Why Spiders Appear to Bungee-Jump from the Sky
by Eric Leach
It felt like the perfect spot to see some wildlife. So I plopped down in the sun-soaked grasses among the widely scattered ponderosa pines and waited. I was squinting, or possibly had my eyes closed, and when my vision came back into focus I saw a visitor. One of the most creative, silent and lethal predators of the natural world was directly in front of me.
In fact, this marvel of evolution hung less than a foot from the tip of my nose, suspended in mid-air. Not until the breeze stirred could I see the miniature bungee cord from which the spider twirled upside down. High above there were several strands of silk floating on air, but only blue sky beyond. Without a single tree limb overhead, where could my visitor have descended from?
When attempting to span long distances, like those between scattered ponderosa pine trees, aerial spiders will deliberately drift lengths of dragline silk on air currents. You can often see these glistening strands high overhead, streaming from tree limbs. Spiders tug occasionally on the thread to determine when their horizontal scaffold has made contact with a sold object and is firmly in place. From anywhere along this bridge the spider can rappel towards the ground, via another dragline.
For the last 220 million years, all spiders have had the ability to manufacture silk. The class Arachnida, to which spiders belong, is named after the Greek maiden Arachne. According to myth, the loom work of the mortal Arachne surpassed that of the jealous goddess Athene. Arachne was therefore condemned to an eternity of perpetual spinning in the form of a spider.
The filaments that spiders spin consist primarily of a protein called keratin, which is also found in eyelashes, nails, and feathers. The large abdomens of spiders are filled with the components of silk in liquid form. On demand spiders can extrude silk through apertures called spinnerets, wringing out the water and producing dry, insoluble fibers. Having numerous spinnerets, like tiny spigots, along the bottom of its abdomen allows a spider to produce threads of different diameters, much like the various attachments placed on pasta machines and cake decorators. Additionally, by controlling the ratio of materials supplied by multiple glands, spiders can vary the ingredients and physical properties of their product.
The thickest and most versatile of the materials crafted by spiders is called dragline silk. Dragline silk is one of the strongest and most versatile materials known. It is five times stronger than steel and thirty percent more flexible than nylon. This combination of properties is very useful to the spider.
Evolutionary theory suggests that the earliest spiders were all ground-dwellers, until they began to explore the world vertically in the pursuit of new insect food sources. With dragline silk attached along the way as their safety rope, they began to climb plants and trees and eventually specialized in the capture of flying insects. These predators now employ many strategic web designs to facilitate the capture of their insect prey, and dragline silk serves as the structural framework for these traps.
When attempting to span long distances, like those between scattered ponderosa pine trees, aerial spiders will deliberately drift lengths of dragline silk on air currents. You can often see these glistening strands high overhead, streaming from tree limbs. Spiders tug occasionally on the thread to determine when their horizontal scaffold has made contact with a sold object and is firmly in place. From anywhere along this bridge the spider can rappel towards the ground, via another dragline. The spider descends headfirst, controlling the silk with a claw on one of its hind legs. This is when spiders appear to bungee -jump from the sky.
Silk is an extremely valuable resource, and spiders seem to know it, for they are into recycling. It is common practice for them to eat their own silk and reuse all the materials. So the next time you want to see some fascinating wildlife, you probably won’t need to look very far.
Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.
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