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1. Caddisflies (Trichoptera) that shred leaf detritus preferentially ingested lipid-coated detritus during the last larval instar. This is the interval when these insects accumulate the triglyceride reserves necessary for metamorphosis and reproduction. Early instar larvae, which do not accumulate lipids, showed no preference for any lipids. This behaviour indicates food selection based on changing nutritional needs.2. Pure unsaturated 18-and 20-carbon fatty acids, either as free acids or triglycerides, were most strongly preferred. Crude lipid and the neutral lipid fraction from aquatic hyphomycete fungi, an important component of aquatic leaf detritus, were also preferred.3. Tipula abdominalis (Diptera: Tipulidae) did not prefer any lipids during the last instar.

A negative exponential model with one independent variable, days or accumulated time, was examined for adequacy as a descriptive equation for aquatic leaf litter processing. The effect of adding a second independent variable, degree days or accumulated temperature, to the model was also examined. The two variable negative exponential model was shown to have two advantages over the single variable model. The expanded model provided an adequate fit of litter processing data for more cases than the single variable model. Also, the two variable model allowed determination of rate coefficients corresponding to each temperature level of the experiment rather than assuming a single, constant rate coefficient as with the one variable model. The trends of the temperature dependent rate coefficients were useful for examining processing differences between experiments for different sites and seasons.

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