Direct observations of the activity responses of mayfly nymphs to relative light change and light intensity


Studies were conducted in a laboratory stream to examine changes in locomotor activity and vertical movement on tile substrates of Stenonema modestum nymphs (Ephemeroptera: Heptagenidae) in response to relative light decrease and acclimation (adaptation) light intensity. Evening twilight, a period during which activity and location changes of mayfly nymphs including S. modestum occur in nature, was simulated by reducing light at constant rates of relative light decrease using a computer-controlled lighting system. Nymphs were kept at one of two light levels prior to the light reduction to test for changes in their response to light caused by visual adaptation to ambient light intensity. The timing of both increased locomotor activity and changes in vertical location on the substrate of S. modestum was strongly correlated with the strength of the relative decrease in light intensity (light stimulus). Timing of both activities took place earlier for nymphs adapted for 60 min at a constant, but reduced light intensity at the water's surface of 8 µW/cm2 than for those adapted for 60 min at a constant noon-time light intensity of 800 µW/cm2 Individual nymph responses to light change were contingent upon their initial activity during the light-adaptation periods. Relative light change was the controlling factor initiating both the photokinetic (the non-directional locomotor activity on the lower substrate surfaces) and phototactic (light-directed vertical movements between the lower and upper substrate surfaces) activity changes in these stream invertebrates.


Earth Systems Research Center

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Archiv für Hydrobiologie


Schweizerbart Science Publishers

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