The Humpless Casemaker

By Clarke Garry

Many Kinnickinnic River visitors are familiar with the small (~10 mm), dark, tube-shaped cases commonly seen attached to rocks on the river bottom. Although these cases may be vacated, they are an unmistakable indication of high quality water in our river. These tubecases are fabricated by a caddisfly known as the humpless casemaker, Brachycentrus occidentalis (Trichoptera: Brachycentridae), an extremely common benthic (stream bottom) organism in our watershed.

This insect is one of the twenty species of caddisflies I've collected as larvae from the Kinnickinnic River. I presently estimate this species as the second most common caddisfly in the system. Of caddisfly larvae collected to date 23% (491 specimens) are B. occidentalis; the more common netspinner, Ceratopsyche slossonae, composes ~33% of total caddisfly larvae. The humpless casemaker has been collected as larvae at all 17 collection sites established along the length of the Kinni from Kinnickinnic River State Park to sites north of I-94.

The common name of this caddisfly comes from the fact that larvae of this species lack both the dorsal and lateral spacers (humps) on the first abdominal segment. (These adaptations play a role in water flow into and out of the cases of most caddisflies.) The observer will likely notice the dark head and thoracic plates, unusually long middle and hind legs, and green body. Technical identification requires observation of (in addition to the absence of humps): 1) small, widely separated plates on the third thoracic segment, 2) a strong furrow across the first thoracic segment, and 3) the presence of four dark setae (hairs) on the ventral (belly) side of the first abdominal segment. Useful keys for technical species identification are Hilsenhoff (1985) and Flint (1984).

Caddisflies (order Trichoptera) are close relatives of butterflies and moths (order Lepidoptera) and as such spin silk that is used for a variety of purposes. One of the major uses is in formation of the case in which the larva lives, which varies in size and composition in the humpless casemaker. In the first instars (early summer) the cases are assembled from small fragments of vegetation which are attached, using silk secretions, transversely; these cases are square in cross-section. As subsequent instars develop (later summer and fall), case enlargement occurs with a mixture of mineral and silk (Gallepp 1974) and lesser amounts of vegetation; at this time the cases become progressively rounder in profile. As the larva begins transformation to the pupa, larger sand grains are added to the case rims (Gallepp 1974). Silk is also used by larvae to do a tethered drift as well as to make cases and attach them to rocks or plant substrate.

Brachycentrus larvae ingest diatoms, algae, plant detritus, and other insects (Wiggins 1998). They feed both by filtering and by grazing. In the first approach, with the case attached to the substrate, they extend the middle and hind legs to extract food particles from the current. The grazing approach (case unattached) is based on scraping of algae from elements of the substrate.

Hilsenhoff in his 1985 summary of the Brachycentridae of Wisconsin records five species of Brachycentrus in the state. He notes, "All of them tended to be absent from streams that had been subjected to even small amounts of organic pollution, and probably for that reason they were mostly absent from agricultural counties in the south and east." B. occidentalis has an assigned tolerance value of 1 (based on a ten point scale, 0=excellent, 10=very poor) (Hilsenhoff 1987) which supports determinations of high quality water when biotic indexing is carried out. B. occidentalis is an inhabitant of cold streams, ". . . mostly in those with a significant flow from springs." (Hilsenhoff 1985).

My survey records indicate that mature B. occidentalis larvae disappear entirely from the river in late March (the latest record I have is 20 March) and reappear as small larvae in early June. This suggestion of adult emergence (hatch) agrees well with Hilsenhoff's (1985) emergence record of 13 April to 27 May, given a pupal duration of 31-34 days (Gallepp 1974).

Some additional behavioral aspects of Brachycentrus life are of interest. Borger (1980) discusses the availability of cased caddis larvae, including B. americanus to trout during daytime drift. This, of course, is in direct contrast to the classic nocturnal behavioral drift phenomenon. Waters (1972) references a report of brook trout feeding on day-drifting B. americanus larvae. Gallepp (1974) in laboratory studies of B. occidentalis demonstrated that, as pupation approached, filtering behavior ceased and individuals began moving about. With this movement came increased drift in his laboratory stream. An additional curious symbiotic relationship was discovered by Gallepp in studies of wild-collected B. occidentalis pupae from a trout stream in central Wisconsin. Thirty-two percent of these pupae were infested with a larva of the chironomid midge, Eukiefferiella. It appears that if the midge larva is small, it does no harm to the caddisfly. If it is larger, however, it may kill the host by crowding the case or changing the flow of water necessary for respiration.

Borger (1980) relates that trout scrape caddisfly larvae from the river bottom, ingesting case and all. Certainly the pupae, as they escape their cases and swim to the surface to emerge from the pupal skin, are highly vulnerable to predation. And as in all caddisflies, when females return to the water to lay eggs, yet another life stage of the caddisfly is subject to the attention of feeding trout.

References:

Borger, G. A. 1980. Naturals, A Guide to Food Organisms of the Trout. Stackpole Books, Harrisburg, Pennsylvania, 223 pp.

Flint, O. S. 1984. The genus Brachycentrus in North America, with a proposed phylogeny of the genera of Brachycentridae (Trichoptera). Smithsonian Contributions to Zoology 398:1-58.

Gallepp, G. W. 1974. Behavioral ecology of Brachycentrus occidentalis Banks during the pupation period. Ecology 55:1283-1294.

Hilsenhoff, W. L. 1985. The Brachycentridae (Trichoptera) of Wisconsin. Great Lakes Entomologist 18:149-154.

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

Waters, T. F. 1972. The drift of stream insects. Annual Review of Entomology 17:253-272.

Wiggins, G. B. 1998. Larvae of the North American Caddisfly Genera (Trichoptera), 2nd ed., University of Toronto Press, 457 pp.

Dr. Clarke Garry is a professor of biology at the University of Wisconsin – River Falls.