Silk is created as a liquid in special silk glands within the spider's abdomen and released from three pairs of spinnerets before it instantly solidifies.
Nature's architects and engineers
Natural construction by spiders, birds and beavers is equally impressive as any manmade marvel.
Kathryn A. Kahler
From the time we are youngsters, we have a fascination with construction, building and excavation. We start with our hands and simple tools like sticks. Then we move on to hand tools and machines giving our children toy hammers, drills, screwdrivers, or dump trucks and earth movers. We visit architectural landmarks and gaze with awe at the brilliance of those who built them. We are amazed at the marvels of engineering like roads, rails, bridges and tunnels we travel every day.
But consider for a moment the far more fascinating architects and engineers of nature. They don't work with blueprints, tools or computers, advanced reasoning or hands with opposable thumbs. Yet their construction techniques are no less awesome than ours. In some cases, we even have a hard time duplicating their elegance.
Take spider silk, for example. Scientists have studied it for years and only recently uncovered some of the mysteries of its molecular structure. It is made up of complex protein molecules that give it tensile strength greater than high-grade steel or fibers like Kevlar, yet it is as light as air – hence, its more fanciful name, gossamer. On top of that, it's as ductile (or stretchable) as modern manmade fibers like nylon.
All spiders possess spinning glands and make silk for different purposes. There are seven or eight kinds of silk, but most species typically use fewer than six types of varying strength and viscosity. Silk absorbs moisture and, generally, the more water it contains, the more elastic it is. Silk is created as a liquid in special silk glands within the spider's abdomen and released from three pairs of spinnerets before it instantly solidifies.
Spiders use silk to spin webs of varying design, usually to catch prey. Some webs are spiral, some are flat sheets or funnel-shaped, and others have an irregular shape, like the cobwebs in attics and basements. Insects get caught in the sticky silk and are held long enough for the spider to wrap them up for a later meal. Spiders also use silk to make sacs to protect their eggs; to pull together the sides of leaves to form a tunnel to rest or hide in, or to protect their eggs; to lay down a dragline that helps them find their way back home; as a safety line if they fall; or to hang from a leaf when they molt.
One of the most interesting uses of silk is for dispersal, or ballooning. Young spiderlings climb to the top of a tall plant and throw out a silk strand, almost like releasing a kite or parachute. The wind takes the silk and stretches it until it pulls the tiny spider from its perch. It floats on the wind until it lands, sometimes only inches or other times many miles away.
The burrowing wolf spider doesn't spin a web but uses silk to reinforce its nest and to form an egg sac, which it carries behind high on its abdomen. These spiders, found on dunes and sandy beaches in the upper Great Lakes states, dig neat sand burrows about 3/8-inch across and line the holes with silk to cement the sand grains together and prevent the burrow – which can be three feet deep – from collapsing. At night, the spider patiently sits at the burrow entrance and waits for unsuspecting prey to pass.
The insect world offers many other examples of amazing engineering feats of nature, especially in the Hymenoptera order of wasps, bees and ants. Well known for their construction techniques, these insects were given names like masons, potters, diggers, carpenters and miners.
This order includes insect families that deliberately build nests – some solitary and others part of social communities. They may nest in holes in the ground – either excavated themselves or abandoned by small rodents – or construct nests in hollow trees, on tree branches and around our houses.
The group includes solitary nesters, each having sole responsibility for building a nest to protect its young, and provide it with food during the larval period. Social bees, wasps and ants, on the other hand, live together in a colony with a queen and castes of workers that perform functions such as food gathering, nest enlargement, cleaning, temperature regulation and brood maintenance.
One type of solitary nester is a potter wasp native to Wisconsin, Eumenes fraternus. After choosing a nest site in a burrow or hollow twig, the female potter wasp begins mixing mud by bringing bits of dirt or sand alternately with drops of water in her jaws. She pounds them together to the right consistency, and forms them into a hollow sphere with a flask-like opening. The potter then busies herself collecting spiders, caterpillars or other larvae, stinging them (to paralyze them) and pushing them into the opening. They will serve as the food supply for her larva as it develops and paralyzing them assures they will last longer than if they were dead. Once she decides there is sufficient food supply in the flask, the wasp lays a single egg, suspends it from the roof with a thread-like filament just above the food cache, and seals up the opening with mud. The wasp may add more cells adjacent to the first. When the egg hatches, the larva stays in its hanging egg case, reaches into the pile of caterpillars and begins feeding. After a few days – and once the danger of being squashed by an escaping caterpillar has passed – the larva falls from the thread and eats until it pupates. The new adult wasp chews its way out of the pot.
