Silk filaments enhance the settlement of stream insect larvae

Many aquatic organisms need to settle in suitable benthic habitats while being transported via water currents. Such settlement is especially challenging for organisms that encounter complex benthic topography and lack the ability to move easily from the water column to the bed (e.g., via swimming). We conducted flume studies to examine whether the settlement of drifting stream insects is facilitated by adhesive filaments that extend from their bodies. Using a new tripwire visualization technique, we found that neonatal black flies (Simulium tribulatum) drifted with silk threads averaging six times their body length. These threads allowed larvae to contact or snag the bed from a greater height than would be possible through direct body-to-bed contact alone, and instantly arrested their downstream movement. Thus, silk increased their probability of settlement. We then performed an experiment to examine how settlement varied with bed topography and velocity. We tested whether settlement rate differed between a flat bed and an irregular bed that mimicked key aspects of their natural cobble-bed habitat. Velocities were similar for both bed treatments. Settlement on the irregular bed was 40 times greater than on the flat bed due to silk use. Settlement rate also exhibited a marginally significant decline with increasingly velocity on the flat bed, but not on the irregular bed. Silk threads should greatly increase the settlement rate of these nonswimming larvae on coarse-grained stream beds. Thus, silk snagging can potentially reduce the downstream distance that individuals are transported during a drift event, although the effects of silk on other phases of larval dispersal may differ.