MTPR

Unravel The Silky Mysteries Of Spiderweb 'Stabilimenta'

Nov 18, 2015

I hardly would have noticed her among the dry grasses if it were not for an unusual structure incorporated into her web: a thick white zig-zag of silk descending from the center which drew my attention before I noticed the spider waiting in the middle. What was this silken aberration? Wouldn’t it make the otherwise cryptic web more conspicuous to spider-seeking predators or web-avoiding insects? Why would a spider take such risks?

Eight banded legs, eight beady eyes, one intricately constructed silk web. From a contemplative distance of three inches, the banded garden spider is a remarkably beautiful creature. She can measure an inch and a half in diameter and is one of many orb-weaving spiders common to forest, field and urban garden.

Her back is delicately striped black, brown and gold and the intricate black and yellow pattern of her underside resembles one of those sanity-determining ink-blot tests of the 1960s. Her web – and it is typically the female who builds the largest and most elaborate webs – is the classic orb design: a sticky silken spiral radiating from a hub of support lines.

I hardly would have noticed her among the dry grasses if it were not for an unusual structure incorporated into her web: a thick white zig-zag of silk descending from the center which drew my attention before I noticed the spider waiting in the middle. What was this silken aberration? Wouldn’t it make the otherwise cryptic web more conspicuous to spider-seeking predators or web-avoiding insects? Why would a spider take such risks?

These zig-zags of silk found in many orb webs are termed stabilimenta, as they were once thought to aid in stabilizing and strengthening the web. It turns out, though, that this extra silk is laid too loosely on the web to add any extra strength or absorb shock. In addition to the zig-zag design that I noticed, stabilimenta can take on a variety of shapes including spiral discs, crosses and vertical lines. Numerous ideas about the function of stabilimenta have been proposed over the years; some are more plausible than others.

Perhaps this additional silk helps to support the spider’s weight. Is it simply extra silk left over from web construction? Maybe the silk has special properties to reflect UV light and thus attract insect prey to the web. Some have even suggested that it serves as a “love path” to direct the male to the female. Today, most arachnologists (those scientists who study the eight-legged) mostly set aside such romantic notions in favor of theories of predator defense and web protection.

So, back to my original question: why would a spider risk making her web more visible? As it turns out, this may be precisely the point. The prevailing theory favors predator-defense. It seems plausible that stabilimenta act to conceal or distort the outline of the spider or to increase the apparent size of the spider by adding irregular shapes to the web. This may effectively ward off potential predators. Stabilimenta may also act to advertise the web itself so birds can avoid flying into the sticky silk, a bonus to bird and spider alike.

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Fieldnotes," 10/20/15. Listen on air or online Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or via podcast.)