A banded garden spider in her orb web. Dana Wilde photo
Late summer. I’m sitting in the kitchen drinking coffee with two friends. The back door is open, and western sunlight is filtering through the trees.
I’m hardly following what my friends are saying, interesting though it is. For my attention is arrested by bluish glints of light bouncing off miniature cables strung from tree to tree in the woods. It’s as if the glints are lights from sylvan construction projects.
The cables are made of the toughest, most durable material known. The project engineers are spiders.
Some of the silk wires reflecting sunlight are the initial bridges for the spiral-shaped orb webs often seen in trees or brush. To set one up, an orbweaving spider trails a strand of silk from a prominent spot like a tree branch or a deck chair. The breeze catches the strand and tacks it serendipitously to another leaf or a blade of grass or the corner of the house. The spider then walks this bridge back and forth paying out silk to reinforce it. When the bridge feels sturdy and taut, the spider ties another line to it and drops down to secure it below.
Next, in a methodical, craftsman-like process, the spider builds two sets of spirals — one temporary scaffolding, then one permanent — adding radial spokes as it goes. A black-and-yellow garden spider takes a half-hour to an hour to complete the engineering of an orb web 1 to 2 feet in diameter.
Some orb weavers take down the web in the morning and build a new web every night. In some species, taking down the web involves eating the silk, whose proteins take about 45 minutes to get processed through the spider’s system and ready to be respun into new silk.
Orb webs stand out partly because of their beauty and partly because they’re conspicuous. Other kinds of capture webs include the flat ones you see in dewy grass on summer mornings. These are funnel webs, spun by grass spiders with a funnel-like retreat in the center where the spider hides from danger while it waits for bugs to get entangled.
Sheetweb-weaving spiders spin a flat sheet of silk in brush or in an outdoor corner. Sometimes, the sheetweb is slightly convex with a silk dome overhead. Some spiders drop lines of silk from the dome to the sheet in order to knock flying prey down onto the sheet. Many sheetweb weavers wait upside down underneath the sheet, and when an insect gets knocked down and entangled, they cut a hole in the sheet and pull the bug down through. Often, they’ll return to mend the hole.
The cobwebs you see clinging stubbornly in the corners of ceilings or on light fixtures were probably spun by house spiders. Those webs look like random, messy tangles, but researchers have shown that these space-filling webs are constructed with deliberation, too.
Spiders who hunt for a living don’t normally build capture webs, but all spiders make silk. Depending on its species’ needs, a spider may have as many as seven silk glands, each making a different type of silk. There are special silks for egg sacs and for wrapping prey. Dragline silk helps a spider maintain orientation to its surroundings and provides attractive scents to prospective mates. Jumping spiders don’t usually build capture webs, but one held in a vial is apt to spin a retreat in which to hide. Spiders that construct capture webs produce one kind of silk for joining lines, another for safety lines, another for frame lines, another of nonsticky thread used in the preliminary scaffolding.
An individual spider can adjust the properties of its silk to adapt to immediate needs. Some orb-weaving spiders spin thicker draglines as they grow larger, to ensure the line is strong enough to hold them when they fall. Some cobweb weavers can adjust the diameter of wrapping silk to make it stronger in the event of unusually energetic or panicky prey.
Spider silk is tough almost beyond belief. In general it has about half the tensile strength of steel. Tensile strength is a measure of the stress a length can withstand before tearing under its own weight. A strand of typical dragline silk could be nearly 50 miles long before breaking under its own weight. This is longer than the breaking point of a length of steel of the same hair-like width, because the silk is less dense and weighs less than the steel.
Silks also have extraordinary elasticity, meaning the ability to stretch and then return to a resting shape. The huge webs of golden silk orbweavers in the tropics have been used for fishing nets. Some silks can take seven to 10 times more energy to fracture than an equivalent volume of Kevlar, a material used in bulletproof clothing. An engineer calculated that in theory, web silk scaled up to the width of a pencil has the elasticity and recoiling properties to stop a 747 aircraft.
So far, no one has figured out how to farm silk for human use, mainly because it’s not possible to keep large assemblages of the appropriate spiders stably operational — they eat one another.
A whole other world of construction is glinting there in the late summer woods.
Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. His book “A Backyard Book of Spiders in Maine” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.
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