Untangling Spiders' Evolutionary Web

October 31, 2003

Chicago has a new resident—a species of recluse spider found elsewhere in Illinois but not in the city until recently, when a pest control company spotted it.

The spider is harmless to humans but invaluable to scientists embarked on a project called the Spider Tree of Life. The project's goal is to sketch evolutionary relationships among the 37,500 known species of spiders via a representative sampling of 500 species.

In his Origin of Species, in 1859, English biologist Charles Darwin wrote of "the great Tree of Life" that connects all organisms past and present and "covers the surface with its everbranching and beautiful ramifications."

In a contemporary manifestation of Darwin's vision, the National Science Foundation in Arlington, Virginia, has launched the Assembling the Tree of Life project and awarded U.S. $17 million in grants last year to 25 institutions to trace branches of flora and fauna evolution.

"Spiders are the dominant terrestrial carnivore," said Ward Wheeler, curator in the division of invertebrate zoology at the American Museum of Natural History in New York and lead investigator for the U.S. $2.7 million Spider Tree of Life project. "They are all predators—they have a huge influence on the ecosystem."

The Spider Tree of Life quest has led Petra Sierwald, assistant curator of zoology and insects at Chicago's Field Museum and a colleague of Wheeler, to prowl downtown basements. Recently she found a family of four females, four males and three "teenage" recluse spiders in an apartment building.

"The basement is a really nice place for these spiders," Sierwald said. "The heating ducts bring warmth, leaky pipes moisture, and the rats and cockroaches provide a great food delivery service."

From Chicago to Myanmar

Last summer Sierwald traveled much farther from home—to Myanmar. As a guest of the forestry department there, she taught courses on arthropod collection to local students and then found specimens of nursery web spiders and wolf and lynx spiders in a park near Yangon.

Different spiders required different capture techniques. For well-camouflaged webs, Sierwald uses an atomizer to spray cornstarch, which "paints" the webs. She finds the spider and either flicks it into a vial with a spoon or uses a suction device.

A flashlight turns up the wolf spider, whose eyes shine turquoise in the beam. "The reflections are so striking that you expect a very big spider—but often they are just little guys the size of the nail of my pinky finger," Sierwald said.

The need to retrace the Tree of Life stems from the belief that earlier methods of taxonomic classification prior to about 1970 were flawed.

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