Crafty Signs Spun in Web Say to Prey, 'Open Sky'
By NATALIE ANGIER
Published: April 19, 1994
NEW HAVEN— THE room beckons warmly and widely, lighted high overhead by gladdening bulbs that recapitulate the brilliance and spectral range of the sun. But as you step into this parlor of a laboratory just try not to jump, or at the very least stiffen.
In every corner, under every surface, dangling into one's hair, brushing against one's shoulder, are large, dainty, lacy, spiraling, glittering spider webs. And tending each of these webs is a large, undainty, big-bellied, generously appendaged spider. There are dozens and dozens of spiders, some of them yellow, some brownish-black, some pale caramel. And all of them are weirdly frozen in place because -- what do you know! -- spiders really are more scared of humans than most humans are of them.
"A spider's first impulse when something larger comes along is to stop moving and hope it goes away," said Dr. Catherine L. Craig, an evolutionary ecologist at Yale University, who is studying the evolution of spider webs. She taps on a web to prod the little architect from its stupor. It skitters briefly and freezes again.
"There are cultural biases against spiders," Dr. Craig said, with some understatement. "Most people look at my roomful of hanging spiders and it's a nightmare for them. I had one student who came to my lab and volunteered to feed the spiders, but when she saw the spider room, her face contorted like this" -- Dr. Craig gives her freckled face a Munchian twist -- "and she said, 'EEEUUUWWW!' "
No such squeamishness for Dr. Craig. "I can't think of any place I'd rather be," she said, "than sweating in the sun in Panama playing with spiders."
Dr. Craig divides her time between field studies at the Smithsonian Tropical Research Institute on Barro Colorado Island in Panama and laboratory work at Yale with her in-house collection of tropical species of spiders that weave orb webs, including Argiope argentata -- a relative of the common garden spider found in the Eastern United States -- and Nephila clavipes. These are the spiders that generate the cobwebs of Halloween fame, as well as the less-familiar ladder webs, funnel webs, hanging webs and other lattices plain or fancy. Their webs are called orbs in the old-fashioned sense of the word, meaning circular.
To Dr. Craig, a spider web is not a passive structure or a simple sieve that catches insects that blindly fly into it, as had long been believed. Instead, she views the web as among the spider's most dynamic and responsive traits, a cunning weapon designed to lure prey by exploiting an insect's fundamental need for food, flowers and open spaces.
She proposes that spiders incorporate into their webs visual signals like attractive zigzag designs and faux floral colors that are irresistible to a wide range of insects. And because these signals are the cues insects use to forage for food, the creatures are not likely to evolve a means of detecting and avoiding the trickster webs without simultaneously jeopardizing their skill for finding a meal.
In an outpouring of reports put together over the last several months, Dr. Craig offers evidence for a genuine spider revolution at an unknown point in the past, resulting from minor modifications in the silk proteins of which webs are built. She focuses on the subtle details of the webs, trying to understand how the visual and mechanical properties of the silks that make up the webs may have changed over evolutionary time and contributed to the extraordinary success and explosive diversification of orb-weaving spiders.
The molecular changes improved the strength, elasticity and versatility of web silks and allowed the ancestral orb-weaving arachnids to emerge from their obscurity in the dim forest understory and begin laying traps in the open sun and other theretofore forbidden habitats, where whole new classes of prey became available for their dining pleasure. As a result of the evolution of a refined type of silk, she argues, the number of orb-weaving species increased by a stupendous 37-fold over the more primitive orb spinners that predated them. They have branched off into at least 10,000 different species, which means they represent almost a third of all spiders described to date.
The latest results will be presented in three papers to be published over the next several weeks in the journals Animal Behaviour; Evolution, and Behavioral Ecology and Sociobiology. In the studies, Dr. Craig and her colleagues combine detailed spectral analysis of spider silk with field studies of how stingless bees -- a major source of prey for orb weavers -- perceive and respond to variations in spider web design.