Paper wasps are social insects that construct nests of a papery material they make by collecting plant and wood fibers, moistening them with saliva and chewing the mix to a pulp. One species common in the Midwest is Polistes fuscatus, the native "golden paper wasp." Its nest is suspended from a single, central stalk, or pedicle, and is shaped like an upside-down umbrella made up of perfectly shaped hexagons. These wasps build nests in protected places, like under eaves of buildings, under bridges or in dense vegetation. Paper production requires water, which the wasps collect in droplets and bring back to the nest in their jaws. They also use these water drops to moisten the nest surface. When the moist paper surface is fanned by their wings it also serves as a very effective cooling system for the nest.
Carpenter bees are solitary bees that build nests by tunneling into wood – often eaves, window trim and decks of our homes. They don't eat the wood, but instead vibrate their bodies to create a kind of power saw effect as their mandibles rasp against the wood. They deposit the bits of sawdust outside the hole and use it later mixed with saliva to partition a tunnel. The bees start a tunnel hole of about one-half inch in diameter, about two inches deep, then turn 90 degrees and tunnel with the grain, sometimes for more than a foot. The female uses the tunnel for a series of brood cells for her six to eight eggs, depositing a mixture of pollen and nectar with each egg and sealing up each cell with a partition of chewed wood.
In terms of architectural diversity, birds get a four-star rating. Some nests are down-to-basics efficiencies with few amenities; others are awesome in their elegant grace. Birds excavate tunnels in trees and stream banks, build floating platforms on marshes, plaster mud under eaves and hang intricately woven cups from tree branches. They use all manner of vegetation – branches, twigs, grasses, cattails, sedges, leaves, plant fibers, moss and lichens – as well as animal materials – feathers, spider silk and hair. Tufted titmice have been known to pluck hair from live woodchucks, dogs and even humans.
The master craftsmen are the woodpeckers, and chief among them is the pileated. Like other woodpeckers, their bills are flattened laterally and make excellent chisels. After choosing an appropriate spot – often in a dead snag, but sometimes in a live tree – both sexes begin excavation. The birds deliberately swing their heads back and forth to achieve the best angle, chipping out pieces of wood. The entrance hole is about 3½ inches in diameter and the cavity can be two feet deep. The birds are Spartan in their interior design and don't line the cavity with anything but wood chips. Pileateds build a new nest each year and their old nests are often used by other birds, flying squirrels or other animals. Pileated woodpeckers' favorite food is carpenter ants, which they pursue with a vengeance, scaling the bark off trees and chipping out large rectangular holes so big they sometimes cause the tree to break in half.
Water birds tend to build large platform nests, often floating or anchored to standing live plants. The Virginia rail builds a platform of cattails, reeds and grasses, usually with living plants forming a canopy over it. King rails will weave a canopy over the nest and add a ramp leading to the entrance. The tiniest of marsh birds – the marsh wren – weaves a football-shaped mass of wet grass, cattails and rushes, lined with fine grass, plant down and feathers. The nest stands above the water, anchored to standing cattails or bulrushes, and has a side entrance.
Birds are masters at eco-friendly design, their homes blending so well into the landscape as to become invisible to protect the nest and young. Ovenbird nests are so well concealed they are seldom seen. The female builds the nest in a depression of dead leaves on the forest floor using grasses, plant fibers, weed stems, leaves, rootlets, mosses or bark. She shapes an arch of dead leaves and surrounding vegetation over the top with an opening at ground level, making it invisible from above. The nest is lined with fine rootlets, fibers and hair.
Some birds take advantage of the strength and elasticity of spider silk by incorporating it as a building material. Ruby-throated hummingbirds use the silk to tie their nests to the upper surface of a branch or twig. The stickiness and elasticity of the silk helps the rubythroat mold its perfectly shaped cup and adorn the exterior with lichens.
