Current stoichiometric theory predicts that consumers typically use post-absorption mechanisms (i.e., excretion rates) rather than pre-absorption mechanisms (i.e., consumption rates and assimilation efficiencies) to help maintain body elemental composition.  Post-absorption mechanisms are also predicted to exhibit more adaptability to resource quality changes than pre-absorption mechanisms, which has led to the largely untested assumption that pre-absorption mechanisms are unimportant to consumer elemental composition.  Here, we determined the effects of altered resource stoichiometry (decreased C:N:P of detritus) on the stoichiometry of a leaf-shredding caddisfly (Pycnopsyche) via pre-absorption mechanisms.  Previous research in a nutrient-enriched headwater stream demonstrated that Pycnopsyche C:P decreased by 55% in response to a 37% decrease in leaf detritus C:P.  We hypothesized that Pycnopsyche’s consumption rate would decline with decreased detrital C:N:P because they would need less resource mass to obtain a similar mass of nutrients.  However, because increased leaf-associated microbial biomass decreases detrital C:N:P, we also predicted that Pycnopsyche assimilation efficiencies would increase.  Results indicated that increased detrital quality altered both Pycnopsyche leaf consumption rates and assimilation efficiencies.  Similar pre-absorption changes in feeding efficiencies may occur in other systems and are a potentially overlooked pathway through which variation in resource quality alters consumer elemental composition.