The ballast stones in Silo nigricomis cases (Insecta: Trichoptera): drift resistance and ecological benefits, investigated by acoustic Doppler velocimetry

Hydraulic stress parameters of a cased caddis larva (Drusus biguttatus) using spatio-temporally filtered velocity measurements

“…In fact, this is much higher than published instantaneous threshold velocities for drift entry in other caddisflies, e.g., 0.28 ± 0.01 m s −1 in fifth instar and 0.13 ± 0.01 m s −1 in first instar larvae of Allogamus auricollis (Pictet, 1834) (Waringer, 1989). The highest threshold for drift entry for caddisflies so far has been reported for final instar larvae of Silo nigricornis (Pictet, 1834), which is up to 1.26 m s −1 ( König & Waringer, 2008 ). Given the risks associated with drifting, such as mechanical injuries by smashing against boulders, increased fish predation, or the increased possibility of landing in unfavurable microhabitats ( Waringer, 1992 ), withstanding the ever-changing flow stresses due to the velocity fluctuations is of vital importance for benthic invertebrates in streams.…”

“…The reasons why benthic animals expose themselves to high flow velocities are manifold. According to the data available, one of the driving forces is feeding behavior which heavily depends on feeding type: Drusus biguttatus is a typical scraper ( Vitecek et al, 2015 ) feeding on epilithic algae and biofilm, which, in turn, reach their highest densities midstream where hydraulic stress and water flow are highest ( König & Waringer, 2008 ).…”

“…On average, only 5.1% (range 3–29%; Tables 2 , 3 ) of the drag force is compensated by submerged weight in Drusus biguttatus , whereas the remainder has to be counterbalanced by the active efforts of the larvae to remain attached to the substrate, such as muscular strength and the effectiveness of claw usage. In Silo nigricornis , where lateral ballast stones are integrated into the case design, this compensation is up to 40% ( König & Waringer, 2008 ), which greatly expands the area of drift-save microhabitats: field studies have shown that in Allogamus auricollis and Silo nigricornis , 0% of the population was out of their range of adhesive friction on the stream bed. As a consequence, those species were significantly ( P < 0.001) under-represented in the drift.…”

Description of the larval stage of Drusus mixtus (Pictet, 1834) (Trichoptera: Limnephilidae: Drusinae) with notes on its ecology and zoogeography

“…However, these new ecological niches are not without challenges and riks: epilithic algae are most abundant at lotic stream sections and grow at the upper surface of the substrate. This forces grazers and other scraper species to expose themselves more to higher current velocities during feeding than omnivorous generalists that feed near the banks (König & Waringer, 2008). This results in a significant over-representation of scraper Drusinae species in the drift when compared with their relative abundance on the stream bed (Bacher & Waringer 1996).…”

Dynamic microhabitat shifts in space and time of caddisfly larvae (Insecta: Trichoptera) in a first‐order calcareous mountain stream

“…Large particles favour biofilm growth and respiration (Parker et al 2018), and increase the biomass of water mosses (Glime and Clemons 1972;Skuja 2011) and (semi-) sessile macrozoobenthic taxa, such as Gastropda, and Hydropsychidae, Simuliidae and Elmidae larvae (Bosco and Stanford 1996;Dietrich and Waringer 1999;Scheder and Waringer 2002;Zieritz and Waringer 2008). On the other end, small sediment particles, such as sand grains, are essential as habitats for Chironomid larvae (Chaloner and Wotton 1996) or for constructing cases and shelters in Trichoptera (König and Waringer 2008;Waringer and Graf 2011;Morse et al 2019). These examples corroborate the importance of substratum size as a prime determinant of the structure of lotic macroinvertebrate communities (Reice 1980;Lester et al 1996).…”