Positioning on substrates, positioning changes, and diel drift periodicities in mayflies

Diel changes in the positioning of several mayflies (Ephemeroptera) on the top and bottom surfaces of artificial substrates were examined. Stream drift samples were taken in conjunction with substrate samples. No significant diel changes in positioning were observed while all taxa exhibited nocturnal increases in drift. All taxa had apparent surface preferences. Baetis species occurred largely on top surfaces while Ephemerella spp., Paraleptophlebia mollis, and Heptageniidae largely occupied bottom surfaces. Diel positioning changes did not appear to be an important mechanism producing diel drift periodicities. Drift propensity showed large variation between taxa and was positively correlated with the proportion of individuals on top surfaces. For most taxa, drift probability varied markedly between dates and was correlated with high growth rates.