Asking the pikas politely…
Diel changes in the numbers and activity of mayfly (Ephemeroptera) nymphs on the substrate surface in a stony‐bottom stream were determined by direct observations, to investigate the relationship between benthic activity and drift. Individuals were viewed through a glass box (0.1 m2 area) at 2‐h intervals for 24 h; dim red light was used during darkness. Baetis was relatively unaffected by the presence of the observer, whereas Cinygmula was very easily disturbed. The number of individuals in view, activity per individual, and total activity all were greatest by day and least during the night for both species. Each of these measures was significantly correlated with temperature, which varied 6°–8°C over the diel cycle. Drift activity, in contrast, was strongly nocturnal and generally peaked immediately after nightfall. Because of the clear lack of correspondence between drift and the activity of mayfly nymphs on stone tops, behavioral drift in this system cannot be explained as the passive consequence of foraging.