The BugLady is a big fan of carnivores, and she admires spiders for their singlemindedness in that respect. She doesn’t know a lot about how they conduct their daily lives, and she knows even less about their taxonomy, but she can’t pass by a good spider web without checking it (next time, BugFan Mike). Webs are amazing structures, but spiders have about a half-dozen other uses for silk besides catching prey (and the silk itself is pretty amazing). “Spider Webs” is a huge topic, so this is, necessarily, an overview.
The Who, What, When, How, and Why of spider silk.
WHO? Everybody knows that spiders spin trap webs to catch their supper, but that’s a generalization, because not all spiders pursue their prey passively (from the edge of a web) and not all trap webs look like the classic disc spun by Charlotte. Most spiders can spin silk (yes, males do, too), even though they may not use it to make webs. Seven or eight kinds of silk have been catalogued, but the average spider doesn’t need or have the full array.
WHAT? Spider silk/fiber is a long, complex molecule made of protein with a few other ingredients thrown in, like sugars, fats, and pigments. Silk is made up of protein, and each strand is also covered by a layer of protein. In the right environmental conditions, it’s stronger than steel, even at very cold or very hot temperatures, tougher than Kevlar, antimicrobial, flexible, almost as stretchy as rubber, durable, biodegradable, hypoallergenic, and, according to one source, so lightweight that a strand of silk long enough to circle the world would weigh about a pound.
WHEN? Spiders hatch out of their eggs with the ability to produce silk and they do so all their lives. Spiders have been spinning trap webs for three or four million years.
HOW? Spiders carry a gel called unspun silk dope in their silk glands, and when the liquid is released, it travels through projections called spinnerets located on the underside of the abdomen’s tip and becomes solid when it hits the air. Spinnerets are the external extension of the silk glands; most spiders have between two and eight spinnerets, tipped by “spigots” that control the diameter of the emerging thread. They can release silk from several spinnerets at once to make a stronger strand. Spiders can grab the new silk with their legs and pull on it, and it looks like they’re pulling fully-formed silk from their body (humans have tried pulling on it, too – more about that later).
WHY? Silk is not “one size fits all;” the various demands for silk have resulted in a variety of types of silk and a variety of kinds of silk glands to produce it. Silk is used:
To capture prey (trap webs) – the insect-catching webs of some primitive spiders consists only of a single, sticky line, which they monitor to see if it’s vibrating. Orb weavers make a two-dimensional web, and funnel-web and other species’ webs are three-dimensional. About half of the 37,000+ species of spiders worldwide use web to capture prey.
In reproduction – males may deposit sperm onto a small bit of web before fertilizing the female, and she uses silk to make and attach her egg sacs. In some species, newly hatched spiderlings form a nursery web in which they stay for a few days.
For locomotion – young spiders may travel by “dynamic kiting” (ballooning). They let out some very fine silken lines (gossamer), and when the wind picks up the lines, it picks up the spiderling, too.
For protection – spiders may spin a nest close to a web, where they can hide while waiting for supper, or they may web a few leaves together for the same purpose.
To immobilize prey (picture Frodo being wrapped up by Shelob).
As a lining for their nests or shelters – one wolf spider that lives in sand dunes deposits her eggs in a tunnel that she lines with silk.
As a road map – spiders generally leave a non-sticky dragline behind them as they travel so that they can find their way home – one source said that unless the strand is broken, a spiders’ lifelong journey can be read in its silk trail.
As a safety net – a dragline acts like a belaying rope to catch them if they jump off of a flower, whether escaping from danger or launching themselves after an insect, or simply dangling.
To monitor their surroundings, some spiders deploy an “alarm” line. If the animal that trips it is small, it becomes dinner; if large, the spider hides.
As an attractant – a female may lay down a pheromone-laced trail of silk for her potential mate to follow. The “perfumed” drag line of the six-spotted fishing spider is laid on the water’s surface.
