Clarke Garry is a professor of biology at the University of WIsconsin - River Falls.

by Clarke Garry

Pick up almost any rock from the bottom of the Kinni and one will discover a dark, olive-green, worm-like creature crawling on the underside. This larva could be any of several species of netspinning caddisfly immatures. What makes these larvae especially noteworthy, beyond their obvious numbers, is that they lack the case typical of the majority of caddis species. Netspinners are members of the order Trichoptera, family Hydropsychidae. The family name translates from its Greek origins as "water spirit," although some translations would have it as "water butterfly."

The netspinners account for four of the 20 known species of caddisflies in the Kinnickinnic River. The most common of the 1272 hydropsychid specimens collected in my 1999 Kinnickinnic benthic inventory were: Ceratopsyche slossonae (33% of total caddisfly larvae collected), Ceratopsyche alhedra (22%), and Ceratopsyche alternans (4%). [For quantitative comparison the next most common kind of caddis collected in the inventory was the humpless casemaker, Brachycentrus occidentalis, representing an additional 23% of the Trichoptera.] C. alhedra and C. alternans tolerate water with only the slightest organic enrichment; these have tolerance values of 3 (based on a ten point scale, 0=excellent, 10=very poor) (Hilsenhoff 1987). C. slossonae (tolerance value = 4) can apparently survive in streams with moderate amounts of organic matter. And that brings up a very interesting point. Based on the 1999 dataset, C. slossonae is found in the Kinni from Kinnickinnic State Park and 15 of 16 additional evenly-spaced collection sites ending just north of I-94. Collection records of C. alhedra and C. alternans begin in the park and progress upstream only as far as Site 9 (Quarry Road bridge area). One must be cautious with negative data, but no collections of the latter two species have yet been made in the 8 sites upstream from this point. This distribution pattern has similar examples in certain mayflies and stoneflies.

Take a closer look at these larvae and one sees a strongly curving body with three pairs of thoracic legs and a pair of prolegs on the last abdominal segment. The prolegs have a distinctive tuft of hairs arising from them. Each of the three thoracic segments is covered dorsally with a dark, protective plate, a characteristic, along with numerous branched, ventral, thoracic and abdominal gills, that separates this caddisfly family (in the larval stage) from others.

The three common species of netspinners can be separated from each other by various patterns of light markings on the brown background of the head capsule. C. slossonae can be very distinctive with a single median yellow spot on its head. However, some of them (the "dark form" of C. slossonae) have a uniformly dark head as pointed out to me by Guenter Schuster and David Etnier, two well-known hydropsychid authorities, at the 1999 North American Benthological Society Meeting last year, when I showed them an array of Kinni netspinners. C. alhedra is distinctive because of a pair of light patches in a side-by-side pattern midway down the front of the head. And C. alternans is a classic "checkerboard" species with numerous intermixed light and dark areas on the head.

These caddisflies construct a fine-meshed silken catchnet attached to a rock or piece of woody material and oriented to the current. This net functions to collect suspended organic food materials including debris, various invertebrates, algae, and diatoms (Schefter and Wiggins 1986). Next to the net the larva lies concealed in a spun silken retreat, camouflaged with sand or organics, from which it exits to feed on filtered materials. The worm-like larval stage molts several times, progressively increasing in size. The pupa is then transformed to an adult within its reinforced retreat. Exiting from the case is the pharate adult (still in a pupal cover) heading for the surface to emerge. This emergence behavior, and the return of the females to the water to lay eggs, make these life stages particularly vulnerable to feeding trout.

Dr. Ralph Holzenthal of the University of Minnesota estimates that any given watershed in Minnesota or Wisconsin may have 50 to 75 species of caddisflies, as determined by adult collections. Therein lies the key to further understanding this fauna, a systematic collection of adults. The author has an adult study planned for the Kinnickinnic Watershed based on portable UV (blacklight) attraction. This will, additionally, provide detailed emergence patterns that are not available from larval studies.

References:

Hilsenhoff, W. L. 1987. An improved biotic index of organic stream pollution. Great Lakes Entomologist 20:31-39.

Schefter, P. W. and G. B. Wiggins. 1986. A systematic study of the Nearctic larvae of the Hydropsyche morosa group (Trichoptera: Hydropsychidae). Royal Ontario Museum, Life Sciences Miscellaneous Publications, 94 pp.