EMERGENCE AND UPSTREAM FLIGHT OF LOTIC MAYFLIES AND CADDISFLIES (EPHEMEROPTERA AND TRICHOPTERA) IN A LAKE OUTLET, CENTRAL FINLAND

Temporal and spatial occurrence and upstream flight of lotic mayflies and caddisflies were studied by means of kick netting, slit traps and floating emergence traps in Lake Konnevesi and its outlet stream (Siik akoski), mainly in 1983. Twenty species of mayflies and 78 species of caddisflies were recorded. Clearly lotic mayflies in the material were Baetis subalpinus, Heptagenia sulphurea and Ephemerella mucronata, wh ich occurred only in the stream samples. Females of lotic Baetis rhoda ni had a long distance upstream flight in the area and occurred freque ntly in the floating emergence traps especially in sandy littoral habi tats of L. Konnevesi. Procloeon bifidum and Nixe joernensis, which in some regions occur mainly in streams, were frequently found in the sto ny littoral belt of L. Konnevesi. Most of the mayflies recorded in the area were univoltine, but Baetis rhodani and Centroptilum luteolum at least had two generations in 1983. Strictly lotic species among caddi sflies were Ceratopsyche nevae, Hydropsyche saxonica, Halesus digitatu s, Ceraclea perplexa, Ylodes detruncatus, Athripsodes commutatus and H ydroptila forcipata. These were mainly found in stream samples. A shor t distance upstream flight of females (c. 0.1-0.4 km) was observed in the populations of H. siltalai, Psychomyia pusilla and Oxyethira frici and a moderate distance flight (at least 0.6 km) by females of Hydrop syche pellucidula, Cheumatopsyche lepida and Polycentropus irroratus. The most abundant caddisflies of the stream Siikakoski (Brachycentrus subnubilus, Neureclipsis bimaculata, Rhyacophila nubila, Agapetus ochr ipes, Micrasema setiferum and Hydroptila cornuta) were frequently foun d also in floating emergence traps in the lake (3.7 km from the rapids ). Females of Brachycentrus, Neureclipsis and Rhyacophila favoured tra ps on sandy substrata while females of Hydroptila were usually found i n the stony belt. In the trap material of Agapetus ochripes, males wer e more abundant than females. According to emergence data, all caddisf lies recorded in the area were univoltine though the emerging period o f several species such as Rhyacophila nubila and Neureclipsis bimacula ta was long. Maximum emergence was observed in early June when brachyc entrids were swarming and another peak in August when limnephilids and leptocerids were abundant.