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SUMMARY

1. The consumption of microcrustacea by two polyphagous predators, larvae of the caddisfly Plectrocnemia conspersa (Curtis) and the alderfly Sialis fuliginosa Pictet, was investigated in an English stream with a well-known macro- and microinvertebrate fauna. Benthic samples were collected in August, November, December and April, and the gut contents of all individuals of both predators were examined.

2. All the microcrustacean groups (Cyclopoida, Harpacticoida, Chydoridae and Ostracoda) were identified in gut contents. Of the ten taxa present in the benthos, all occurred in the diet of P. conspersa; nine were found in S. fuliginosa.

3. Ontogenetic shifts in the diets of both predators were found, and microcrustacea were consumed more frequently by small than large instars.

4. There was little evidence of selective feeding by P. conspersa, whereas ostracods were over-represented in the diet of S. fuliginosa, compared with benthic relative densities. The Chydoridae were under-represented in the diet of both predators.

5. The food web of Broadstone Stream is perhaps the most detailed web available for any running water habitat. Increased taxonomic resolution produced marked changes in values of connectance and predator-prey ratios. Linkage density remained fairly constant at different levels of resolution and were high, indicative of a web of generalist species. Omnivory was pronounced and may be characteristic of donor-controlled systems where organic detritus is the primary energy base.