The orb web, the classical wheel-shaped net that made Charlotte and countless other spiders famous, is spun by two different groups of spiders. For a long time, scientists debated whether these two groups had evolved this marvel of engineering independently. Now, new evidence reported in the 23 June issue of Science, published by AAAS, the nonprofit science society, suggests the orb web had a single evolutionary origin and may have been snagging flying insects as early as 136 million years ago.
New genetic evidence from one set of orb-web spinners called the Deinopoidea confirms that the group shares some key silk proteins with its fellow orb weavers the Araneoidea, according to postdoctoral Jessica Garb of the University of California, Riverside and her colleagues. Based on fossil evidence, the ancestor of the two spider groups probably lived at least 136 million years ago, making the orb web an ancient adaptation.
In addition to spiders that construct typical orb-webs, deinopoids include the ogre-faced, net-casting spiders that throw a modified orb web stretched between their legs over their prey. Araneoids include the orb weavers such as golden silk spiders with their traditional spiraling web as well as sheet-web weaving spiders.
Garb says the finding "does not support a double origin for the orb web," but indicates that the unique design evolved only once.
"A lot of people had said over the years that the orb web was a pinnacle of adaptive design. Our work confirms that not only is this web type very old, it was also lost in certain lineages of spiders," Garb says, noting that some of today's descendants of the early orb weaver, such as the familiar black widow spider, weave a tangled web instead of the orb.
In a second study, Enrique Peñalver of the American Museum of Natural History and colleagues provide a unique glimpse of an early weaver's handiwork. The researchers found pieces of a web with several insects still entangled encased in a 110-million year old piece of amber from a site in Spain.
The amber contains 26 web strands with a mite, a wasp leg and a beetle adhering to some sticky thread with visible droplets of web "glue." The find is the oldest known example of a web with trapped insects, according to co-author David Grimaldi of the American Museum of Natural History.
Although Grimaldi and colleagues say there is not enough of the web preserved to know for sure, the parts that are preserved suggest an orb web design. In any case, Grimaldi says, the find indicates that the challenging silken traps of Early Cretaceous spiders may have put the pressure on many types of flying insects to evolve. Although the insects in the ancient web belong to extinct groups, "their size and diversity are precisely what one would expect to see in modern webs--small wasps, flies and beetles, groups that are abundant and diverse today and that are important pollinators as well," Grimaldi says. "Apparently, spiders have been fishing insects from the air for a very long time."
But do orb weavers all use the same sort of fishing line? To find out, Garb and colleagues focused on the genetics of several key silk proteins that form the building blocks of an orb web, including the outside wheel, spokes and sticky capture threads of the orb's spiral.
The capture spirals of deinopoids and araneoids use different techniques to achieve stickiness. Araneoids produce glue droplets to make their capture spirals adhesive. Deinopoids, on the other hand, wrap their capture threads with a different type of silk fiber that "the spiders comb, until it almost has the appearance of Velcro under a microscope, and they snag insects that way," Garb explains.
The sticky difference was part of what made scientists think deinopoids and araneoids had independently evolved the orb web instead of inheriting it from a common ancestor. But no one had looked at the underlying genetics of the deinopoid web silks. Working with assistant professor Cheryl Hayashi and two undergraduate students, Garb and the team now show that both spider groups use the same set of web-building silks. The protein building blocks of spider silk are of great interest to industry, which would like to duplicate silk's amazing blend of strength, stretchiness and toughness for things like bandages, bulletproof fibers, aerospace tethers and nets. But for these applications to become a reality, researchers need to understand exactly how spider silks differ down to the genetic level.
"Because there's this diversity of spider silks and spiders use them for different functions, they have different mechanical properties," Garb says. "And to understand where this variation in mechanical properties comes from, we have to start with looking at the proteins that make up these silks."
Since most silk studies have been done on one or two species of araneoid spiders, Garb says the new look at deinopoid silk expands the list of "potential models for new materials" designed with the diversity of natural spider silks in mind.
"Silk Genes Support the Single Origin of Orb-Webs" by J.E. Garb et al.
"Early Cretaceous Spider Web with Its Prey" by E. Peñalver et al.
The Garb study was supported by the National Science Foundation and the Army Research Office. The Peñalver study was supported by the Spanish Ministry of Education and Science.
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