abdomen. Most spiders have different silk glands to produce various types of silk. Multiple spinnerets (and tiny tubes called spigots) allow the silk to be drawn outside the spider’s body, with the spider controlling the type of silk, the thickness, and the speed at which it emerges. As the silk leaves the spigots, the silk protein molecules line up parallel to each other and form the strong elastic material.
How strong? We once got an extremely close look at an Anna’s hummingbird while rescuing it from entanglement in a spider’s web. Hummingbirds like to use spider silk to line their miniature nests, and evidently this bird got careless. It was tired from its struggle to escape, but certainly not too tired to fly away after we removed all the silky strands.
Spiders use silk for more than just web-making. Young or small spiders also use it as a mode of transportation. Let’s take a quick look at a spider’s life cycle.
Many female spiders breed only once in their nine- to 12-month lifetime, although some larger spiders, such as the tarantula, may live as long as 20 years. The female may lay several batches of eggs, since delayed implantation permits her to retain part of the sperm to produce several hatches of offspring. Each time the female lays a batch of eggs, she wraps them in a protective pouch of silk.
Upon hatching, the young instinctively chew their way out of the egg packet and scatter, lest they be eaten by a hungry parent, sibling, or passerby. Luckily, they are born knowing how to spin silk, so each one can climb to the tip of a leaf, a blade of grass, or a rock, and lay out a silken line when they sense a slight wind. Now comes the fun part “” ballooning, or their ability to “fly” from here to there. Once the baby spider has produced a silken thread long enough to catch the breeze, it’s off and away. Multiply that action times a batch of several hundred hatchlings, and the air can become filled with gossamer strands. It’s wonderful to see, but may be less enjoyable should you be standing downwind.
Spiders have exoskeletons. As a spider grows it must shed or molt its hard exterior and grow a new one. If you’ve ever seen a “dead” spider, take a closer look. There’s a good chance it’s really just the discarded outer shell. The larger the spider species, the more often it is likely to molt before reaching full adulthood. But in order to grow, spiders have to eat, and for a large proportion of them, that means building webs.
There are many types of webs, from cobwebs to funnel webs to sheet webs to orb webs. Yes, there really are cobweb spiders, although they go by a multitude of names. One that you’ll recognize is the black widow. Their webs are messy and somewhat disorganized. Let’s spend our time, instead, on the construction of the beautiful orb webs, the type that make you ask, “How did they do that?”
Orb web spiders “” approximately 2,500 species have been identified “” create the circular web shape that catches the photographer’s eye. Some of them are built by rather drab-looking spiders, the type that frequent a typical garden. And one’s first thought, after noticing the beauty of the web, is, how did the spider attach it to the widespread anchors at the top?
Until we started studying spiders, we thought it required trips from one top anchor, down to the ground, and back up the other side. But that was before we had heard of ballooning. Usually at night, the spider climbs to a high point and waits for a breeze. It lets out a strand of non-sticky silk and lets it float on the air current, constantly testing to see if the other end has attached itself to another surface. Kind of like a fly-fisherman testing the waters.
When the other end snags, the spider reels in the slack and uses some sticky silk to attach the end. It then crosses this new bridge line while dropping a second line with less tension, so it hangs below the original. Now the spider moves to the center of the lower line and attaches another strand. You can’t see it yet, but this will be the center of the orb web.
Dropping directly down, the spider finds a third attachment point, and the outer limits of the web are set. All of these lines are “dry,” meaning the spider can safely move across them to reach all corners of the new construction. And it does so quickly, joining each anchor point to form the triangle that makes up the finished product.
Of course, there is still a lot to do. Radius threads are stretched from the center to the sides of the frame, and then a circular orb spirals out from the center. So far, all of the silk has been dry, so it’s not going to catch much in the way of insects. But once the framework is complete, the spider backtracks, using the spiral as a guide as it lays down a layer of sticky silk. The result is a web with safe radius threads for walking and a spiral to catch dinner. Then it’s just a matter of being patient until an insect is caught in the web.
Many spiders rebuild their webs each night. Some eat the silk “” it is mostly protein “” and others roll it in a ball and discard it. We watch for them on our morning walks to add a little enchantment to our day. And knowing how webs are made doesn’t diminish the magic a bit.