The crowds of spiders of Browns Canyon

By Jeff Mitton

Each year in early June I travel to Salida to give a presentation on natural history to 120 river guides attending a workshop organized by the Headwaters Institute.

I use this trip as an excuse to stay at Hecla Junction Campground in the Browns Canyon Wilderness on the Arkansas River. Technically, it is a Wilderness Study Area, but it is a gem, fully worthy of wilderness designation.

Browns Canyon is a naturalist's delight.

Canyon wrens call from the cliffs along the river, and bighorn sheep graze serenely on the steep rocky hillside across the river. Tree lizards skitter dexterously over rocks. Cedar waxwings comb the junipers for berries, and violet-green swallows soar, arc and climb in aerobatic pursuit of insects. Cacti andyucca bloom brilliantly. Tiger beetles prowl the narrow beaches, and western swallowtails sip water and glean minerals from puddles.

On my first visit to Browns Canyon I came upon a spring and seep system in a meadow 200 yards west of the campground. The primary spring fills a shallow pool, irregular in shape and approximately 15 feet across, fringed with cattails and grasses. A tiny stream exits the pool and meanders downhill for 100 yards or so before losing speed and volume, leaving only salt crusts.

In several other places in the same meadow water makes its way to the surface but flows as a thin film over the ground. In August, when other meadows have turned brown and crunchy, this meadow is green. The water flows year-round.

A ranger told me that the water flows through the winter without freezing. Perhaps the water has a geothermal component, or perhaps it is salty, though not to my taste.

Green plants and a constant source of water support a diverse and abundant community of insects. Dragonflies and damselflies perch on the grasses around the pool and hunt over the entire meadow. Iridescent long-legged flies, conspicuous for their bright colors, are visible wherever the ground is moist. Many species of butterflies fly erratically over the meadow, and small grasshoppers and moths climb on the grasses in the water seeps.

The abundant insects feed a dense aggregation of spiders in the genus Pardosa,commonly referred to as thin-legged wolf spiders. Indeed, their legs are slender and each leg is bristling with long spines pointing up and out. These spiders are neither large nor tiny; in their normal posture they cover more than a dime but less than a quarter. I think most of them are a single species, maybe Pardosa valens,but unambiguous identification of species in this group requires examination under a dissecting microscope.

Two color morphs are common. The more colorful morph is occasionally seen with an egg sac attached to its spinnerets, so these are females. They have a white band around the side of the cephalothorax (the first body part), a tawny stripe down the center of their abdomens and elaborate patterns of silver, blueish gray and brown on the body and legs. Males also have a light band around the cephalothorax, but they are darker with some light flecking on the abdomens and their legs are more conspicuously banded with white, black grey and tan.

As I walk across the meadow, they scatter before me so I can usually see up to a dozen at a time. I visit them every year, and every year I think to myself, "I have never seen so many spiders."

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.


Jan. 20, 2011