Ontogenic changes in prey consumption by the stonefly Paragnetina media inrelation to temporal variation in prey nutrient content

In Duffin Creek, Ontario, nymphs of the predatory stonefly Paragnetina medi a (Perlidae) commonly feed on detritus and three prey types: hydropsychid l arvae (Hydropsyche sparna and Hydropsyche slossonae), nymphs of the mayfly Baetis tricaudatus, and chironomid larvae belonging to subfamilies Tanypodi nae and Orthocladiinae. This study examined temporal changes in the nutrien t (lipid and protein) content of these prey to determine if the predator tr acks food resources by selecting prey on the basis of nutrient requirements at different stages in its own development. All three common prey types ex hibited temporal variation in lipid levels, with peaks occurring at differe nt times: June for the hydropsychids, August for the chironomids, and Octob er for B. tricaudatus. Prey protein levels were less variable. The proporti ons of the prey types that were eaten varied throughout the year and accord ing to predator size. For example, while small P. media ate mostly detritus , they also consumed early-instar hydropsychids during the larval recruitme nt period of the latter; mid-sized nymphs included more animal matter in th eir diet, primarily chironomid larvae; and larger nymphs primarily ate B. t ricaudatus. Paragnetina media nymphs in all size categories showed an incre ase in body lipid level in the autumn, suggesting a general accumulation of lipid reserves in readiness for the winter, although dietary adjustment to accomplish this was detectable only in small P. media. There was no eviden ce to suggest that P. media selected prey on the basis of the latter's prot ein content. Male P. media nymphs preyed predominantly on chironomid larvae and included mites in their diet, whereas female nymphs preferred B. trica udatus and hydropsychids. On a per milligram body mass basis, male nymphs h ad the higher nutrient gain, since, for both lipid and protein, intake by m ales was between two and three times that by females. However, male and fem ale bodies had similar lipid contents. We conclude that whereas there is te mporal variation both in the nutritional (lipid and protein) content of the common prey of P. media and in this predator's diet, there is only weak ev idence for nutrient-resource tracking.