SEASONAL AND DIEL CHANGES IN POSITIONING OF GLOSSOSOMA-NIGRIOR BANKS (TRICHOPTERA-GLOSSOSOMATIDAE) ON ARTIFICIAL SUBSTRATES

Seasonal and diel changes in the positioning of larvae of G. nigrior on bricks are described. Between July and Nov. the proportion on the top face increased, the proportion on the front (upstream) face decreased and the proportion on other faces remained relatively unchanged. These changes were related to seasonal changes in water temperature. The positioning of larvae was dependent on current velocity only at higher water temperatures (> 19.degree. C). At lower current velocities (< 70 cm/s) larvae were predominantly on the front; at higher current velocities they were more equally distributed on the top, front and sides. Diel changes in positioning were apparent only at higher water temperatures (> 19.degree. C) and were characterized by an increase in the proportion on the front and decrease in the proportion on the top at night. Diel changes in positioning were not related to phototactic responses. G. nigrior appeared to use positioning to help satisfy respiratory requirements. At high water temperatures and at low current velocities the largest proportion were on the front where O2 richness was greatest. Nocturnal increases in the proportion on the front are probably controlled by diel rhythms of the O2 consumption rate.