Another spider silk user is the yellow warbler who crafts a small cradle like nest in the fork of a shrub. She brings beakfuls of milkweed fibers, hemp, grasses and plant down to the site, using her body to form a deep cup. She uses her beak to weave the pieces together and spider silk to cement them to the supporting branches. The nest is an amazingly strong structure for its delicate appearance. Sometimes, the yellow warbler builds second or third stories on her nest, but not because she needs the space. Unfortunately, this species is one of the prime victims of brown-headed cowbird parasitism. If she has only one of her own eggs in the nest or if the cowbird egg is laid first, she will reject the cowbird egg and build a new nest on top of the first. If the parasitism continues, the yellow warbler continues to build layer upon layer, sometimes as many as six times. If she has two or more eggs in the nest before the cowbird egg is laid, she accepts the parasite egg and dooms her own; the cowbird egg hatches before the warblers and grows larger and faster, forcing the smaller warbler chicks out of the nest.
The heavy equipment operators of the natural Wisconsin landscape are the beavers, capable of dramatically altering their landscape in a matter of days. They are best adapted to living in a deep-water environment, and when none exists, they set about to create it using the only tools in their tool belts – their teeth, feet and special physiological adaptations that help them work underwater. Their persistent habits are often the bane of farmers, homeowners, road crews and trout managers.
If a pair of beavers looking for a site to build a lodge doesn't find a suitable place, they will often start by damming a stream to create the lake or pond they need. A dam is constructed of logs, sticks, stones and mud, all of which the beavers tow or carry to the dam site with their teeth or front paws. If they don't find logs or sticks lying around, they cut branches and fell trees on their own. Their four chisel like incisors and strong jaws are perfectly suited for the task.
When felling a tree, the beaver stands with its front paws against the tree, turns its head sideways and bites a 45-degree cut through the wood. It then bites a series of cuts down the tree a distance about equal to the tree diameter, then turns its head the other way and bites an opposing 45-degree cut. To complete the chip, it bites through the chunk between the two cuts, tears it loose and begins another cut. It continues to work its way around the tree, taking gradually smaller bites as it reaches the center. If the tree is on a steep slope, the beaver will more often cut the tree on the uphill side. Once the tree falls, the beaver uses its incisors to pull the log to the dam or floats it into position.
When constructing their dam, the beavers grasp the butt ends of cut brush in their teeth and front paws and poke them into the stream bed or river bank. They continue to add brush in this fashion until they have a base across the stream, adding rocks, stones and mud to reinforce it. Next, they use their teeth to add smaller diameter sticks to the mass of brush, securing them to the base and chewing them off level with the top of the dam. Once enough wood and brush is piled to raise the dam above the water level, the beavers switch to cementing mode. They dive beneath the surface, bringing loads of mud to the dam site under their chins and between their front paws, and pushing mud between the spaces in the brush. As the water level in the pond rises, the beavers continue to add material to the top. Beaver dams can be massive, reaching heights of several feet, spanning distances of a quarter mile or more, and so sturdy that a person can walk on top.
Beavers are aided in their underwater work by special adaptations. Between the front incisors and back molars they have a flap of skin that prevents water and wood chips from entering their throats when cutting wood underwater. They also seal off their noses and ears, and have nictitating membranes to cover their eyeballs like goggles. Slightly webbed front paws deftly dig and hold sticks, stones and mud, and large, thickly webbed hind feet propel them through the water. When digging tunnels, beavers start out using their front paws only, but as the tunnel lengthens, they use hind feet to kick the dirt out of the entrance. Beavers use their flat, rudder like tails for balance when felling trees, and for propulsion as they drag branches through the water.
Beavers have physiological adaptations as well that aid in their aquatic endeavors. They can slow their heartbeat to reduce blood flow to extremities and conserve oxygen for the brain. They have larger than normal lungs to hold more air, and their livers have a greater capacity for oxygenated blood. While a human can only exchange at most 20 percent of his lung capacity, a beaver can exchange 75 percent when it reaches the surface to breathe, allowing it to stay underwater for up to 15 minutes and swim distances of one-half mile.
So as you admire the results of your next do-it-yourself construction project made from step-by-step plans and precut lumber, take a humbling step back and think about nature's architects and engineers that accomplish their building projects with little more than teeth, jaws, beaks and feet, driven only by instinct.
Kathryn A. Kahler designs and constructs feature stories from Madison for Wisconsin Natural Resources magazine.