Nymphal development of Baetis vagans McDunnough (Ephemeroptera: Baetidae) and drift habits of large nymphs

Nymphal development of Baetis vagans McDunnough was studied at the Forks of the Credit River, Peel County, Ontario. Here, B. vagans is a bivoltine species inhabiting fast water; one generation occurs in the stream from November until late May and a second generation of smaller nymphs from July until mid-October. Nymphs continued to grow over winter although growth was slower than in the summer generation.In laboratory drift studies, the presence of a stonefly predator, Paragnetina media (Walker), significantly increased the drift of large Baetis vagans nymphs during dark periods. By drifting at night, mayfly nymphs may be less susceptible to predation by both invertebrates and fish.