PATTERNS OF MORPHOLOGICAL AND REPRODUCTIVE VARIATION IN STENONEMA MAYFLIES (EPHEMEROPTERA: HEPTAGENIIDAE) IN EASTERN LAKE HURON

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Abstract
Significant variation in the morphology of late-instar nymphs of Stenonema tripunctatum (Banks) in Lake Huron could not be ascribed to their proximity to warm-water plumes originating from the Bruce Nuclear Power Development, by either multivariate or univariate analyses of size and shape. Major sources of variation in S. tripunctatum were developmental state and sex. Nymphs collected just prior to emergence had low values of transverse measurements in anterior regions (inter-orbital width, prothoracic width), whereas males and females were separated by these measurements and other components of head geometry.No significant effects of thermal exposure were found in the reproductive development of S. tripunctatum, but this species is most commonly found below depths at which thermal-plume effects are expected. Based on a small but representative sample, ultimate-instar nymphs of Stenonema vicarium (Walker) from a depth of 3 m contained significantly fewer eggs and allocated a significantly lower proportion of biomass (dry weight) to reproduction in locations with higher temperatures.