The Life Histories and Drifting of the Plecoptera and Ephemeroptera in a Dartmoor Stream

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Abstract
Monthly samples of invertebrate drift and benthos were taken from the Walla Brook, a small stream on Dartmoor (south-west England). A shovel sampler and pond net were used for the bottom samples, and both surface nets and modified plankton samplers for the drift samples. The drift samplers are briefly described. Both drift samplers and an insect trap provided information on the imagines. The physical and chemical conditions in the Walla Brook are described. This study started after one of the coldest winters ever recorded on Dartmoor and the climate of Jan. and Feb. 1963 was very severe. Life histories are described for the following species: Plecoptera: Protonemura meyeri, Amphinemura sulcicollis, Leuctra inermis, L. hippopus, Perlodes microcephala, Isoperla grammatica and Chloroperla terrentium. Ephemeroptera: Ephemerella ignita, Ecdyonurus venosus, Baetis niger, B. rhodani and B. scambus. Notes are given on the following species: Plecoptera: Nemoura cambrica, Leuctra fusca, Brachyptera risi and Dinocras cephalotes. Ephemeroptera: Leptophlebia vespertina, Paraleptophlebia submarginata, Rhithrogena semicolorata, Heptagehia lateralis, Caenis rivulorum and Baetis pumilus. The results of other workers are discussed. The number of nymphs (N) taken in a drift sampler in 24 h is given by the equation: N = V [T.D1 + (24-T)D2] where the volume of water passing through the sampler = Vm3/h, length of night = Th, mean density of nymphs in the drift = D1 individuals/m3 at night and D2 individuals/m3 during the day. The various factors which affect D1 and D2 are discussed. It is concluded that the density of nymphs in the drift reflects both the number of nymphs moving over the exposed parts of stones and aquatic plants, and the extent of the competition between nymphs for food and space. More nymphs are on the tops of stones at night and competition appears to be greatest between rapidly-growing nymphs.