Humans have uses for it, too. Medicinally, it been used as a styptic (it’s rich in Vitamin K) and as a bandage, and in contemporary medicine, silk is being tested as a stimulant for neuronal regeneration. Some Pacific Islanders make nets from webs of the big Nephila spiders, and web has been used to make crosshairs in telescopic rifle sights and in other optics, including microscopes. And for some highly technical applications that the BugLady doesn’t understand, like N-slit interferometric signals and inertial confinement fusion (astonishing that the optimal material for some 21st century technology is a fiber made by a spider).
And then there’s fabric. A while back, BugFan Diane sent this link to a story about spinning the silk of Madagascar Golden orb spiders into fabric, an intersection, she said, of bugs and fiber art. It took 82 people four years to extract silk from over a million Golden orb spiders (no spiders were harmed in making the fabric) and create the 4’ x 11’ piece of cloth. https://www.newscientist.com/blogs/shortsharpscience/2012/01/the-science-of-the-spider-silk.html. See two wonderful videos about its construction at http://www.vam.ac.uk/content/articles/g/golden-spider-silk/. Wow! Thanks, Diane!
Not surprisingly, humans are interested in growing some of this amazing substance, but (Spiderman notwithstanding) results have been mixed. Humans have manually pulled on the strands of spider silk to get a longer fiber and, along the way, have discovered that because spiders are carnivores, “farming” a whole slew of them in a big cage is a non-starter. And, the molecule has proven difficult to duplicate.
Which brings us to the brave new world of GMOs. After some tinkering in the lab, scientists have come up with silk moths, E. coli bacteria, and tobacco and potato plants that have been inoculated with spider DNA and have yielded decidedly mixed (and sometimes surprising) results. Two real-life applications that were mentioned are “spider” fiber for IPad covers and car seats. Most bizarre is a genetically modified goat that secretes silk fibers in its milk (the BugLady is not kidding). Fascinating article at http://www.wired.com/2015/06/bolt-threads-spider-silk/.
A few cool web-facts:
- Trap webs lose their stickiness over the course of a day. Spiders may reclaim some protein by eating their old, ragged web before replacing it with a new one. There’s a group of kleptoparasitic spiders that live at the edges of other spiders’ webs and nibble on the silk spun by their hosts.
- Orb weavers in the genus Aranea use four different kinds of silk to spin a single web. Let’s watch one at work. First, she raises her abdomen, releases several strands of stiff, non-stick silk (dragline) and waits until they snag on something down wind. She repeats the process, laying down foundation lines. She makes a “skeleton” of the web and then constructs its hub, spinning radius strands that look like spokes on a wheel. A second kind of silk builds a temporary, auxiliary spiral that serves as a pattern for the eventual larger spiral. She uses a very flexible, sticky silk for the spiral portion of the web. A special, gluey silk is used to fix the web to its substrate and at the web’s joints. Finally, she spins a trap line that extends into the vegetation where she will monitor her web. When she’s done, she eats the web’s hub and the initial scaffolding. The whole process takes only a few hours.
- A few species of orb weavers that stay on their trap webs in the daytime embroider the center with a thick, zigzag line of silk called a stabilimentum, which is much more visible to us than it is to an insect. Its purpose has been the topic of much discussion – To make the web stronger? To hide the web-maker? To confuse an attacker long enough for the spider to bail? To give birds and bats a “visual” or an echo so they can avoid it? To reflect UV light and attract insects?
- Wandering spiders (those that hunt actively, like crab and jumping spiders) have the best eyesight (a few can see objects a foot away, but for most, it’s three or four inches, max); sedentary spiders have the worst eyesight.
- BugLady found tantalizing references to buck moths being attracted to the webs of orb-weaving spiders because of the orb-weavers’ use of “pheromone mimicry.”
- The Bolas spider catches its prey by lobbing at it a length of silk tipped by a sticky glob.
- A species of assassin bug purposely strums the edge of a trap web, grabbing the spider when she comes out to investigate.
- https://www.sciencedaily.com/releases/2007/09/070912145919.htm. Spiders are ordinarily solitary critters, but sometimes……..
Kate Redmond, The BugLady
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