The drag coefficient of cased caddis larvae from running waters: experimental determination and ecological applications

SUMMARY 1 Drag coefficients of cylindrical‐case caddis larvae from running waters were investigated in the laboratory using an artificial stream channel. 2 Dead larvae of the limnephilids Allogamus auricollis (first to fifth instars), fifth instars of Potamophylax cingulatus, Chaetapteryx fusca, Drusus monticola, Metanoea rhaetica and fourth instars of the brachycentrid Micrasema minimum, were exposed to different current speeds. When the heads of the larvae were directed towards the water flow (frontal position), the current necessary to dislodge the larvae ranged from 3.00cms‐1 (A. auricollis, first instar) to 70.50cms‐1 (P. cingulatus). With flow normal to the long axis of the case (lateral position), these speeds ranged from 2.20cms‐1 (M. minimum) to 20.80cms‐1 (P. cingulatus). 3 In frontal position, individual Reynolds numbers at the moment of dislodgement ranged from 74 (A. auricollis, first instars) to 14100 (P. cingulatus), and from 14 (M. minimum) to 1143 (P. cingulatus) in lateral position. Regression equations correlating case length, mean case width or fresh weight with Reynolds numbers at the moment of dislodgement (frontal and lateral position) were very highly significant with r20.91. 4 For the range of Reynolds numbers given above, the drag coefficient varied between 5.05 (A. auricollis, first instars) and 0.26 (P. cingulatus) in frontal case position and from 2.97 (M. minimum) to 0.69 (C. fusca) in lateral case position. Furthermore, the relationship between Reynolds number and drag coefficient was found to be linear on a In/In scale for both frontal and lateral case position.