Abstract
Seasonal and diel changes in the positioning of larvae of Glossosoma nigrior on bricks are described. Between July and November 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 °C). At lower current velocities (< 70 cm/s) larvae were predominantly on the front, whereas 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 °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 oxygen richness was greatest. It was hypothesized that nocturnal increases in the proportion on the front were controlled by diel rhythms of oxygen-consumption rate.
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