Wading for Bugs: Exploring Streams with the Experts

8. Mystery of the Spine-Adorned Caddisfly

48 8 Mystery of the Spine-Adorned Caddisfly Marilyn Myers Tucked into the arid Great Basin lands of eastern California and western Nevada, the desert springs I explored for my graduate research were full of surprises. Finding the springs proved to be only the first challenge. We knew scarcely anything about what lived in them; my goal was to discover and study the invertebrate diversity that lived in these tiny oases. At first, I collected adult insects. There were the conventional approaches: setting out blacklight traps to draw insects in at night, placing emergence traps over the springs to catch adults as they emerged from the water, hanging sticky traps to catch adults on the fly, and sweeping with my wide net across plants growing next to the water. And there were the not-so-conventional techniques: putting out pans of water mixed with antifreeze to catch insects as they laid their eggs in my pans instead of the spring, and snatching by hand or by net the adult insects resting below undercut banks. At a few of the springs, adults of the caddisfly Pedomoecus sierra were common. But the mystery was that my benthic collections rarely contained larvae of this species, and I had not collected any pupae. Adult Pedomoecus were small (only about a half inch long) and not known to be strong fliers. Because the desert springs were separated by miles of inhospitable habitat, I figured that the larval stages must be hiding somewhere in the spring runs. Benthic sampling required innovation because the spring systems were so small. A Surber sampler, typically used for quantitative Mystery of the Spine-Adorned Caddisfly 49 sampling, with its one-foot-square frame, was too big for many of the springs, and usually there was insufficient flow to carry the insects into the collection bag. Instead, I pushed a four-inch PVC pipe into the substrate and removed everything within the pipe. With these samples I could describe the animals I’d collected in a given volume and quantitatively compare across the many springs. For qualitative sampling I kicked samples into a D-frame or a small aquarium net. I sampled other microhabitats typically occupied by caddisfly larvae, like leaf packs, aquatic plants, and woody debris, by picking or scooping them up by hand. Often I lay belly-down next to the spring run and just watched the flowing water and rocky bottom substrates, in the same manner a birdwatcher looks for movement in trees or shrubs. By watching quietly, I could easily see a large number of aquatic invertebrates going about their business in the crystal clear water, oblivious to my observation. Despite these efforts, my samples never contained more than an occasional Pedomoecus larva, and I never observed them on the rocky surfaces of the springs. To help solve the mystery of the missing larvae, I went to the “encyclopedia” for trichopterists, Larvae of the North American Caddisfly Genera (Trichoptera) (Wiggins 1996), to find clues. Describing Pedomoecus, Wiggins wrote, “[t]he unusual morphological features suggest that the larvae have some specialized way of life, not yet understood.” He was referring to an amazing assortment of stiff spines Pedomoecus larvae have on their head and legs: they appear as if they are armored for battle. In addition, the thorax and first two abdominal segments have girdles of long setae (hair-like structures) that give the larva a furry appearance. But the case or home of the larva, slightly curved and made of little sand particles, is unremarkable. It provided no clues about where the animal might reside. One day, despite conventional wisdom about caddisflies, I found larval Pedomoecus in the last place I expected. The Glass Mountains of Mono County, California, were formed by volcanic forces, and most are composed of rhyolite, a rock that decomposes into angular, glass-like sand. After finding many adults at a small spring brook in those mountains, I knew 50 Sleuthing for Caddis the larvae had to be present, and I was determined to find them. Sitting bank-side along the spring, I observed discrete areas of sand that appeared as mini-dunes in the middle of the channel, constantly moving and changing. It suddenly dawned on me: that was one place I hadn’t adequately sampled. Moving sand: no one would expect to find a caddisfly in that kind of microhabitat. I grabbed a soil sieve, scooped up about a...