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Sticky Subjects: Insights into ancient spider diet, kinship

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6:48pm, June 21, 2006
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Remnants of a spider web embedded in ancient amber suggest that some spiders' diets haven't changed much in millions of years. Separate research indicates that some groups of modern spiders that spin webs in the same pattern didn't stumble upon that design independently, as scientists had suspected, but evolved from a common ancestor. Both studies provide glimpses of spiders' evolutionary history.

Spider silk is made of proteins, so it degrades quickly and rarely fossilizes. When old specimens are found, they're most often preserved as single strands in amber, says David A. Grimaldi, an entomologist at the American Museum of Natural History in New York City. But in a piece of 110-million-year-old Spanish amber, he and his colleagues discovered a collection of silk strands. They describe this oldest known example of a multi-strand web, which still contained several insects, in the June 23 Science.

Grimaldi, his museum colleague Enrique Peñalver, and Xavier Delclòs from the University of Barcelona found pieces of at least 26 strands of silk preserved in the amber fragment, which measured about 18 millimeters in length. The longest strand of silk is about 5.7 mm long, and the strands generally measure between 0.6 and 1.9 micrometers in diameter.

Most of the silk strands are straight or slightly curved, and some are connected in a pattern similar to that seen in webs spun by modern orb-web spiders. In such webs, long strands of silk connect at the center, like spokes of a wheel, and the sticky threads that capture prey spiral out from that center.

Small droplets, presumably of spider-produced glue used to make the web sticky, adorn two of the strands preserved in amber, another sign that the ancient web maker probably was an orb weaver, says Grimaldi.

Regardless of its lineage, that spider apparently was successful—it had snagged a wasp, a fly, and a mite in its web. Those insects represent the three insect groups most commonly captured by today's orb-web spiders, Grimaldi notes.

There are two groups of modern-day orb-web spiders, says Jessica E. Garb, an evolutionary biologist at the University of California, Riverside. Their webs are remarkably similar in shape, but they differ by one telling detail. The araneoids snare prey by embellishing their webs with droplets of glue, as in the web found in amber. The deinopoids spin silk strands that they then comb until the surface is covered with sticky fibers—"like tubular Velcro," says Garb. Most scientists have presumed that the distinctive shapes of these spiders' webs arose independently in the two groups.

However, new research by Garb and her colleagues, also reported in the June 23 Science, makes the opposite case. The researchers analyzed genetic material in the silk-making glands of two species of deinopoid spiders. Along with 12 new kinds of silk proteins, they identified proteins called MaSp2 and Flag, which had been known to be produced only by araneoid spiders.

MaSp2 appears in the spider glands that generate silk for the orb web's radial threads, and Flag protein is found in the glands that spin the sticky threads, says Garb.

Garb's findings are strong evidence that the orb-web design evolved only once, says Samuel Zschokke, a zoologist at the University of Basel in Switzerland. "Evolution is a big puzzle, and this is another piece that fits into it," he notes.

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