Role of silk threads in the dispersal of larvae through stream pools

The slow flow found in stream pools should lead to increased larval deposition in habitat unsuitable to many benthic invertebrates such as the larvae of the black fly Simulium tribulatum. To increase their chances of successfully transiting a pool, these larvae may use silk threads previously thought to facilitate settlement in fast-flowing habitat. In this study, we tested two ways in which these threads can affect a larva's transport in slow flow. First, a significant decrease in the fall velocity, which should lead to increased drift distance, was found between live (with silk) and dead (without silk) larvae falling in a laboratory chamber. The reduction due to the presence of silk varied over the natural range of larval sizes from a maximum of a 36% reduction for neonates to only 14% for the largest late-instar larvae. A second possible role of silk, that of increasing the chance of resuspension in slow flow due to increased drag forces, was tested using dynamically scaled models of neonate and late-instar larvae. The coefficient of drag (C(d)) was determined for both model types, with and without scaled silk threads, under conditions recreating a range of naturally occurring Reynolds numbers (Re). A significant increase in C(d) was found due to the presence of silk for both life stages across relevant ranges in Re. The results of this study show that the presence of silk threads should significantly increase the distance larvae travel and their chances of being resuspended should they be deposited on the bed of a pool. Further, the same threads may act to both limit transport in regions of preferred fast flow, while maximizing transport through regions of unsuitably slow flow.