Net-Building Behaviour of Six Hydropsyche species (Trichoptera) in Relation to Current Velocity and Distribution along the Rhône River

A longitudinal succession of species of Hydropsyche along a stream continuum is a common pattern in Palearctic as well as in Nearctic regions. Using a laboratory channel, we studied the relationship between current velocity and six Hydropsyche species distributed from small streams to large rivers in the Rhône Basin (France). Different phases of net-building behaviour were used to test the reactions of larvae to variations in current speed over a range of 4-40 cm/s. Hydropsyche angustipennis had more than 30% of its larvae wandering at the end of the experiment and was the least adapted species to the experimental conditions; the proportion of wandering larvae for the other five species was less than 10%. The largest proportion of catchnets with regular meshes occurred at velocities between 28 and 40 cm/s for the upstream species (H. siltalai and H. angustipennis) and for a species of the middle reaches of stream (H. pellucidula). Another middle-reach species (H. exocellata) had the highest proportion of catchnets with regular meshes between 8 and 40 cm/s, and for the downstream species (H. contubernalis and H. modesta), this proportion occurred at 15 cm/s. Our results suggest that the species are ordered in a downstream sequence according to their ability to spin a normal catchnet when the natural current velocity decreases. Downstream species seem to be more tolerant than upstream species to low-velocity conditions that may occur over long periods in the lower reaches of large rivers.