The Shark and the Jellyfish: More Stories in Natural History

Spider on the Fly

43 Spider on the Fly Though spiders are walkers, nimble afoot and wingless, they are not shy about taking to the sky. They spend much of their time suspended weightless and are always ready to take off. They may patrol gravity bound across acres of trackless foliage, but if their course leads them to a precipice so steep their tiny eyes cannot see the bottom, they step over the edge and off into space without breaking stride. As quick as a spider step, they affix a lifeline at their feet, jump as far as they can, and sail through the air with eight legs spread wide. Then they pull their tether tight to arrest their fall, turning their trajectory into a broad, swinging arc. Spiders fly—on diaphanous wings of spider web. They spin that web constantly—fine protein strands recycled from their high-protein diet. The line functions as a ninth appendage upon which they are utterly dependent. Even on solid ground, spiders prefer to stand on quivering strands of silk. The most terrestrial wolf spider spins a few strands around his bivouac and places a forefoot on one of them to serve as his ears. Telltale vibrations transmitted down the line tell of wind and weather conditions, signal the passage of potential mates or prey, and warn of the approach of danger. A spider makes minimal weights of web at a time—mere micrograms— yet she can play it out yard after yard. At daybreak, she collapses all the web in an orb that covers a square meter, gathers it into a ball no bigger than she is, and then eats it all in a few minutes (unless it has captured too much dust). The following evening she recycles her work, extending a very small mass of web very far by spinning it very thin. The web is but a fraction of a micron thick, too slender for prey to see (or easily avoid). We see the invisible threads only from the way they bend—diffract— light. At low sun angles, a meadow’s contours come alight, outlined in scores of glistening filaments—spectral wrapping that traces spider paths across the land. The silver strands rappel diagonally through space from branch to branch to blade of grass. A hundred spiders could hunt each square meter of meadow, and all of them would escape notice but for their shimmering draglines. 44 Spider web is the quintessential expression of the forces that hold the molecules of life together. These life forces follow an inverse distance law: they are stronger where the distances they span are shorter, like the force in the surface tension of water. The power of molecular bonds is relatively immense over extremely short spaces, such as the distance between two surfaces in touch with each other. A small moth accidentally flattened onto the surface of a streamside puddle will be held there forever by a power much greater than the insect’s strength could ever overcome. Bonds like those between water molecules hold the proteins of spider silk together in their polymers. They provide the adjustable links through which the elastic protein units instantaneously re-form when they slide past each other. And they provide the force that holds the web to whatever it touches. The momentum of a hurtling insect sends ripples out across a transparent orb of spider web from the point of impact—ripples that echo back and forth through the network, gradually fading until all motion ceases. The insect has just splashed into a virtual vertical sheet of clear water and is held to each strand just as firmly as it would be by the surface tension of a streamside pond. But the web lacks the total surface area of a pond, and an entangled insect works the hundreds of microscopic bristles on each leg against the strands to free itself. The spider must find and secure its prey before it escapes . Finding that prey is complicated by the geometry of the web—the spiral structure distributes the vibrations of a struggling insect uniformly across the disk, just as it distributed the energy of impact from one strand to all. Nonetheless, the spider at the center can interpret the omnidirectional signal. She has an appreciation of vibrational harmonics as keen as that of a maestro violinist. She will not be drawn to low-frequency vibrations—she ignores the snagged feather twisting in the wind or the ant testing a holdfast point. But signals...