The role of responses to light in the selection and maintenance of microhabitat by the nymphs of two species of mayfly.

In a field investigation of the role played by shade and direct illumination on the distribution of stream fauna (Hughes, in press), two species of mayfly nymph, Baetis harrisoni Barnard and Trieorythus discolor (Burro.), were found predominantly in reaches exposed to sunlight and reaches shaded by dense vegetation respectively. The results for Baetis harrisoni bear out the conclusions reached by Harrison & Elsworth (1958) who, during their Berg River survey, compared the faunas of a shady tributary and a physically similar open reach of the main Berg River at the same altitude and found higher incidences of this species at the open sampling station. Light and shade are generally tfiought (Welch, 1952; Coker, 1954) to limit or favour the distribution of stream fauna through such indirect factors as branch and leaf debris, algal growth and temperature. In the case of these two species of mayfly, however, it is postulated that their distribution within the stream (with respect to regions of light and shade) is a function of the responses of the nymphs to light stimuli. The following series of experiments was designed to investigate the responses of both species to light and thereby to determine the role played by these responses in the selection and maintenance of the microhabitat.