The results have far-reaching implications for understanding essential questions of ecology and evolution, among them the nuances of predator-prey interactions, and the mechanism through which new species arise. Dr. Craig sees the spider web as a beautiful means for weaving a molecular approach to biology with a more holistic view of animal behavior. "It's a way to tie together evolutionary studies of macroprocess like foraging behavior with considerations at the level of the genes," Dr. Craig said, in this case the genes that direct the spider's all-important silk production.
Dr. Craig also believes that by understanding the complex spectral features of a spider web, scientists can get a handle on how insects see their world.
"Her work has been very interesting for a lot of us," said Dr. George W. Uetz, a professor of biology at the University of Cincinnati who has studied spider foraging behavior. "It's made us look at insect-spiderweb interactions differently."
Dr. Uetz said that his earlier research had suggested that spider webs were passive filters for collecting insects, rather than real attractants for prey. "Her work has overturned that whole set of ideas," he said. "You might think I'd be depressed about it, but I'm not."
Dr. Uetz added that a more sophisticated knowledge of spider foraging strategies would help him in his own investigations of an unusual type of tropical spider in Mexico, which gathers in groups of up to 165,000 spiders per colony, building enormous communal webs across trees or power lines, and interacting socially, a rarity among spiders.
Dr. Craig began her study of spider webs nearly a decade ago, inspired by the intellectual possibilities of the subject rather than any sort of lifelong love of the beasts. "The last time I stepped on a spider," she said, "was the day before I began working on them."
She saw the web as a relatively neglected area of research, one that had been subjected to rather silly and uninformative manipulations -- like giving spiders hallucinogenic drugs and observing the mess they made of their webs.
Her first step was to investigate the common assumption that webs were simple colanders, invisible to the luckless insects that barreled into them. Such a supposition meant that interactions between web-building spiders and insects were passive and indirect, very different from the active encounter between, say, a lion and the gazelle it covets.
Dr. Craig soon proved these assumptions wrong. Through a series of experiments in which she examined and quantified insect responses to webs in different lighting conditions, she found that the potential prey could indeed see the silk meshes under certain circumstances. And often, when they got close enough to the web to make it out in all its filamentous horror, they made unmistakable efforts to fly around the trap. Seeing the (Ultraviolet) Light
Those findings prompted her to wonder whether spiders might seek to counteract that repulsion and, if so, how they did it. She first showed that many web silks have a noteworthy feature: they reflect light in the ultraviolet range of the spectrum. This property may serve to attract insects, which unlike humans can see ultraviolet light and are drawn to it from afar as part of a so-called open-space response: insects need open space to help them navigate, and because ultraviolet light can come only from the sun or the sky, a bit of it glittering is like a billboard proclaiming free range ahead.
But in her most recent work, some of it done with Dr. Gary D. Bernard of the University of Washington and Dr. Jonathan A. Coddington at the Smithsonian Institution, Dr. Craig has proved that ultraviolet shimmering is only one part of the orb weaver's story. As it turns out, this simple reflectance feature is a primitive characteristic of spider webs, and the more advanced orb weavers have come up with clever variations on the theme.
In the new experiments, the scientists have focused on two big web-weavers, Argiope and Nephila. Both spiders hunt in broad daylight, building new webs each morning to catch fast-flying, keen-eyed meaty creatures like bees, and consuming the silk traps each night to use their protein. The spiders' webs display markedly different optical properties, yet Dr. Craig has shown that both approaches effectively seduce -- and confuse -- the prey.
With artful flair, the Argiope spiders decorate their webs, placing thick strands of silk in the middle to create zigzags or cross-hatch patterns. The decorations appear white to us, but the scientists have shown that the adornments reflect ultraviolet light brightly, just as do the web silks of primitive spiders. At the same time, the silks making up the rest of the web, all the circling and radiating strands designed to catch and hold prey, show a marked reduction in their ultraviolet-reflecting properties.
Dr. Craig suggests that the decoration entices prey, not by stimulating a generalized open-space response, but by resembling blossoming grasses or nectar guides on flower petals, which also glow with ultraviolet light in a more benign effort to invite pollinators. Because the surrounding web is essentially invisible to the insect, the prey would be drawn toward the decoration, then get tangled up in the unseen catching threads around it.
