Size related shifts in the physical habitat of two mayfly species (Ephemeroptera)

1. The spatial niche shifts of the nymphs of two mayfly species (Rhithrogena semicolorata and Ecdyonurus sp. gr. venosus) in relation to their growth were investigated in two lowland springs of northern Italy where they are dominant species. 2. Ordination of the different size classes of the two species revealed a canonical trend related to season and bottom roughness. 3. Larger nymphs of both species colonized rougher substrata. Differences between species in the roughness of areas colonized by small nymphs, which were very numerous, may have been due to competition for space.