Hydraulic niche utilization by larvae of the three Drusinae clades (Insecta: Trichoptera)

Abstract

Hydraulic niche descriptors of final instar larvae of nine Drusus species (Trichoptera) were studied in small, spring-fed, first-order headwaters located in the Mühlviertel (Upper Austria), Koralpe (Carinthia, Austria), and in the Austrian and Italian Alps. The species investigated covered all three clades of Drusinae: the shredder clade (Drusus franzi, D. alpinus), the grazer clade (D. biguttatus, D. chauvinianus, D. dudor, D. monticola), and the filtering carnivore clade (D. chrysotus, D. katagelastos, D. muelleri). Flow velocity was measured at front center of 68 larvae, head upstream, on the top of mineral substrate particles at water depths of 10–30 mm, using a tripod-stabilized Micro propeller meter (propeller diameter = 10 mm). Each data series consisted of a sampled measurement lasting 30 s (measuring interval = 1 s). In total, 2040 single velocity measurements were taken. Instantaneous flow velocities and drag at the sites of the 68 larvae varied from 0 to 0.93 m s⁻¹ and 0 to 8346 *10⁻⁶ N, respectively. Flow velocities and drag between the three clades were highly significantly different (p < 0.001); mean velocity (+ 95% confidence limits) for the three clades were 0.09 + 0.00 m s⁻¹ for the shredder, 0.25 + 0.00 m s⁻¹ for the grazer, and 0.31 + 0.01 m s⁻¹ for the filtering carnivore clade; the corresponding data for drag were (85 + 18)*10⁻⁶ N, (422 + 61)*10⁻⁶ N and (1125 + 83)*10⁻⁶ N, respectively. Adhesive friction ranged from (41.07 + 53.03)*10⁻⁶ N in D. franzi to (255.24 + 216.87)*10⁻⁶ N in D. chrysotus. Except in D. franzi and D. dudor adhesive friction was always well below drag force, indicating that submerged weight alone was not sufficient to stabilize the larvae in their hydraulic environment. Reynolds numbers varied between 0 in D. franzi and D. alpinus, and 12,634 in D. katagelastos, with 7% of the total in the laminar (R < 500), 30% in the transitional (R = 500–2000), and 61% in the fully turbulent stage (R > 2000). Froude numbers (Fr) varied from 0 to 2.97. The two Drusus species of the shredder clade and three out of four species of the grazer clade were exposed to subcritical Fr < 1, one species of the grazer clade and two out of three species of the filtering clade to supercritical Froude numbers >1.