Significantly, the spiders vary the placement of the decorations from one day to the next. In other new experiments, Dr. Craig has demonstrated that the variation is important to the spiders' foraging success. It turns out that many bees intercepted by webs manage to shake themselves free. By shifting the pattern of the central embellishment from day to day, the spider seems to interfere with the bees' ability to learn from their mistakes.
That is good news for the spider, not solely because the next time the repeat visitor may get stuck for good and thereby serve as a substantial meal but also because just getting caught benefits the spider: she appears to consume as a vegetarian side dish the pollen packets that escaping bees leave behind as they frantically disengage from the web.
For its part, the giant Nephila uses pigments to make its web appealing to prey. The scientists have learned that the spider will incorporate more or less yellow pigment into its silk strands depending on how bright the sun is over a period of several days: the more sunshine, the more yellow the web becomes with each passing day. Dr. Craig proposes that yellow is a brilliant choice of colors from a strategic perspective.
"Yellow is a very generalized visual signal that both herbivorous and pollinating insects associate with flowers and new leaf material," she said. If the spider web likewise has the color of the most abundant food resource, she added, the insects may have a much harder time evolving a mechanism to avoid it.
In field experiments with bees trained to forage for sugar water at the same spot several times a day, Dr. Craig and her assistants presented the bees with webs, hung on a small hoop and artificially colored blue-violet, green, yellow or unpigmented. With most of the webs, the bees quickly learned to fly up and over the hoops to get to the sugary reward. But in the case of the yellow-tinted web, the bees were confused, either repeatedly getting caught, or flying up to it and away. Advanced Weavers
Putting together the various strands of evidence, the scientists propose that the suppleness of their silks distinguishes the advanced -- or as scientists prefer to call them, derived -- orb-weaving spiders, like Argiope, from their primitive cousins, a small group of spiders called the deinopoids. The deinopoid spiders, the scientists suggest, are comparatively older and less varied in their web-making techniques.
Their silk uniformly reflects ultraviolet light. It is relatively weak and rigid. As a result, the deinopoids are limited to building webs at night or in dim light, where they may be hazily attractive to insects but not seen in any sharp, repellant particulars. They can catch only slow or small insects.
Not surprisingly, then, the primitive orb-weaving spiders count a mere 300 species or so in their group.
By contrast, the advanced orb weavers, called the araneoids, possess silk of great optical versatility and superior strength and elasticity. The spiders have moved everywhere, learned to do everything and made practically the entire insect kingdom their oyster.
Dr. Craig next plans to investigate the molecular and genetic differences between the primitive and newer types of silks, but already it is clear that the triumph of the great orb-weaving spiders hangs by a few slender threads.
Photo: Dr. Catherine L. Craig, evolutionary ecologist at Yale University, in the field in Panama studying spider webs. (pg. C13) Diagrams: "Building a Better Insect Trap" Some spiders, like the Silver Argiope above, weave struts called stabilimenti into their webs to entice flying insects by reflecting ultraviolet light. Part of the spider's body is also reflective in the same ultraviolet range. The rest of the sticky web has markedly reduced reflectivity, making it invisible to prey like this 0.4-inch stingless bee. "Elegant and Deceptive Variations" By shifting the pattern of the stabilimenti from day to day, the spider seems to interfere with the bees' ability to learn from their mistakes. Here are four variations on a two-bar embellishment of a web. (Sources: Dr. Catherine L. Craig; "Biology of Spiders," R.F. Foelix (Harvard University Press); "Spiders and Their Kin," Levi and Levi (Golden Press) (pg. C1) "Testing Reactions To Web Colors" Bees were trained to feed at specific locations, flying through an opening to reach a dish of sugar water (a). Confronted with webs, some bees were intercepted (b); some approached and retreated (c); some bypassed webs (d, e and f). (The New York Times; Illustration by Sandra Olenik and Michael Rothman) (pg/ C13)