Microdistribution and secondary production of Cyrnellus fraternus (Trichoptera : Polycentropodidae) from snag habitats in the Elm Fork of the Trinity River, Texas

A population of Cyrnellus fraternus (Banks) from an 8-km segment of the Elm Fork in north Texas was sampled once to twice monthly from October 1994 through September 1995. Distributions of C. fraternus were restricted to submerged wood habitat, or snag, the most stable substratum for retreat building and for resisting periodic high waters. Larvae were not found in the soft sand and mud benthic habitats near submerged wood. The insects were multivoltine with a minimum of 3 generations per year. Pupae were collected from July to September and adults emerge from May to October. The population overwintered as larvae. Individual dry mass of autumn 1994 fifth instars was significantly larger than that of summer 1995 fifth instars. Estimated annual production on snags for this population was 1,015.4 mg/m(2)/yr, mean standing stock biomass was 61.24 mg/m(2), cohort production:biomass ratio was 4.59, and the annual production/biomass rate was 16.58/yr. Standing stock biomass ranged from 1.6-319.1 mg/m(2) during the year. Standing stock biomass values were significantly higher in autumn and. summer than in winter. Larvae were more abundant in the lower portions of the study area, where now was reduced for much of the year, than in the upper reaches, characterized by higher current velocities. C. fraternus larvae were found in their highest numbers on snags with dense deposits of silt and sand