Food habits, net-spinning activity, energetics, and mercury accumulation in Hydrospsyche morosa were examined over a one year period on the South River in central Virginia. Feeding nets were observed as early as April and were widespread by May. Nets were virtually absent from late November through March. Gut content analysis revealed seasonal patterns in the consumption of various food items. From April through October, when feeding nets were widespread, detritus formed the bulk of the diet in terms of both numbers of particles and volume occupied. From November through March however, the algal component dominated in terms of numbers of particles although the detritus component still occupied a greater volume. Ivlev's preference index was employed and indicated that the seasonal differences in the relative amount of the three food types were not simply a matter of changing seston concentrations, but rather suggested a shift from a filter-feeding mode of feeding in the summer months to grazing on diatoms in the winter. H. morosa was bivoltine on the South River. The estimate of secondary production for the summer cohort was 3,246 mg AFDW/m²/yr, while the estimate for the winter cohort was 2,145 mg AFDW/m²/yr. The secondary production also was estimated for each season based on food habits to determine the impact of the observed seasonal switch in feeding habits on production and egestion rates. During the summer, the detritus component contributed most to production averaging about 50 percent. Animal and algal material contributed 30 and 20 percent, respectively. During the winter, algal material contributed most to the production, averaging just over 62 percent. Detritus also contributed during the winter averaging over 30 percent. Monthly rates of production and egestion were between 3 and 3.5 times faster during the summer. The concentrations of total mercury in seston, periphyton, and in the body tissue of H. morosa were analyzed each month. Mercury concentrations were between four and six times higher in the seston than in the periphyton. The concentration of mercury in the body tissue of H. morasa ranged from 0.14 ppm in March to over 1.20 ppm in July. Differences in Mercury concentration in the insects between seasons were significant. Regression analysis revealed a significant relationship between Hg concentration in the insects and the relative amount of detritus found in the guts. / Master of Science