Signs for Spiders' Diners

One thing I've always appreciated about northern wildlife is that our spiders are small. Mind, I'm in favor of the local spiders---anything that reduces the number of mosquitoes is a good creature in my book---but I'm glad they don't come in tropical (or even temperate-zone) sizes. An eight-legged strider as big as my fingertip is a welcome hunter of pests, but I can't shake the feeling that one the size of my hand might hunt me.

Though individually small, spiders in Alaska and the Yukon are a diverse group. Before much in the way of greenery has broken through the soil, dark-colored hunting spiders are out roaming for early insects. Scientists who study spiders---arachnologists---consider these wanderers to be fairly primitive examples of their kind. One clue is that they do not build webs to snare prey; that ability came later in spider evolution.

During summer, gardeners may find little spiders lurking in their flowers, mimicking the color of the blooms. This is a feature of the family of crab spiders, named for their appearance, not their edibility. Crab spiders aren't web builders either, the dense weaving that may appear among the leaves of the same flowers belongs to other kinds of spiders, considered to be a bit more advanced than the wait-and-pounce varieties.

Later in the season, the superb webs of the orb weavers show up, when morning dew makes these usually near-invisible nets glisten. The regular, geometric forms of these webs mark the efforts of recently evolved spiders.

Some of them are not perfectly regular. I've been saved many times from putting my nose through webs across woodland paths because of highly visible zigzag patterns running diagonally near the middle of the webs, To me, these looked like excessive reinforcements near the spiders' central resting place. I speculated that the thick zigzag served a double purpose--to warn off larger animals like me that would damage the web, and to distract the spider's potential prey. A mosquito might avoid the reinforced portion and be caught by the invisible threads nearby.

Like many perfectly reasonable-sounding theories, my speculation apparently has been disproved by research. It turns out that the thicker portions of those webs actually serve to attract the spider's dinner.

It's long been known that insects can see in the ultraviolet portion of the spectrum, and ultraviolet light attracts many kinds of them. Some flowers take advantage of this by drawing what amount to landing patterns on their petals to guide pollinating insects---patterns invisible to human eyes unless photographed with UV-sensitive film. What was unknown until very recently is that spider silk, like the patterns on the flowers, reflects ultraviolet light.

Scientists Catherine Craig of Yale University and Stephen Nowicki of Duke were interested in how the optical properties of spider webs changed with evolution. They found that silk from the earliest kinds of spiders, hunters that spin only to line their homes or cover their eggs, reflects ultraviolet light naturally. Silk from primitive web-weavers is even more enhanced in its ability to reflect UV.

The researchers then discovered that one of these basic webs illuminated by white light with an ultraviolet component, like sunlight, snared more fruit flies than one lit only by visible light. When they repeated the experiments with the zigzag-containing web produced by one of the most recently evolved spiders, they found that only the heavy pattern reflected ultraviolet light--quite brightly--while the rest of the web was invisible. They also discovered that the zigzag-decorated webs caught more than half again as many fruit flies as did the undecorated ones.

The researchers interpret their findings in terms of evolution. The capacity of spider silk to reflect ultraviolet light did not arise for the purpose of luring prey, but the pressure of natural selection preserved and enhanced the feature because it did attract insects.

For me, an eerie image comes to mind: in effect, spiders discovered that porch lights draw bugs many millennia before people stumbled on the same principle.