Fall 2002

New Spider Found in Swallow Cliff Woods

For more than five years, Petra Sierwald, entomologist for the Field Museum of Natural History in Chicago, and other specialists have inventoried the fungi, lichen, vascular plants, arachnids, birds, and mollusks — among other organisms — of Swallow Cliff Woods Forest Preserve in southwestern Cook County. Sierwald and museum volunteer Nina Sandlin, who joined the project last year, were specifically looking for spiders. One of the great many they found brought the county into the national limelight.

"As far as I know, this is the first time in the world that a group has attempted to do an entire inventory at one site," explained Sierwald, whose fascination with the complex reproductive systems of spiders drew her into a career in systematics and evolutionary biology.

Swallow Cliff is an 800-acre mix of oak savanna and prairie, a hilly landscape formed thousands of years ago by glaciers. When the inventory at Swallow Cliff began, ground spiders were sampled biweekly between 1996 and 1999. Using leaf-litter traps, team members collected one-kilogram increments of leaf litter that were placed in special cloth bags, which allowed the trapped arthropods to breath. At the museum, the leaf litter was then placed under a lamp. "With the exception of the mollusks that retreated into their shells, the other collected arthropods would dig down into the leaf litter to escape the heat," Sierwald explained. "They went through the litter, into a funnel, and then into a container filled with a solution that allowed them to be preserved for later identification."

In the end, the Field Museum team identified 14,166 individual spiders representing 21 families and 159 species at four sites within the preserve. Those that were not easily identifiable were taken to Mike Draney, a spider expert at the University of Wisconsin-Green Bay. "In March, I went up to Green Bay with a gym bag full of little vials of spiders," Sandlin recalled, "and Mike and I slaved over them. It was pretty clear that one was 'Walckenaeria something,' but it was hard to see the details we needed." Easy to understand, considering the spider was only two millimeters long.

The specimen turned out to be Walckenaeria palustris, a member of Linyphiidae, or the sheet web spider family. It is the first of its kind to be found in the United States. Sheet web spiders construct their dense, sheet-like webs close to the ground. "This was exactly the kind of find we were hoping to encounter," Sierwald stated. "We know that at least 40 percent of the organisms living in the soil have not been described. Discovering the Walckenaeria palustris is a reminder of how little we know and it brings us closer to our goal of knowing about every species in one given habitat."

— Jayne Bohner