Life Histories of Hexagenia limbata and Ephemera simulans (Ephemeroptera) in Dauphin Lake, Manitoba

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Abstract
The life histories of Hexagenia limbata (Serville) and Ephemera simulans Walker were investigated in Dauphin Lake, a large (surface area of 519.3 km2) but shallow (mean depth of 2.1 m) prairie lake in south-western Manitoba. Wing pad developmental stages helped to separate overlapping cohorts of H. limbata. During the 2-yr study, seven cohorts of this species were distinguished, indicative of three different life history patterns. Four of the seven cohorts had a 2-yr life history in which the eggs hatched soon after oviposition. Another type of 2-yr pattern was evident in two cohorts which spent nine months as overwintering eggs, and fifteen months in the nymphal stage. One cohort required 1.16 to 1.25 years (14 to 15 months) growth. The weighted average for all seven cohorts was 1.9 years (22.6 mo). The degree days above 10°C accumulated by each life history type were 1848, 2030 and 1806, respectively. Three or four cohorts were present at all times of the year in the population, which provides a large range of nymphal sizes and emergence times for feeding fish. In contrast, Ephemera simulans had a simple 2-yr life history in Dauphin Lake.

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